Address two Clang-Tidy findings the custom container tests exposed

Both come from instantiating basic_json with containers other than the
default ones, and neither shows up with the Clang-Tidy version available
outside CI:

- insert(const_iterator, basic_json&&) forwards its by-value iterator to
  the const-reference overload. performance-unnecessary-value-param asks
  for the copy to be a move; it only fires for an iterator that is not
  trivially copyable, as std::deque's is not. The NOLINT on the function
  does not cover it, because the finding is reported where the parameter
  is used rather than where it is declared. Move it, which is what the
  check asks for and is a (very small) improvement in its own right.

- cppcoreguidelines-use-enum-class rejects the unnamed enum that shadowed
  the inherited key_compare member type. An enum class would not do, since
  it declares a type of that name and the probe would find it again; a
  member function declaration hides the name just as well.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/api/basic_json/array_t.md

@@ -14,7 +14,11 @@ To store objects in C++, a type is defined by the template parameters explained
 ## Template parameters
 
 `ArrayType`
-:   container type to store arrays (e.g., `std::vector` or `std::list`)
+:   container type to store arrays. It must be a vector-like container: the library uses `operator[]`, `at()`, and
+    `resize()`, and requires random-access iterators. `#!cpp std::vector` and `#!cpp std::deque` qualify;
+    `#!cpp std::list` does not. See
+    [Template Parameter Requirements](../../features/types/template_parameters.md#arraytype) for the full list of
+    requirements.
 
 `AllocatorType`
 :   the allocator to use for objects (e.g., `std::allocator`)
@@ -66,3 +70,4 @@ Arrays are stored as pointers in a `basic_json` type. That is, for any access to
 ## Version history
 
 - Added in version 1.0.0.
+- Made `capacity()` optional, so that array types such as `#!cpp std::deque` can be used, in version 3.13.0.

docs/mkdocs/docs/api/basic_json/binary_t.md

@@ -42,7 +42,9 @@ represent a byte array in modern C++.
     `value_type` must additionally be exactly one byte wide (e.g., `std::uint8_t`/`char`/`std::byte`): the binary
     serializers (CBOR, MessagePack, BSON, UBJSON) read and write the container's raw bytes via
     `reinterpret_cast`, which is only correct for byte-sized elements -- a container like
-    `#!cpp std::vector<std::intptr_t>` will not work as `BinaryType`.
+    `#!cpp std::vector<std::intptr_t>` will not work as `BinaryType`. The elements must be stored contiguously, and
+    the binary readers additionally require `resize()` and `operator[]`. See
+    [Template Parameter Requirements](../../features/types/template_parameters.md#binarytype) for the full list.
 
 ## Notes
 
@@ -50,6 +52,11 @@ represent a byte array in modern C++.
 
 The default values for `BinaryType` is `#!cpp std::vector<std::uint8_t>`.
 
+#### Supported byte types
+
+`#!cpp std::vector<std::uint8_t>`, `#!cpp std::vector<char>`, and `#!cpp std::vector<std::byte>` are supported.
+Regardless of which of them is configured, [`dump`](dump.md) writes the bytes as the numbers 0..255.
+
 #### Custom BinaryType behavior
 
 When a custom `BinaryType` is configured (other than the default `#!cpp std::vector<std::uint8_t>`), you can assign
@@ -126,3 +133,6 @@ type `#!cpp binary_t*` must be dereferenced.
 ## Version history
 
 - Added in version 3.8.0. Changed the type of subtype to `std::uint64_t` in version 3.10.0.
+- Fixed [`dump`](dump.md), [`std::hash`](std_hash.md), and [`to_ubjson`](to_ubjson.md) for byte types that are not
+  integers (e.g., `#!cpp std::byte`) in version 3.13.0. `dump` now writes the bytes of a signed byte type (e.g.,
+  `#!cpp char`) as 0..255 rather than as negative numbers.

docs/mkdocs/docs/api/basic_json/boolean_t.md

@@ -11,6 +11,14 @@ literals `#!json true` and `#!json false`.
 
 To store boolean values in C++, a type is defined by the template parameter  `BooleanType` which chooses the type to use.
 
+## Template parameters
+
+`BooleanType`
+:   the type to store booleans. As it is stored directly inside a `basic_json` value (in a union), it must be a
+    trivially default-constructible, trivially copyable, and trivially destructible type that is convertible to and
+    from `#!cpp bool`. See
+    [Template Parameter Requirements](../../features/types/template_parameters.md#booleantype).
+
 ## Notes
 
 #### Default type

docs/mkdocs/docs/api/basic_json/index.md

@@ -35,6 +35,10 @@ class basic_json;
 | `BinaryType`         | type for binary arrays                                                    | [`binary_t`](binary_t.md)                   |
 | `CustomBaseClass`    | extension point for user code                                             | [`json_base_class_t`](json_base_class_t.md) |
 
+The library imposes a number of requirements on these types that are not expressed as C++ concepts, such as the
+container operations `object_t` and `array_t` must provide, or the fact that `StringType` must be `char`-based. They
+are collected in [Template Parameter Requirements](../../features/types/template_parameters.md).
+
 ## Specializations
 
 - [**json**](../json.md) - default specialization

docs/mkdocs/docs/api/basic_json/json_base_class_t.md

@@ -21,8 +21,11 @@ The default value for `CustomBaseClass` is `void`. In this case, an
 
 #### Limitations
 
-The type `CustomBaseClass` has to be a default-constructible class.
+The type `CustomBaseClass` has to be a default-constructible, non-`final` class.
 `basic_json` only supports copy/move construction/assignment if `CustomBaseClass` does so as well.
+A `CustomBaseClass` with non-static data members forfeits `basic_json`'s
+[standard layout](https://en.cppreference.com/w/cpp/named_req/StandardLayoutType) guarantee. See
+[Template Parameter Requirements](../../features/types/template_parameters.md#custombaseclass).
 
 ## Examples
 

docs/mkdocs/docs/api/basic_json/json_serializer.md

@@ -19,6 +19,12 @@ using json_serializer = JSONSerializer<T, SFINAE>;
 
 The default values for `json_serializer` is [`adl_serializer`](../adl_serializer/index.md).
 
+#### Requirements
+
+A custom serializer must provide `#!cpp static void to_json(basic_json&, T)` for every type it serializes, and either
+`#!cpp static void from_json(const basic_json&, T&)` or `#!cpp static T from_json(const basic_json&)` for every type it
+deserializes. See [Template Parameter Requirements](../../features/types/template_parameters.md#jsonserializer).
+
 ## Examples
 
 ??? example

docs/mkdocs/docs/api/basic_json/number_float_t.md

@@ -20,6 +20,16 @@ used.
 To store floating-point numbers in C++, a type is defined by the template parameter `NumberFloatType` which chooses the
 type to use.
 
+## Template parameters
+
+`NumberFloatType`
+:   the type to store floating-point numbers. Parsing and serialization are implemented in terms of
+    `#!cpp std::strtof`/`#!cpp std::strtod`/`#!cpp std::strtold` and `#!cpp std::snprintf`, so the type must be
+    `#!cpp float`, `#!cpp double`, or `#!cpp long double`. The
+    [binary formats](../../features/binary_formats/index.md) additionally require `#!cpp float` or `#!cpp double`,
+    because they have no encoding for `#!cpp long double`. See
+    [Template Parameter Requirements](../../features/types/template_parameters.md#numberfloattype).
+
 ## Notes
 
 #### Default type

docs/mkdocs/docs/api/basic_json/number_integer_t.md

@@ -20,6 +20,13 @@ used.
 To store integer numbers in C++, a type is defined by the template parameter `NumberIntegerType` which chooses the type
 to use.
 
+## Template parameters
+
+`NumberIntegerType`
+:   the type to store signed integers. It must be a **signed integral** type (`#!cpp std::is_integral`) with a
+    `#!cpp std::numeric_limits` specialization, and it is stored directly inside a `basic_json` value. See
+    [Template Parameter Requirements](../../features/types/template_parameters.md#numberintegertype-and-numberunsignedtype).
+
 ## Notes
 
 #### Default type

docs/mkdocs/docs/api/basic_json/number_unsigned_t.md

@@ -20,6 +20,14 @@ used.
 To store unsigned integer numbers in C++, a type is defined by the template parameter `NumberUnsignedType` which chooses
 the type to use.
 
+## Template parameters
+
+`NumberUnsignedType`
+:   the type to store unsigned integers. It must be an **unsigned integral** type (`#!cpp std::is_integral`) with a
+    `#!cpp std::numeric_limits` specialization, and it must be able to represent the absolute value of every
+    [`number_integer_t`](number_integer_t.md) value. See
+    [Template Parameter Requirements](../../features/types/template_parameters.md#numberintegertype-and-numberunsignedtype).
+
 ## Notes
 
 #### Default type

docs/mkdocs/docs/api/basic_json/object_comparator_t.md

@@ -30,3 +30,5 @@ and [`default_object_comparator_t`](default_object_comparator_t.md) otherwise.
 - Added in version 3.0.0.
 - Changed to be conditionally defined as `#!cpp typename object_t::key_compare` or `default_object_comparator_t` in
   version 3.11.0.
+- Fixed the fallback to `default_object_comparator_t`, which previously failed to compile for object types without a
+  `key_compare` member type, in version 3.13.0.

docs/mkdocs/docs/api/basic_json/object_t.md

@@ -18,7 +18,11 @@ To store objects in C++, a type is defined by the template parameters described
 ## Template parameters
 
 `ObjectType`
-:   the container to store objects (e.g., `std::map` or `std::unordered_map`)
+:   the container to store objects. Its template parameters must have the same order and meaning as those of
+    `std::map`; in particular, the third parameter is a comparator. `#!cpp std::unordered_map`, whose third parameter
+    is a hash function, therefore needs an adapter -- see
+    [Template Parameter Requirements](../../features/types/template_parameters.md#objecttype) for the full list of
+    requirements, an adapter example, and the containers that are known to work.
 
 `StringType`
 :   the type of the keys or names (e.g., `std::string`). The comparison function `std::less<StringType>` is used to
@@ -122,3 +126,4 @@ the object is silently converted as an array of key-value pairs, which is incorr
 ## Version history
 
 - Added in version 1.0.0.
+- Allowed object types whose `erase(iterator)` returns `#!cpp void` in version 3.13.0.

docs/mkdocs/docs/api/basic_json/string_t.md

@@ -23,6 +23,11 @@ JSON class into byte-sized characters during deserialization.
     `StringType`. To work with wide-character data, convert it to/from UTF-8 at the boundary instead -- see the
     FAQ's [wide string handling](../../home/faq.md#wide-string-handling) section for a conversion recipe.
 
+    Beyond the character type, the library expects a substantial part of the `#!cpp std::string` interface (contiguous
+    null-terminated `data()`, `substr()`, `find()`, `append()`, ...). See
+    [Template Parameter Requirements](../../features/types/template_parameters.md#stringtype) for the full list and
+    for the string types that are known to work.
+
 ## Notes
 
 #### Default type
@@ -78,3 +83,5 @@ and an example.
 ## Version history
 
 - Added in version 1.0.0.
+- Removed the requirement that `string_t` be implicitly convertible from `#!cpp std::string`, which the BSON writer and
+  the UBJSON reader relied on, in version 3.13.0.

docs/mkdocs/docs/api/basic_json/unflatten.md

@@ -37,7 +37,14 @@ Linear in the size of the JSON value.
 ## Notes
 
 Empty objects and arrays are flattened by [`flatten()`](flatten.md) to `#!json null` values and cannot unflattened to
-their original type. Apart from this example, for a JSON value `j`, the following is always true:
+their original type.
+
+A flattened array and a flattened object whose keys are array indices are indistinguishable, because both are
+described by the same JSON pointers. A value is therefore restored as an array if and only if one of its keys is the
+reference token `0`, and as an object otherwise: `#!json {"2": 1}` is restored unchanged, whereas `#!json {"0": 1}` is
+restored as `#!json [1]`. This decision does not depend on the order in which the flattened object is iterated.
+
+Apart from these two cases, for a JSON value `j`, the following is always true:
 `#!cpp j == j.flatten().unflatten()`.
 
 ## Examples
@@ -63,3 +70,4 @@ their original type. Apart from this example, for a JSON value `j`, the followin
 ## Version history
 
 - Added in version 2.0.0.
+- Made the array/object decision independent of the object's iteration order in version 3.13.0.

docs/mkdocs/docs/api/macros/json_throw_user.md

@@ -12,9 +12,11 @@
 Controls how exceptions are handled by the library.
 
 1. This macro overrides [`#!cpp catch`](https://en.cppreference.com/w/cpp/language/try_catch) calls inside the library.
-   The argument is the type of the exception to catch. As of version 3.8.0, the library only catches `std::out_of_range`
-   exceptions internally to rethrow them as [`json::out_of_range`](../../home/exceptions.md#out-of-range) exceptions.
-   The macro is always followed by a scope.
+   The argument is the type of the exception to catch. The library uses it in a single place: to swallow any exception
+   escaping the parent-pointer check that [`JSON_DIAGNOSTICS`](json_diagnostics.md) adds to the class invariant. The
+   places where the library catches its own [`json::out_of_range`](../../home/exceptions.md#out-of-range) exceptions
+   use `JSON_INTERNAL_CATCH` instead, which `JSON_CATCH_USER` also overrides unless `JSON_INTERNAL_CATCH_USER` is
+   defined. The macro is always followed by a scope.
 2. This macro overrides `#!cpp throw` calls inside the library. The argument is the exception to be thrown. Note that
    `JSON_THROW_USER` should leave the current scope (e.g., by throwing or aborting), as continuing after it may yield
    undefined behavior.

docs/mkdocs/docs/features/object_order.md

@@ -51,7 +51,11 @@ If you do want to preserve the **insertion order**, you can use the type [`nlohm
     --8<-- "examples/ordered_json.output"
     ```
 
-Alternatively, you can use a more sophisticated ordered map like [`tsl::ordered_map`](https://github.com/Tessil/ordered-map) ([integration](https://github.com/nlohmann/json/issues/546#issuecomment-304447518)) or [`nlohmann::fifo_map`](https://github.com/nlohmann/fifo_map) ([integration](https://github.com/nlohmann/json/issues/485#issuecomment-333652309)).
+Alternatively, [`nlohmann::fifo_map`](https://github.com/nlohmann/fifo_map) also preserves the insertion order and, unlike [`ordered_map`](../api/ordered_map.md), keeps a lookup index, so it does not have the quadratic cost described below. It is used through a small adapter ([integration](https://github.com/nlohmann/json/issues/485#issuecomment-333652309)).
+
+If the order does not matter and you only want faster lookup, `boost::unordered_flat_map`, `absl::flat_hash_map`, `absl::node_hash_map`, and several other hash maps work through an adapter that restores the template argument order `basic_json` expects; see [Template Parameter Requirements](types/template_parameters.md#objecttype). Note these are *unordered*, not insertion-ordered.
+
+[`tsl::ordered_map`](https://github.com/Tessil/ordered-map) cannot be used: its iterators expose the mapped value as `const`, while `basic_json` needs to modify it in place.
 
 The [`ordered_map`](../api/ordered_map.md) behind `nlohmann::ordered_json` is deliberately minimal and has no lookup
 index, so every key access is a linear scan and building an object of `n` keys costs O(n²). This is unnoticeable at

docs/mkdocs/docs/features/types/index.md

@@ -79,7 +79,8 @@ template<
     class NumberFloatType = double,
     template<typename U> class AllocatorType = std::allocator,
     template<typename T, typename SFINAE = void> class JSONSerializer = adl_serializer,
-    class BinaryType = std::vector<std::uint8_t>
+    class BinaryType = std::vector<std::uint8_t>,
+    class CustomBaseClass = void
 >
 class basic_json;
 ```
@@ -106,6 +107,10 @@ using number_float_t = NumberFloatType;
 using binary_t = nlohmann::byte_container_with_subtype<BinaryType>;
 ```
 
+Not every type can be passed for these template arguments: the library uses the resulting types in ways that imply a
+number of requirements, for instance that `StringType` is `char`-based or that `ArrayType` is vector-like. These
+requirements are collected in [Template Parameter Requirements](template_parameters.md).
+
 
 ## Objects
 

docs/mkdocs/docs/features/types/template_parameters.md

@@ -0,0 +1,663 @@
+# Template Parameter Requirements
+
+Class [`basic_json`](../../api/basic_json/index.md) is configurable through eleven template parameters. The library
+never formally states what a type passed for one of these parameters has to provide -- the requirements are implied by
+the way the library uses the resulting [`object_t`](../../api/basic_json/object_t.md),
+[`array_t`](../../api/basic_json/array_t.md), [`string_t`](../../api/basic_json/string_t.md), etc. This page collects
+these requirements so they do not have to be discovered by trial and error. Each section lists the concrete types
+that are known to work for that parameter and the ones that do not, checked against Boost 1.83, Abseil 20250127.0,
+Folly, EASTL 3.21, `ankerl::unordered_dense`, `phmap`, `gtl`, `robin_hood`, `tsl::ordered_map`, and Qt 6.
+
+## How to read this page
+
+Requirements are split into two groups:
+
+- **Always required** -- needed to instantiate `basic_json` at all, or needed by functions that virtually every program
+  uses (construction, element access, [`dump`](../../api/basic_json/dump.md)).
+- **Required for ...** -- only needed when a particular part of the API is instantiated. Member function templates are
+  only instantiated when they are used, so a type may be perfectly usable even though it does not satisfy these
+  requirements, as long as the corresponding functions are never called.
+
+!!! warning "Requirements are not checked"
+
+    Apart from a `#!cpp static_assert` on the array iterator category, the requirements below are not diagnosed with
+    dedicated error messages. Violating most of them results in a compiler error somewhere inside the library. Six
+    violations are not caught at compile time at all:
+
+    - A [`StringType`](#stringtype) whose `data()` is not null-terminated compiles and silently misparses numbers,
+      because the lexer hands the buffer to `#!cpp std::strtoull`/`#!cpp std::strtoll`/`#!cpp std::strtod`.
+    - A [`BinaryType`](#binarytype) whose `value_type` is wider than one byte compiles and silently produces wrong
+      results, because the readers and writers reinterpret its storage as raw bytes.
+    - A stateful [`AllocatorType`](#allocatortype) compiles and silently ignores its state: allocation, deallocation,
+      and [`get_allocator()`](../../api/basic_json/get_allocator.md) each use a different default-constructed instance.
+    - A [`NumberUnsignedType`](#numberintegertype-and-numberunsignedtype) too narrow to hold the absolute value of
+      every `NumberIntegerType` value silently corrupts: with `#!cpp std::int64_t`/`#!cpp std::uint32_t`,
+      `#!cpp basic_json(INT64_MIN).dump()` yields `#!json -0`.
+    - The two [cross-specialization conversions](#cross-specialization-conversions) below. These abort on an assertion
+      in a normal build, and only fail silently under `#!cpp NDEBUG`.
+
+## Overview
+
+| Template parameter                                                | Default                           | Notable substitutes                                                    |
+|-------------------------------------------------------------------|-----------------------------------|------------------------------------------------------------------------|
+| [`ObjectType`](#objecttype)                                       | `std::map`                        | [`nlohmann::ordered_map`](../../api/ordered_map.md), Abseil hash maps  |
+| [`ArrayType`](#arraytype)                                         | `std::vector`                     | `#!cpp std::deque`                                                     |
+| [`StringType`](#stringtype)                                       | `std::string`                     | `std::string`-like types over `char`                                   |
+| [`BooleanType`](#booleantype)                                     | `bool`                            | none worth using                                                       |
+| [`NumberIntegerType`](#numberintegertype-and-numberunsignedtype)  | `std::int64_t`                    | any signed integer type                                                |
+| [`NumberUnsignedType`](#numberintegertype-and-numberunsignedtype) | `std::uint64_t`                   | any unsigned integer type                                              |
+| [`NumberFloatType`](#numberfloattype)                             | `double`                          | `float` (`long double`: no binary formats)                             |
+| [`AllocatorType`](#allocatortype)                                 | `std::allocator`                  | stateless allocators                                                   |
+| [`JSONSerializer`](#jsonserializer)                               | `adl_serializer`                  | serializers with the same interface                                    |
+| [`BinaryType`](#binarytype)                                       | `#!cpp std::vector<std::uint8_t>` | `#!cpp std::vector<char>`                                              |
+| [`CustomBaseClass`](#custombaseclass)                             | `void`                            | any default-constructible class                                        |
+
+!!! warning "Third-party containers and incomplete types"
+
+    `object_t` is instantiated inside the definition of `basic_json` -- it is probed for a `key_compare` member to
+    form [`object_comparator_t`](../../api/basic_json/object_comparator_t.md) -- i.e. while `basic_json` is still an
+    incomplete type. `#!cpp std::map` is required by the standard to support incomplete mapped types; most
+    third-party maps are not, and inspecting the mapped type at class scope (for instance with
+    `#!cpp std::is_trivially_move_assignable`) makes them unusable as `ObjectType`, no matter how their template
+    arguments are adapted. This rules out `absl::btree_map`, `phmap::btree_map`, `gtl::btree_map`,
+    `robin_hood::unordered_node_map`, `folly::F14FastMap`, and `eastl::hash_map`.
+
+    `array_t` is only *named* in the class definition and is not instantiated until `basic_json` is complete, so an
+    `ArrayType` that inspects its value type at class scope is generally fine -- `boost::container::small_vector` and
+    `static_vector` both reject incomplete value types yet work here. `absl::InlinedVector` is the exception: the
+    `#!cpp std::is_trivially_move_assignable<basic_json>` it evaluates while instantiating itself re-enters the
+    library's own trait machinery mid-instantiation.
+
+!!! note "Folly requires C++20"
+
+    Folly's headers use `#!cpp consteval` and `#!cpp std::type_identity`, so any `basic_json` specialization that
+    names a Folly type has to be compiled as C++20 or later, whatever the rest of the library supports.
+
+## `ObjectType`
+
+`ObjectType` is instantiated as
+
+```cpp
+using object_t = ObjectType<StringType,                                    // key_type
+                            basic_json,                                    // mapped_type
+                            default_object_comparator_t,                   // key_compare
+                            AllocatorType<std::pair<const StringType,
+                                                    basic_json>>>;         // allocator_type
+```
+
+i.e., the template arguments follow the order and meaning of `std::map`.
+
+### Always required
+
+- The template must be usable with **four** type arguments in the order shown above. The third argument is a
+  **comparator**; containers that expect something else in this position (e.g., a hash function) need an alias template
+  or wrapper -- see [Notes](#notes).
+- An optional member type `key_compare`. If it is present it becomes
+  [`object_comparator_t`](../../api/basic_json/object_comparator_t.md); otherwise
+  [`default_object_comparator_t`](../../api/basic_json/default_object_comparator_t.md) is used.
+- Member types `key_type`, `mapped_type`, `value_type`, and `iterator`.
+- `value_type` must behave like `#!cpp std::pair<const key_type, mapped_type>`; the library accesses `.first` and
+  `.second` on it.
+- `iterator` must be default-constructible and satisfy
+  [LegacyBidirectionalIterator](https://en.cppreference.com/w/cpp/named_req/BidirectionalIterator). The type returned
+  by `cbegin()`/`cend()` must satisfy the same requirements.
+- Constructors: default, copy, move, and from an iterator range `(first, last)`.
+- Member functions `begin()`, `end()`, `cbegin()`, `cend()`, `empty()`, `size()`, `max_size()`, `clear()`,
+  `find(key)`, `count(key)`, `emplace(key, value)`, `insert(value_type)`, `insert(first, last)`, `operator[](key)`,
+  `erase(iterator)`, and `erase(first, last)`. `erase(iterator)` may return the following iterator or `#!cpp void`;
+  in the latter case the library computes the successor itself, before erasing.
+- `erase(key)` is **optional**: if the container does not provide one, the library falls back to `find(key)` followed
+  by `erase(iterator)`.
+- `at(key)` is required only by [`to_ubjson`](../../api/basic_json/to_ubjson.md) and
+  [`to_bjdata`](../../api/basic_json/to_bjdata.md), but every container tried here provides it.
+- `emplace` and `insert(value_type)` must return `#!cpp std::pair<iterator, bool>` and must have **unique-key**
+  semantics; multimaps cannot be used.
+- The type must be swappable (via `std::swap` or an ADL `swap`).
+- The comparison operators `==` and `<`; `!=`, `<=`, `>`, and `>=` are derived from them. Where the library uses
+  three-way comparison (C++20), `==` and `<=>` are required **instead** -- the six two-way operators do not satisfy
+  it. They implement [`basic_json`'s comparison operators](../../api/basic_json/operator_eq.md).
+
+### Required for heterogeneous key lookup
+
+The overloads of [`at`](../../api/basic_json/at.md), [`operator[]`](../../api/basic_json/operator%5B%5D.md),
+[`find`](../../api/basic_json/find.md), [`contains`](../../api/basic_json/contains.md),
+[`count`](../../api/basic_json/count.md), [`erase`](../../api/basic_json/erase.md), and
+[`value`](../../api/basic_json/value.md) that accept a key type other than `object_t::key_type` require
+
+- a **transparent** comparator, i.e. [`object_comparator_t`](../../api/basic_json/object_comparator_t.md) has a member
+  type `is_transparent` (this is why the default comparator is `#!cpp std::less<>` since C++14), and
+- corresponding heterogeneous `find`, `count`, `erase`, and `operator[]` overloads on the container.
+
+### Notes
+
+#### `std::unordered_map` needs an adapter
+
+`#!cpp std::unordered_map` cannot be passed directly: its third template parameter is a hash function, but
+`basic_json` passes a comparator in that position. An alias template or wrapper that restores the expected argument
+order makes it usable:
+
+```cpp
+template<class Key, class T, class IgnoredCompare, class Allocator>
+struct unordered_map_object
+    : std::unordered_map<Key, T, std::hash<Key>, std::equal_to<Key>, Allocator>
+{
+    using base_t = std::unordered_map<Key, T, std::hash<Key>, std::equal_to<Key>, Allocator>;
+    using base_t::base_t;
+};
+
+using unordered_json = nlohmann::basic_json<unordered_map_object>;
+```
+
+Whether `#!cpp std::unordered_map` can be instantiated at all depends on the standard library: `object_t` is formed
+while `basic_json` is still incomplete (see the warning above), and libstdc++ 9 needs the size of the mapped type to
+instantiate the hash map's node type, so the adapter does not compile there. Newer libstdc++ versions, and the hash
+maps listed below, do not have that problem.
+
+The adapter above works verbatim for Abseil's, Boost's, `phmap`'s and `gtl`'s hash maps, which all place the hash
+function third and take a `#!cpp std::pair<const Key, T>` allocator fifth. Two need a different adapter:
+
+- `ankerl::unordered_dense` expects an allocator over `#!cpp std::pair<Key, T>` (non-const key), so the allocator has
+  to be rebound to that or dropped.
+- `robin_hood`'s fifth parameter is the non-type `MaxLoadFactor100`, so its adapter must drop the allocator entirely.
+
+None of these hash maps defines `key_compare`, so all of them additionally rely on `object_comparator_t` falling back
+to [`default_object_comparator_t`](../../api/basic_json/default_object_comparator_t.md); see
+[`object_comparator_t`](../../api/basic_json/object_comparator_t.md).
+
+#### Abseil hash maps
+
+`absl::flat_hash_map` and `absl::node_hash_map` tolerate an incomplete value type, but they take a hash function as
+their third template argument. The same adapter as for `#!cpp std::unordered_map` makes them usable:
+
+```cpp
+template<class Key, class T, class IgnoredCompare, class Allocator>
+struct flat_hash_object
+    : absl::flat_hash_map<Key, T, absl::Hash<Key>, std::equal_to<Key>, Allocator>
+{
+    using base_t = absl::flat_hash_map<Key, T, absl::Hash<Key>, std::equal_to<Key>, Allocator>;
+    using base_t::base_t;
+};
+
+using flat_hash_json = nlohmann::basic_json<flat_hash_object>;
+```
+
+`absl::node_hash_map` keeps references to the mapped values valid across insertions; `absl::flat_hash_map` does not,
+which makes it behave like [`ordered_json`](../../api/ordered_json.md) with respect to
+[iterator invalidation](../../api/basic_json/index.md#iterator-invalidation). Both expose a `capacity()` member
+function, so [`JSON_DIAGNOSTICS`](../../api/macros/json_diagnostics.md) treats them conservatively and keeps the
+parent pointers correct either way.
+
+#### Iteration order
+
+The library never relies on the container's iteration order for correctness; it does determine the order in which
+object keys are serialized by [`dump`](../../api/basic_json/dump.md) and visited by
+[`items`](../../api/basic_json/items.md). See [Object Order](../object_order.md).
+
+#### `capacity()` marks a container as insertion-ordered
+
+With [`JSON_DIAGNOSTICS`](../../api/macros/json_diagnostics.md) enabled, the library detects insertion-ordered maps by
+probing for a `capacity()` member function (`nlohmann::ordered_map` inherits it from `std::vector`) and refreshes all
+parent pointers after every insertion. An `ObjectType` that happens to have a `capacity()` member is therefore treated
+conservatively -- this is correct, but slower.
+
+#### Key order and duplicate keys
+
+The library does not sort or de-duplicate keys itself; the behavior described in
+[`object_t`](../../api/basic_json/object_t.md) is entirely the behavior of the chosen container.
+
+### Compatible containers
+
+| Container                                                                                                     | Notes                                                                    |
+|---------------------------------------------------------------------------------------------------------------|--------------------------------------------------------------------------|
+| `#!cpp std::map` (default)                                                                                    |                                                                          |
+| [`nlohmann::ordered_map`](../../api/ordered_map.md)                                                           | used by [`ordered_json`](../../api/ordered_json.md); keeps insertion order |
+| [`nlohmann::fifo_map`](https://github.com/nlohmann/fifo_map)                                                  | keeps insertion order; adapter puts `fifo_map_compare` in the comparator slot |
+| `boost::container::map`, `boost::container::flat_map`                                                         | no adapter needed                                                        |
+| `#!cpp std::unordered_map`                                                                                    | through the adapter above; not with libstdc++ 9, see the note            |
+| `boost::unordered_map`, `boost::unordered_flat_map`, `boost::unordered_node_map`                              | through the adapter above                                                |
+| `absl::flat_hash_map`, `absl::node_hash_map`                                                                  | through the adapter above; `flat_hash_map` moves mapped values on rehash  |
+| `phmap::flat_hash_map`, `phmap::node_hash_map`, `gtl::flat_hash_map`                                          | through the adapter above                                                |
+| `ankerl::unordered_dense::map` and `segmented_map`                                                            | adapter must rebind or drop the allocator                                |
+| `robin_hood::unordered_flat_map`                                                                              | adapter must drop the allocator                                          |
+| `folly::F14NodeMap`                                                                                           | through the adapter above; requires C++20, see the note above            |
+| `folly::sorted_vector_map`                                                                                    | alias must drop the allocator, whose value type it disagrees on          |
+
+### Containers that cannot be used
+
+| Container                                                                                                     | Reason                                                                   |
+|---------------------------------------------------------------------------------------------------------------|--------------------------------------------------------------------------|
+| `absl::btree_map`, `phmap::btree_map`, `gtl::btree_map`, `robin_hood::unordered_node_map`, `folly::F14FastMap`, `eastl::hash_map` | require a complete mapped type                          |
+| `eastl::map`                                                                                                  | EASTL iterators do not work with `#!cpp std::iterator_traits`            |
+| `tsl::ordered_map`                                                                                            | its iterators expose the mapped value as `#!cpp const`                   |
+| `QMap`                                                                                                        | no `value_type` member type                                              |
+| `QHash`                                                                                                       | its `value_type` is the mapped type rather than a key/value pair, and its iterators dereference to the mapped value |
+| `#!cpp std::multimap`, `#!cpp std::unordered_multimap`                                                         | `emplace` does not return `#!cpp std::pair<iterator, bool>`              |
+
+## `ArrayType`
+
+`ArrayType` is instantiated as
+
+```cpp
+using array_t = ArrayType<basic_json, AllocatorType<basic_json>>;
+```
+
+### Always required
+
+- The template must be usable with **two** type arguments (value type and allocator).
+- Member types `value_type` and `iterator`.
+- Constructors: default, copy, and move; and from an iterator range `(first, last)`.
+- Member functions `begin()`, `end()`, `cbegin()`, `cend()`, `empty()`, `size()`, `max_size()`, `clear()`,
+  `operator[](size_type)`, `back()`, `push_back()`, `emplace_back()`, `pop_back()`, `resize()`,
+  `insert()` (single element, count, and range), `erase(pos)`, and `erase(first, last)`.
+  `basic_json::insert(pos, initializer_list)` goes through the range overload, so no initializer-list `insert` is
+  needed. `at(size_type)` is **not** required: [`basic_json::at(size_type)`](../../api/basic_json/at.md) checks the
+  index itself and then uses `operator[]`.
+- `iterator` must be default-constructible, and it as well as the type returned by `cbegin()`/`cend()` must satisfy
+  [LegacyRandomAccessIterator](https://en.cppreference.com/w/cpp/named_req/RandomAccessIterator).
+  A `#!cpp static_assert` only checks for
+  [LegacyBidirectionalIterator](https://en.cppreference.com/w/cpp/named_req/BidirectionalIterator), but
+  [`dump`](../../api/basic_json/dump.md) (`cend() - 1`),
+  [`erase(idx)`](../../api/basic_json/erase.md) (`begin() + idx`), and the random-access operations of
+  [`basic_json::iterator`](../../api/basic_json/begin.md) require random access.
+- The comparison operators, as for [`ObjectType`](#objecttype): `==` and `<`, or `==` and `<=>` under C++20.
+
+### Required for individual functions
+
+- A member type `value_type`, for [`to_bson`](../../api/basic_json/to_bson.md) of an array.
+- A constructor from `(count, value)`, for
+  [`basic_json(size_type, const basic_json&)`](../../api/basic_json/basic_json.md).
+- Swappability, via `#!cpp std::swap` or an ADL `swap`, for [`swap(array_t&)`](../../api/basic_json/swap.md).
+
+!!! note "`capacity()` is optional"
+
+    With [`JSON_DIAGNOSTICS`](../../api/macros/json_diagnostics.md) enabled, the library reads `array_t::capacity()`
+    to find out whether adding an element reallocated the array and moved its elements, which would invalidate the
+    parent pointers. An array type without a `capacity()` member function is handled conservatively: the parent
+    pointers of all elements are refreshed after every insertion, which makes adding *n* elements cost O(*n*²). Only
+    diagnostics builds pay this; without them `capacity()` is never called.
+
+### Compatible containers
+
+| Container                                                                               | Notes                                                    |
+|-------------------------------------------------------------------------------------------|----------------------------------------------------------|
+| `#!cpp std::vector` (default)                                                           |                                                          |
+| `#!cpp std::deque`                                                                      | references survive appends, but not insertions elsewhere; see the `capacity()` note above |
+| `#!cpp std::pmr::vector`                                                                | through an alias, as the allocator comes from `AllocatorType` instead |
+| `boost::container::vector`, `deque`, `devector`                                         |                                                          |
+| `boost::container::stable_vector`                                                       | the only one tried that keeps references valid across *every* insertion |
+| `boost::container::small_vector`, `folly::small_vector`                                 | through an alias that fixes the inline capacity          |
+| `boost::container::static_vector`                                                       | through the same kind of alias, for arrays that stay within the fixed capacity |
+| `folly::fbvector`                                                                       | requires C++20, see the note above                       |
+
+### Containers that cannot be used
+
+| Container                        | Reason                                                                                       |
+|----------------------------------|----------------------------------------------------------------------------------------------|
+| `#!cpp std::list`                | no `operator[]`, and no random-access iterators                                              |
+| `eastl::vector`, `QList`, `QVector` | no `max_size()`; they handle the incomplete value type fine                                |
+| `absl::InlinedVector`            | requires a complete value type, see the note above                                            |
+| `absl::FixedArray`               | the size is fixed at construction, so `resize`, `push_back`, `insert` and `erase` are missing |
+
+## `StringType`
+
+`StringType` is used **both** for JSON string values and for the keys of JSON objects
+(`string_t` and `object_t::key_type`).
+
+### Always required
+
+- A member type `value_type` that is one byte wide and `char`-compatible. The library stores and processes UTF-8
+  encoded `char` data and hands `data()` to `#!cpp std::strtoull`/`#!cpp std::strtoll`.
+  `#!cpp std::wstring`, `#!cpp std::u16string`, and `#!cpp std::u32string` are **not** valid choices; see the FAQ on
+  [wide string handling](../../home/faq.md#wide-string-handling).
+- Constructors: default, copy, move, from `#!cpp const char*` (which must not be `#!cpp explicit`), from
+  `#!cpp (const char*, size_type)`, and from `#!cpp (size_type, char)`; and copy or move assignment.
+- Member functions `size()`, `clear()`, `resize(n, c)`, `data()`, `push_back(char)`, and `operator[]`
+  (const and non-const, returning references). `c_str()` and `back()` are **not** required.
+- `data()` must return a pointer to a contiguous, **null-terminated** buffer -- the parser hands it to
+  `#!cpp std::strtoull`. A type whose `data()` is not null-terminated does not fail to compile; it silently
+  misparses numbers.
+- `append(const char*, size_type)`, used by [`dump`](../../api/basic_json/dump.md), and `append(const StringType&)`,
+  used by the CBOR reader for indefinite-length strings. The library's internal string concatenation additionally has
+  to append a `#!cpp char` and a `#!cpp const char*`; for each it selects between `append(arg)`, `#!cpp operator+=`,
+  `append(first, last)`, and `append(data, size)`.
+- The comparison operator `==` against another `StringType`, and `<` for use as a key of the chosen
+  [`ObjectType`](#objecttype) (with the default comparator, `#!cpp std::less<>` must be able to compare two
+  `StringType` values, and a `StringType` with the key types used for lookup). `!=` is never applied to a
+  `StringType`, and `==` against `#!cpp const char*` is resolved by the implicit `#!cpp const char*` constructor.
+
+### Required for the binary formats
+
+- `resize(n)`, used by the readers to make room for a block of bytes.
+- Non-const `operator[]`, into which the readers `#!cpp std::memcpy` those bytes.
+
+### Required for JSON Pointer, `flatten`, and `diff`
+
+- A static member `npos` and the member function `find_first_of(char, size_type)` -- together with `data()`,
+  `reserve(n)`, and `append(const char*, size_type)` they implement the escaping and unescaping of reference tokens
+  described in RFC 6901. Neither `find(const StringType&, size_type)`, nor `substr(pos, count)`, nor
+  `replace(pos, count, const StringType&)` is required.
+- `empty()`.
+- `begin()` and `end()` -- used by
+  [`operator[](const json_pointer&)`](../../api/basic_json/operator%5B%5D.md) to decide whether a reference token
+  denotes an array index.
+
+### Required for other functionality
+
+| Functionality                                                                                                    | Additional requirement                                                              |
+|--------------------------------------------------------------------------------------------------------------------|-------------------------------------------------------------------------------------|
+| [`diff`](../../api/basic_json/diff.md), [`items`](../../api/basic_json/items.md), [`std::hash`](../../api/basic_json/std_hash.md) | conversion of a `#!cpp std::size_t` to `StringType`: either assignability from the result of `#!cpp std::to_string`, or an ADL overload `#!cpp void int_to_string(StringType&, std::size_t)` |
+| [`std::hash<basic_json>`](../../api/basic_json/std_hash.md)                                                      | additionally a specialization of `#!cpp std::hash<StringType>`                        |
+| [`to_bson`](../../api/basic_json/to_bson.md)                                                                    | `find(value_type)` and `npos`                                                         |
+| [`parse`](../../api/basic_json/parse.md) from a `string_t`                                                      | the input adapters must accept it; otherwise pass a character range                   |
+| `#!cpp operator<<(std::ostream&, const json_pointer&)`                                                           | streamability to `#!cpp std::ostream`                                                 |
+| exception messages                                                                                               | `data()` and `size()`, or `begin()` and `end()`                                       |
+
+### Compatible types
+
+| Type                                                            | Notes                                                                     |
+|-------------------------------------------------------------------|---------------------------------------------------------------------------|
+| `#!cpp std::string` (default)                                   |                                                                           |
+| `#!cpp std::basic_string` with a custom **stateless** allocator |                                                                           |
+| `#!cpp std::pmr::string`                                        | see the warning below before relying on the memory resource               |
+| `boost::container::string`                                      | needs a user-supplied `#!cpp std::hash` specialization (Boost provides `boost::hash` instead) |
+| `folly::fbstring`                                               | requires C++20, see the note above                                        |
+| `eastl::string`                                                 | needs a user-supplied `#!cpp std::hash` and an ADL `int_to_string` (it is not assignable from a `#!cpp std::string`); [`parse`](../../api/basic_json/parse.md) does not accept it directly -- pass a character range or a `#!cpp std::string` |
+| a custom string class in a user-defined namespace               | if the requirements above are met                                          |
+
+### Types that cannot be used
+
+| Type                                                                | Reason                                                                |
+|-----------------------------------------------------------------------|-----------------------------------------------------------------------|
+| `#!cpp std::wstring`, `#!cpp std::u16string`, `#!cpp std::u32string` | the character type is not one byte wide                               |
+| `#!cpp std::u8string`                                               | one byte wide, but `#!cpp char8_t` is not `#!cpp char`-compatible      |
+| `absl::Cord`                                                        | no `value_type`, and the storage is not contiguous                    |
+| `QString`                                                           | no `append(const char*, size_type)`; its `QChar` is also two bytes wide, though that is never diagnosed |
+
+!!! warning "A `std::pmr::string` mostly does not use the memory resource you choose"
+
+    `basic_json` cannot be given an allocator or a memory resource. `AllocatorType` is default-constructed at every
+    allocation and has to be stateless (see [`AllocatorType`](#allocatortype)), and string values the library creates
+    are constructed with their own default allocator. So:
+
+    - Every string the library itself produces -- from [`parse`](../../api/basic_json/parse.md), from
+      [`dump`](../../api/basic_json/dump.md), or by default construction -- allocates from
+      `#!cpp std::pmr::get_default_resource()`.
+    - **Copying** an arena-backed string into a value silently drops its memory resource: the copy lands on the
+      default resource, because `#!cpp std::pmr::polymorphic_allocator` does not propagate on copy construction.
+      Nothing warns about this.
+    - **Moving** one in does keep it, and later growth still allocates from that arena -- but it does not survive a
+      copy of the enclosing `basic_json`.
+    - Passing `#!cpp std::pmr::polymorphic_allocator` as `AllocatorType` does not work around any of this; it does
+      not compile.
+
+    Apart from moving a string in, the only way to redirect these allocations is the process-global
+    `#!cpp std::pmr::set_default_resource()`.
+
+!!! tip "Reference implementation"
+
+    The unit test `tests/src/unit-alt-string.cpp` contains `alt_string`, a minimal string type that satisfies the
+    requirements needed for the tested subset of the API. It is a good starting point for a custom `StringType`.
+
+## `BooleanType`
+
+`boolean_t` is stored **directly** inside `basic_json`, as a member of an anonymous union.
+
+### Always required
+
+- A literal type that is trivially default-constructible, trivially copyable, and trivially destructible; otherwise the
+  union's special member functions are deleted.
+- **Implicitly** convertible from `#!cpp bool` -- an `#!cpp explicit` constructor is not enough, because the
+  `to_json` overload for a custom `BooleanType` is constrained on `#!cpp std::is_convertible` -- and contextually
+  convertible to `#!cpp bool` (here an `#!cpp explicit operator bool` is fine).
+- Comparison operators `==`, `!=`, `<`, `<=`, `>`, `>=` (or `<=>`).
+- Convertible from and to `#!cpp bool` through the serializer, because
+  [`get<bool>()`](../../api/basic_json/get.md) is used internally.
+
+There is little reason to use anything other than `#!cpp bool` here.
+
+### Compatible types
+
+`#!cpp bool` is the only usable choice. Another trivially copyable type that is implicitly convertible to and from
+`#!cpp bool` -- `#!cpp std::uint8_t`, say -- does compile, and JSON booleans still round-trip, but the type then
+serves as both `boolean_t` and an ordinary integer: `basic_json` can no longer be constructed or assigned from a
+`#!cpp std::uint8_t` at all (the boolean and unsigned-integer `to_json` overloads become ambiguous), and
+[`get<std::uint8_t>()`](../../api/basic_json/get.md) on a number throws
+[`type_error.302`](../../home/exceptions.md#jsonexceptiontype_error302) instead of returning the value.
+
+## `NumberIntegerType` and `NumberUnsignedType`
+
+Both types are stored **directly** inside `basic_json`'s union.
+
+### Always required
+
+- `#!cpp std::is_integral` must be satisfied: `NumberIntegerType` must be a **signed** integer type,
+  `NumberUnsignedType` an **unsigned** integer type. Class types are not supported -- among others, the constructors
+  taking integer values are constrained on `#!cpp std::is_integral`.
+- Trivially default-constructible, trivially copyable, and trivially destructible (union member).
+- `#!cpp std::numeric_limits` must be specialized for both types.
+- `NumberUnsignedType` must be able to represent the absolute value of every `NumberIntegerType` value; serialization
+  of negative numbers converts the value to `NumberUnsignedType`.
+- Both types must fit into the internal 64-character number buffer used by
+  [`dump`](../../api/basic_json/dump.md), which is the case for all standard integer types.
+- [`std::hash<basic_json>`](../../api/basic_json/std_hash.md) additionally requires `#!cpp std::hash` specializations.
+
+### Notes
+
+The number types influence what the parser accepts: an integer literal that does not round-trip through the chosen type
+is stored as [`number_float_t`](../../api/basic_json/number_float_t.md) instead. Choosing types narrower than 64 bits
+therefore silently changes parse results rather than raising an error. See
+[Number Handling](number_handling.md) for details.
+
+### Compatible types
+
+| Type pair                                                                                    | Support                                                             |
+|----------------------------------------------------------------------------------------------|---------------------------------------------------------------------|
+| `#!cpp std::int64_t` / `#!cpp std::uint64_t` (default)                                       | full                                                                |
+| `#!cpp std::int32_t` / `#!cpp std::uint32_t`, `#!cpp long long` / `#!cpp unsigned long long` | full; narrower types change which literals the parser can represent |
+| any other pair of standard signed/unsigned integer types                                     | full                                                                |
+| class types, enumerations                                                                    | not usable; `#!cpp std::is_integral` must hold                      |
+| `#!cpp bool`, or a type already used for another member of the union                         | not usable; `#!cpp std::is_integral<bool>` is in fact `#!cpp true`, but the `get_impl_ptr` overloads for `boolean_t`, `number_integer_t`, `number_unsigned_t` and `number_float_t` would collide |
+
+## `NumberFloatType`
+
+`number_float_t` is stored **directly** inside `basic_json`'s union.
+
+### Always required
+
+- Trivially default-constructible, trivially copyable, and trivially destructible (union member).
+- `#!cpp std::numeric_limits` must be specialized; `max_digits10` is used to size the conversion.
+- `#!cpp std::isfinite` must be applicable to the type.
+
+### Required for parsing and serialization
+
+`NumberFloatType` must be one of `#!cpp float`, `#!cpp double`, or `#!cpp long double`:
+
+- The [parser](../parsing/index.md) converts number literals with `#!cpp std::strtof`, `#!cpp std::strtod`, or
+  `#!cpp std::strtold`; the library provides overloads for exactly these three types.
+- [`dump`](../../api/basic_json/dump.md) falls back to `#!cpp std::snprintf` with the `%g` and `%Lg` conversion
+  specifiers, for which the library likewise provides only `#!cpp double` and `#!cpp long double` overloads
+  (`#!cpp float` is promoted to `#!cpp double`).
+
+If `#!cpp std::numeric_limits<NumberFloatType>` describes an IEEE 754 binary32 or binary64 number, `dump` uses the
+Grisu2 algorithm, which produces the shortest representation that round-trips. Otherwise the `snprintf` fallback with
+`max_digits10` digits is used.
+
+### Required for the binary formats
+
+`NumberFloatType` must be `#!cpp float` or `#!cpp double`. The writers for
+[CBOR, MessagePack, UBJSON, BJData, and BSON](../binary_formats/index.md) map a floating-point value onto an IEEE 754
+binary32 or binary64 field and have no encoding for `#!cpp long double`.
+
+### Compatible types
+
+| Type                        | Support                                                                                             |
+|-----------------------------|-----------------------------------------------------------------------------------------------------|
+| `#!cpp double` (default)    | full; short round-trip output through Grisu2                                                        |
+| `#!cpp float`               | full; short round-trip output through Grisu2                                                        |
+| `#!cpp long double`         | `dump` and `parse` only; the binary format writers do not compile, as they only handle IEEE 754 binary32 and binary64 |
+| any other type              | not usable                                                                                          |
+
+## `AllocatorType`
+
+`AllocatorType` is instantiated with **one** argument, for each of `object_t`, `array_t`, `string_t`, `binary_t`,
+`basic_json`, and `#!cpp std::pair<const StringType, basic_json>`.
+
+### Always required
+
+- The template must be usable with exactly one type argument. The library instantiates `AllocatorType<T>` directly and
+  never uses `#!cpp std::allocator_traits<...>::rebind_alloc`.
+- It must satisfy the [Allocator](https://en.cppreference.com/w/cpp/named_req/Allocator) named requirement so that
+  `#!cpp std::allocator_traits` can be used with it.
+- It must be **default-constructible and stateless**. Objects are allocated with a default-constructed allocator and
+  deallocated with a *different* default-constructed allocator, and
+  [`get_allocator()`](../../api/basic_json/get_allocator.md) returns a default-constructed instance. Allocators
+  carrying state are not supported, so there is no way to tell a `basic_json` where to allocate from; see the note
+  under [`StringType`](#stringtype) for what that means in practice. A stateful allocator is **not diagnosed**: it
+  compiles and silently ignores the state.
+- It must support **incomplete types**: `AllocatorType<basic_json>` is instantiated inside the definition of
+  `basic_json` itself.
+- `#!cpp std::allocator_traits<AllocatorType<basic_json>>::pointer` becomes
+  [`basic_json::pointer`](../../api/basic_json/index.md#container-types), and iterators are constructed from raw
+  `#!cpp basic_json*` values. The `pointer` type must therefore be a plain pointer; fancy pointers are not supported.
+
+### Compatible types
+
+| Type                                                            | Support                                            |
+|-----------------------------------------------------------------|----------------------------------------------------|
+| `#!cpp std::allocator` (default)                                | full                                               |
+| a custom stateless allocator template                           | full                                               |
+| stateful allocators, e.g. `#!cpp std::pmr::polymorphic_allocator`| not usable; see the requirements above             |
+
+## `JSONSerializer`
+
+`JSONSerializer` is instantiated as `JSONSerializer<T, void>` and defaults to
+[`adl_serializer`](../../api/adl_serializer/index.md).
+
+### Always required
+
+- The template must accept **two** type arguments. It does not have to give the second one a default -- `basic_json`
+  declares the parameter as `#!cpp template<typename T, typename SFINAE = void> class JSONSerializer`, so uses such as
+  `#!cpp JSONSerializer<T>` inside the library supply `#!cpp void` themselves. The second parameter exists so that
+  partial specializations can be constrained by SFINAE.
+- For every type `T` that is converted **to** a JSON value, a static member function
+  `#!cpp static void to_json(basic_json&, T)` must exist.
+- For every type `T` that is converted **from** a JSON value, either
+  `#!cpp static void from_json(const basic_json&, T&)` or `#!cpp static T from_json(const basic_json&)` must exist.
+  The latter form is required for types that are not default-constructible; see
+  [Arbitrary Types Conversions](../arbitrary_types.md).
+- To support the [converting constructor](../../api/basic_json/basic_json.md) between different `basic_json`
+  specializations, `to_json` must be available for `boolean_t`, `number_integer_t`, `number_unsigned_t`,
+  `number_float_t`, `string_t`, `object_t`, `array_t`, and `binary_t` of the *source* specialization.
+
+### Compatible types
+
+| Type                                                              | Support                                                                 |
+|-------------------------------------------------------------------|-------------------------------------------------------------------------|
+| [`nlohmann::adl_serializer`](../../api/adl_serializer/index.md) (default) | full                                                             |
+| a class template deriving from `adl_serializer`                   | full; the usual way to change behavior while keeping the defaults        |
+| an unrelated template with the same interface                     | full, but it has to handle every type the library converts               |
+
+## `BinaryType`
+
+`BinaryType` is not a JSON type; it is used for the byte strings of the
+[binary formats](../binary_formats/index.md). It is wrapped as
+
+```cpp
+using binary_t = nlohmann::byte_container_with_subtype<BinaryType>;
+```
+
+### Always required
+
+- A non-`final` class type -- [`byte_container_with_subtype`](../../api/byte_container_with_subtype/index.md) derives
+  from it publicly.
+- A member type `value_type` that is **exactly one byte** wide (e.g., `#!cpp std::uint8_t`, `#!cpp char`, or
+  `#!cpp std::byte`). Readers and writers reinterpret the container's storage as raw bytes. A wider `value_type` is
+  **not diagnosed**: it compiles and silently produces wrong results.
+- Contiguous storage: the binary readers `#!cpp std::memcpy` into `#!cpp &binary[n]`, the writers `reinterpret_cast`
+  `data()`.
+- Default-constructible, copy-constructible, and move-constructible.
+- Member functions `size()`, `empty()`, `data()`, `resize()`, `operator[]`, `back()`, `begin()`, `end()`, `cbegin()`,
+  and `cend()` with random-access iterators, and `insert(pos, first, last)`, which the CBOR reader uses to join the
+  chunks of an indefinite-length byte string. `push_back()` is **not** required.
+- Comparison operators: `==` is used by
+  [`byte_container_with_subtype`](../../api/byte_container_with_subtype/index.md), the relational operators by
+  [`basic_json`'s comparison operators](../../api/basic_json/operator_le.md).
+
+### Required for individual functions
+
+- `clear()`, for [`basic_json::clear()`](../../api/basic_json/clear.md).
+
+`max_size()`, `at()`, `reserve()`, `erase()`, `pop_back()`, and `emplace_back()` are **not** used at all.
+
+See [`binary_t`](../../api/basic_json/binary_t.md) for how a non-default `BinaryType` changes the meaning of assigning
+such a container to a `basic_json` value.
+
+### Compatible containers
+
+| Container                                                                                                                  | Notes                                                              |
+|------------------------------------------------------------------------------------------------------------------------------|--------------------------------------------------------------------|
+| `#!cpp std::vector<std::uint8_t>` (default)                                                                                |                                                                    |
+| `#!cpp std::vector<char>`, `#!cpp std::vector<std::byte>`                                                                  | `dump()` writes the bytes as 0..255 whichever is used              |
+| `boost::container::vector<std::uint8_t>`, `boost::container::small_vector<std::uint8_t, N>`                                |                                                                    |
+| `absl::InlinedVector<std::uint8_t, N>`                                                                                     | usable here, unlike as an `ArrayType`, because the value type is complete |
+| `eastl::vector<std::uint8_t>`                                                                                              | usable here, unlike as an `ArrayType`, because `max_size()` is not needed |
+| `folly::fbvector<std::uint8_t>`                                                                                            | requires C++20, see the note above                                 |
+
+### Containers that cannot be used
+
+| Container                                            | Reason                                                                                  |
+|--------------------------------------------------------|-----------------------------------------------------------------------------------------|
+| `QByteArray`                                         | no `empty()` (it spells that `isEmpty()`); its `insert` takes an index rather than an iterator; and it converts to `string_t`, which makes `to_json` ambiguous between a string and a binary value |
+| `#!cpp std::string`                                  | `binary_t::container_type` and `string_t` would be the same type, so the two [`swap`](../../api/basic_json/swap.md) overloads collide and `basic_json` cannot be instantiated at all |
+| `#!cpp std::deque<std::uint8_t>`                     | storage is not contiguous, so there is no `data()`                                       |
+| containers whose `value_type` is wider than one byte | see above -- accepted by the compiler, wrong at runtime                                  |
+
+## `CustomBaseClass`
+
+`CustomBaseClass` is an extension point: unless it is `#!cpp void` (the default, which selects the empty
+`nlohmann::json_default_base`), `basic_json` publicly derives from it.
+
+### Always required
+
+- A non-`final`, default-constructible class type.
+- `basic_json` is copy-/move-constructible and copy-/move-assignable only if `CustomBaseClass` is.
+
+### Notes
+
+`basic_json` is documented to be a
+[StandardLayoutType](https://en.cppreference.com/w/cpp/named_req/StandardLayoutType). Because `basic_json` has
+non-static data members of its own, a `CustomBaseClass` with non-static data members forfeits this guarantee.
+
+Note the namespace of `CustomBaseClass` becomes an associated namespace of `basic_json` for the purpose of
+argument-dependent lookup.
+
+See [`json_base_class_t`](../../api/basic_json/json_base_class_t.md) for an example.
+
+### Compatible types
+
+| Type                                                     | Support                                                      |
+|----------------------------------------------------------|--------------------------------------------------------------|
+| `#!cpp void` (default)                                   | an empty base class is used; no effect on `basic_json`        |
+| any default-constructible, non-`final` class             | full; see [`json_base_class_t`](../../api/basic_json/json_base_class_t.md) |
+
+## Cross-specialization conversions
+
+Converting a value from one `basic_json` specialization into another (see the
+[converting constructor](../../api/basic_json/basic_json.md)) imposes two additional requirements that are not
+diagnosed at compile time. With assertions enabled they abort on the `#!cpp JSON_ASSERT` at the end of the converting
+constructor; under `#!cpp NDEBUG` they fail **silently** at runtime:
+
+- The target `string_t` must be directly constructible from the source `string_t`. Otherwise the string is converted to
+  an array of character codes.
+- The target `object_t::key_type` must be directly constructible from the source object's key type. Otherwise the
+  object is converted to an array of key/value pairs.
+
+See [issue #3425](https://github.com/nlohmann/json/issues/3425), [`string_t`](../../api/basic_json/string_t.md), and
+[`object_t`](../../api/basic_json/object_t.md).
+
+## See also
+
+- [Types](index.md) -- overview of how JSON values are stored
+- [Number Handling](number_handling.md) -- how the number types affect parsing and serialization
+- [Object Order](../object_order.md) -- using an insertion-ordered `ObjectType`
+- [`basic_json`](../../api/basic_json/index.md) -- API documentation of the class template

docs/mkdocs/mkdocs.yml

@@ -98,6 +98,7 @@ nav:
       - Types:
           - features/types/index.md
           - features/types/number_handling.md
+          - features/types/template_parameters.md
   - Integration:
       - integration/index.md
       - integration/migration_guide.md

include/nlohmann/detail/exceptions.hpp

@@ -101,7 +101,10 @@ class exception : public std::exception
                     {
                         if (&element.second == current)
                         {
-                            tokens.emplace_back(element.first.c_str());
+                            // data() is null-terminated, so a key containing
+                            // a null byte is cut short here rather than
+                            // truncating the whole message at what()
+                            tokens.emplace_back(element.first.data());
                             break;
                         }
                     }

include/nlohmann/detail/hash.hpp

@@ -114,7 +114,9 @@ std::size_t hash(const BasicJsonType& j)
             seed = combine(seed, static_cast<std::size_t>(j.get_binary().subtype()));
             for (const auto byte : j.get_binary())
             {
-                seed = combine(seed, std::hash<std::uint8_t> {}(byte));
+                // the cast is needed for binary types whose value type is not
+                // an integer (e.g., std::byte)
+                seed = combine(seed, std::hash<std::uint8_t> {}(static_cast<std::uint8_t>(byte)));
             }
             return seed;
         }

include/nlohmann/detail/input/binary_reader.hpp

@@ -2899,7 +2899,10 @@ class binary_reader
                                number_string,
                                out_of_range::create(406, concat("number overflow parsing '", number_string, '\''), nullptr));
                 }
-                return sax->number_float(parsed_float, std::move(number_string));
+                // number_string is a std::string, while the SAX interface takes a
+                // string_t; convert explicitly, as the two are only implicitly
+                // convertible for some string types
+                return sax->number_float(parsed_float, string_t(number_string.data(), number_string.size()));
             }
             case token_type::uninitialized:
             case token_type::literal_true:

include/nlohmann/detail/iterators/iter_impl.hpp

@@ -88,8 +88,13 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
 
     iter_impl() = default;
     ~iter_impl() = default;
-    iter_impl(iter_impl&&) noexcept = default;
-    iter_impl& operator=(iter_impl&&) noexcept = default;
+    // the exception specification is left to be computed rather than declared:
+    // an array or object type whose iterator is not nothrow move constructible
+    // (std::deque's is not before libstdc++ 11) would make a declared noexcept
+    // differ from the implicit one, which deletes the function -- and is an
+    // error outright with older compilers
+    iter_impl(iter_impl&&) = default; // NOLINT(hicpp-noexcept-move,performance-noexcept-move-constructor,cppcoreguidelines-noexcept-move-operations)
+    iter_impl& operator=(iter_impl&&) = default; // NOLINT(hicpp-noexcept-move,performance-noexcept-move-constructor,cppcoreguidelines-noexcept-move-operations)
 
     /*!
     @brief constructor for a given JSON instance

include/nlohmann/detail/json_pointer.hpp

@@ -17,6 +17,7 @@
 #endif  // JSON_NO_IO
 #include <limits> // max
 #include <numeric> // accumulate
+#include <set> // set
 #include <string> // string
 #include <utility> // move
 #include <vector> // vector
@@ -71,7 +72,7 @@ class json_pointer
                                string_t{},
                                [](const string_t& a, const string_t& b)
         {
-            return detail::concat(a, '/', detail::escape(b));
+            return detail::concat<string_t>(a, '/', detail::escape(b));
         });
     }
 
@@ -265,7 +266,7 @@ class json_pointer
             JSON_THROW(detail::parse_error::create(109, 0, detail::concat("array index '", s, "' is not a number"), nullptr));
         }
 
-        const char* p = s.c_str();
+        const char* p = s.data();
         char* p_end = nullptr; // NOLINT(misc-const-correctness)
         errno = 0; // strtoull doesn't reset errno
         const unsigned long long res = std::strtoull(p, &p_end, 10); // NOLINT(runtime/int)
@@ -300,19 +301,35 @@ class json_pointer
     }
 
   private:
+    /*!
+    @brief the reference token sequences that denote arrays
+
+    @ref unflatten collects the pointer prefixes that have a reference token 0
+    among their children; @ref get_and_create creates arrays exactly below
+    those prefixes and objects everywhere else. Deciding this up front keeps
+    the result independent of the order in which the flattened object is
+    iterated, which is unspecified for some object types.
+    */
+    using array_parents_t = std::set<std::vector<string_t>>;
+
     /*!
     @brief create and return a reference to the pointed to value
 
     @complexity Linear in the number of reference tokens.
 
+    @throw parse_error.106 if an array index begins with '0'
     @throw parse_error.109 if array index is not a number
     @throw type_error.313 if value cannot be unflattened
     */
     template<typename BasicJsonType>
-    BasicJsonType& get_and_create(BasicJsonType& j) const
+    BasicJsonType& get_and_create(BasicJsonType& j, const array_parents_t& array_parents) const
     {
         auto* result = &j;
 
+        // the reference tokens that have been consumed so far; used to look up
+        // whether the value to be created below is an array or an object
+        std::vector<string_t> prefix;
+
         // in case no reference tokens exist, return a reference to the JSON value
         // j which will be overwritten by a primitive value
         for (const auto& reference_token : reference_tokens)
@@ -321,10 +338,11 @@ class json_pointer
             {
                 case detail::value_t::null:
                 {
-                    if (reference_token == "0")
+                    if (array_parents.find(prefix) != array_parents.end())
                     {
-                        // start a new array if the reference token is 0
-                        result = &result->operator[](0);
+                        // some reference token below this position is 0, so the
+                        // value is an array
+                        result = &result->operator[](array_index<BasicJsonType>(reference_token));
                     }
                     else
                     {
@@ -364,6 +382,8 @@ class json_pointer
                 default:
                     JSON_THROW(detail::type_error::create(313, "invalid value to unflatten", &j));
             }
+
+            prefix.push_back(reference_token);
         }
 
         return *result;
@@ -823,7 +843,8 @@ class json_pointer
         {
             // use the text between the beginning of the reference token
             // (start) and the last slash (slash).
-            auto reference_token = reference_string.substr(start, slash - start);
+            const auto count = (slash == string_t::npos ? reference_string.size() : slash) - start;
+            auto reference_token = string_t(reference_string.data() + start, count);
 
             // check reference tokens are properly escaped
             for (std::size_t pos = reference_token.find_first_of('~');
@@ -939,6 +960,24 @@ class json_pointer
 
         BasicJsonType result;
 
+        // collect the pointer prefixes that have a reference token 0 among
+        // their children; the values below them are arrays, all others are
+        // objects (see array_parents_t)
+        array_parents_t array_parents;
+        for (const auto& element : *value.m_data.m_value.object)
+        {
+            json_pointer ptr(element.first);
+            std::vector<string_t> prefix;
+            for (auto& reference_token : ptr.reference_tokens)
+            {
+                if (reference_token == "0")
+                {
+                    array_parents.insert(prefix);
+                }
+                prefix.push_back(std::move(reference_token));
+            }
+        }
+
         // iterate the JSON object values
         for (const auto& element : *value.m_data.m_value.object)
         {
@@ -951,7 +990,7 @@ class json_pointer
             // that if the JSON pointer is "" (i.e., points to the whole value),
             // function get_and_create returns a reference to the result itself.
             // An assignment will then create a primitive value.
-            json_pointer(element.first).get_and_create(result) = element.second;
+            json_pointer(element.first).get_and_create(result, array_parents) = element.second;
         }
 
         return result;

include/nlohmann/detail/meta/type_traits.hpp

@@ -172,17 +172,18 @@ struct has_to_json < BasicJsonType, T, enable_if_t < !is_basic_json<T>::value >>
 template<typename T>
 using detect_key_compare = typename T::key_compare;
 
-template<typename T>
-struct has_key_compare : std::integral_constant<bool, is_detected<detect_key_compare, T>::value> {};
-
-// obtains the actual object key comparator
+// obtains the actual object key comparator: object_t::key_compare if the
+// object type defines it, and default_object_comparator_t otherwise
+//
+// note detected_or_t is used rather than std::conditional, because the latter
+// names both of its type arguments eagerly; object_t::key_compare would then
+// be a hard error for an object type that does not define it
 template<typename BasicJsonType>
 struct actual_object_comparator
 {
     using object_t = typename BasicJsonType::object_t;
     using object_comparator_t = typename BasicJsonType::default_object_comparator_t;
-    using type = typename std::conditional < has_key_compare<object_t>::value,
-          typename object_t::key_compare, object_comparator_t>::type;
+    using type = detected_or_t<object_comparator_t, detect_key_compare, object_t>;
 };
 
 template<typename BasicJsonType>
@@ -778,6 +779,13 @@ using has_erase_with_key_type = typename std::conditional <
                                 std::true_type,
                                 std::false_type >::type;
 
+template<typename T>
+using detect_capacity = decltype(std::declval<const T&>().capacity());
+
+// type trait to check if a type has a capacity() member function
+template<typename T>
+struct has_capacity : std::integral_constant<bool, is_detected<detect_capacity, T>::value> {};
+
 // a naive helper to check if a type is an ordered_map (exploits the fact that
 // ordered_map inherits capacity() from std::vector)
 template <typename T>

include/nlohmann/detail/output/binary_writer.hpp

@@ -261,7 +261,7 @@ class binary_writer
 
                 // step 2: write the string
                 oa->write_characters(
-                    reinterpret_cast<const CharType*>(j.m_data.m_value.string->c_str()),
+                    reinterpret_cast<const CharType*>(j.m_data.m_value.string->data()),
                     j.m_data.m_value.string->size());
                 break;
             }
@@ -581,7 +581,7 @@ class binary_writer
 
                 // step 2: write the string
                 oa->write_characters(
-                    reinterpret_cast<const CharType*>(j.m_data.m_value.string->c_str()),
+                    reinterpret_cast<const CharType*>(j.m_data.m_value.string->data()),
                     j.m_data.m_value.string->size());
                 break;
             }
@@ -798,7 +798,7 @@ class binary_writer
                 }
                 write_number_with_ubjson_prefix(j.m_data.m_value.string->size(), true, use_bjdata);
                 oa->write_characters(
-                    reinterpret_cast<const CharType*>(j.m_data.m_value.string->c_str()),
+                    reinterpret_cast<const CharType*>(j.m_data.m_value.string->data()),
                     j.m_data.m_value.string->size());
                 break;
             }
@@ -887,7 +887,9 @@ class binary_writer
                     for (size_t i = 0; i < j.m_data.m_value.binary->size(); ++i)
                     {
                         oa->write_character(to_char_type(bjdata_draft3 ? 'B' : 'U'));
-                        oa->write_character(to_char_type(j.m_data.m_value.binary->data()[i]));
+                        // the cast is needed for binary types whose value type
+                        // is not an integer (e.g., std::byte)
+                        oa->write_character(to_char_type(static_cast<std::uint8_t>(j.m_data.m_value.binary->data()[i])));
                     }
                 }
 
@@ -948,7 +950,7 @@ class binary_writer
                 {
                     write_number_with_ubjson_prefix(el.first.size(), true, use_bjdata);
                     oa->write_characters(
-                        reinterpret_cast<const CharType*>(el.first.c_str()),
+                        reinterpret_cast<const CharType*>(el.first.data()),
                         el.first.size());
                     write_ubjson(el.second, use_count, use_type, prefix_required, use_bjdata, bjdata_version);
                 }
@@ -1011,8 +1013,11 @@ class binary_writer
     {
         oa->write_character(to_char_type(element_type));
         oa->write_characters(
-            reinterpret_cast<const CharType*>(name.c_str()),
-            name.size() + 1u);
+            reinterpret_cast<const CharType*>(name.data()),
+            name.size());
+        // the terminating null byte is written explicitly rather than taken
+        // from the buffer, so that string_t::data() need not be null-terminated
+        oa->write_character(to_char_type(0x00));
     }
 
     /*!
@@ -1053,8 +1058,9 @@ class binary_writer
 
         write_number<std::int32_t>(to_bson_length(value.size() + 1ul), true);
         oa->write_characters(
-            reinterpret_cast<const CharType*>(value.c_str()),
-            value.size() + 1);
+            reinterpret_cast<const CharType*>(value.data()),
+            value.size());
+        oa->write_character(to_char_type(0x00));
     }
 
     /*!
@@ -1145,7 +1151,8 @@ class binary_writer
 
         const std::size_t embedded_document_size = std::accumulate(std::begin(value), std::end(value), static_cast<std::size_t>(0), [&array_index](std::size_t result, const typename BasicJsonType::array_t::value_type & el)
         {
-            return result + calc_bson_element_size(std::to_string(array_index++), el);
+            const auto key = std::to_string(array_index++);
+            return result + calc_bson_element_size(string_t(key.data(), key.size()), el);
         });
 
         return sizeof(std::int32_t) + embedded_document_size + 1ul;
@@ -1172,7 +1179,11 @@ class binary_writer
 
         for (const auto& el : value)
         {
-            write_bson_element(std::to_string(array_index++), el);
+            // the index is built as a std::string, while write_bson_element takes
+            // a string_t; convert explicitly, as the two are only implicitly
+            // convertible for some string types
+            const auto key = std::to_string(array_index++);
+            write_bson_element(string_t(key.data(), key.size()), el);
         }
 
         oa->write_character(to_char_type(0x00));

include/nlohmann/detail/output/serializer.hpp

@@ -135,7 +135,7 @@ class serializer
                     auto i = val.m_data.m_value.object->cbegin();
                     for (std::size_t cnt = 0; cnt < val.m_data.m_value.object->size() - 1; ++cnt, ++i)
                     {
-                        o->write_characters(indent_string.c_str(), new_indent);
+                        o->write_characters(indent_string.data(), new_indent);
                         o->write_character('\"');
                         dump_escaped(i->first, ensure_ascii);
                         o->write_characters("\": ", 3);
@@ -146,14 +146,14 @@ class serializer
                     // last element
                     JSON_ASSERT(i != val.m_data.m_value.object->cend());
                     JSON_ASSERT(std::next(i) == val.m_data.m_value.object->cend());
-                    o->write_characters(indent_string.c_str(), new_indent);
+                    o->write_characters(indent_string.data(), new_indent);
                     o->write_character('\"');
                     dump_escaped(i->first, ensure_ascii);
                     o->write_characters("\": ", 3);
                     dump(i->second, true, ensure_ascii, indent_step, new_indent);
 
                     o->write_character('\n');
-                    o->write_characters(indent_string.c_str(), current_indent);
+                    o->write_characters(indent_string.data(), current_indent);
                     o->write_character('}');
                 }
                 else
@@ -208,18 +208,18 @@ class serializer
                     for (auto i = val.m_data.m_value.array->cbegin();
                             i != val.m_data.m_value.array->cend() - 1; ++i)
                     {
-                        o->write_characters(indent_string.c_str(), new_indent);
+                        o->write_characters(indent_string.data(), new_indent);
                         dump(*i, true, ensure_ascii, indent_step, new_indent);
                         o->write_characters(",\n", 2);
                     }
 
                     // last element
                     JSON_ASSERT(!val.m_data.m_value.array->empty());
-                    o->write_characters(indent_string.c_str(), new_indent);
+                    o->write_characters(indent_string.data(), new_indent);
                     dump(val.m_data.m_value.array->back(), true, ensure_ascii, indent_step, new_indent);
 
                     o->write_character('\n');
-                    o->write_characters(indent_string.c_str(), current_indent);
+                    o->write_characters(indent_string.data(), current_indent);
                     o->write_character(']');
                 }
                 else
@@ -265,7 +265,7 @@ class serializer
                         indent_string.resize(indent_string.size() * 2, ' ');
                     }
 
-                    o->write_characters(indent_string.c_str(), new_indent);
+                    o->write_characters(indent_string.data(), new_indent);
 
                     o->write_characters("\"bytes\": [", 10);
 
@@ -274,14 +274,14 @@ class serializer
                         for (auto i = val.m_data.m_value.binary->cbegin();
                                 i != val.m_data.m_value.binary->cend() - 1; ++i)
                         {
-                            dump_integer(*i);
+                            dump_integer(to_byte_value(*i));
                             o->write_characters(", ", 2);
                         }
-                        dump_integer(val.m_data.m_value.binary->back());
+                        dump_integer(to_byte_value(val.m_data.m_value.binary->back()));
                     }
 
                     o->write_characters("],\n", 3);
-                    o->write_characters(indent_string.c_str(), new_indent);
+                    o->write_characters(indent_string.data(), new_indent);
 
                     o->write_characters("\"subtype\": ", 11);
                     if (val.m_data.m_value.binary->has_subtype())
@@ -293,7 +293,7 @@ class serializer
                         o->write_characters("null", 4);
                     }
                     o->write_character('\n');
-                    o->write_characters(indent_string.c_str(), current_indent);
+                    o->write_characters(indent_string.data(), current_indent);
                     o->write_character('}');
                 }
                 else
@@ -305,10 +305,10 @@ class serializer
                         for (auto i = val.m_data.m_value.binary->cbegin();
                                 i != val.m_data.m_value.binary->cend() - 1; ++i)
                         {
-                            dump_integer(*i);
+                            dump_integer(to_byte_value(*i));
                             o->write_character(',');
                         }
-                        dump_integer(val.m_data.m_value.binary->back());
+                        dump_integer(to_byte_value(val.m_data.m_value.binary->back()));
                     }
 
                     o->write_characters("],\"subtype\":", 12);
@@ -596,7 +596,7 @@ class serializer
             {
                 case error_handler_t::strict:
                 {
-                    JSON_THROW(type_error::create(316, concat("incomplete UTF-8 string; last byte: 0x", hex_bytes(static_cast<std::uint8_t>(s.back() | 0))), nullptr));
+                    JSON_THROW(type_error::create(316, concat("incomplete UTF-8 string; last byte: 0x", hex_bytes(static_cast<std::uint8_t>(s[s.size() - 1] | 0))), nullptr));
                 }
 
                 case error_handler_t::ignore:
@@ -703,6 +703,19 @@ class serializer
         pos += 6;
     }
 
+    /*!
+    @brief convert a single element of a binary value to its byte value
+
+    The elements of a binary value are dumped as the numbers 0..255, regardless
+    of the value type of the configured BinaryType: that type may be signed
+    (`char`), unsigned (`std::uint8_t`), or not an integer at all
+    (`std::byte`), none of which @ref dump_integer can handle uniformly.
+    */
+    static std::uint8_t to_byte_value(binary_char_t x) noexcept
+    {
+        return static_cast<std::uint8_t>(x);
+    }
+
     // templates to avoid warnings about useless casts
     template <typename NumberType, enable_if_t<std::is_signed<NumberType>::value, int> = 0>
     bool is_negative_number(NumberType x)
@@ -728,8 +741,7 @@ class serializer
     template < typename NumberType, detail::enable_if_t <
                    std::is_integral<NumberType>::value ||
                    std::is_same<NumberType, number_unsigned_t>::value ||
-                   std::is_same<NumberType, number_integer_t>::value ||
-                   std::is_same<NumberType, binary_char_t>::value,
+                   std::is_same<NumberType, number_integer_t>::value,
                    int > = 0 >
     void dump_integer(NumberType x)
     {

include/nlohmann/detail/string_escape.hpp

@@ -8,50 +8,56 @@
 
 #pragma once
 
+#include <cstddef> // size_t
+
 #include <nlohmann/detail/abi_macros.hpp>
 
 NLOHMANN_JSON_NAMESPACE_BEGIN
 namespace detail
 {
 
-/*!
-@brief replace all occurrences of a substring by another string
-
-@param[in,out] s  the string to manipulate; changed so that all
-               occurrences of @a f are replaced with @a t
-@param[in]     f  the substring to replace with @a t
-@param[in]     t  the string to replace @a f
-
-@pre The search string @a f must not be empty. **This precondition is
-enforced with an assertion.**
-
-@since version 2.0.0
-*/
-template<typename StringType>
-inline void replace_substring(StringType& s, const StringType& f,
-                              const StringType& t)
-{
-    JSON_ASSERT(!f.empty());
-    for (auto pos = s.find(f);                // find the first occurrence of f
-            pos != StringType::npos;          // make sure f was found
-            s.replace(pos, f.size(), t),      // replace with t, and
-            pos = s.find(f, pos + t.size()))  // find the next occurrence of f
-    {}
-}
-
 /*!
  * @brief string escaping as described in RFC 6901 (Sect. 4)
  * @param[in] s string to escape
  * @return    escaped string
  *
  * Note the order of escaping "~" to "~0" and "/" to "~1" is important.
+ *
+ * The string is rebuilt in a single pass, appending whole runs between the
+ * characters that need escaping. Scanning with find_first_of() keeps the
+ * common case -- nothing to escape -- as fast as a single search, while
+ * repeated replace() calls would move the tail of the string once per
+ * escaped character.
  */
 template<typename StringType>
-inline StringType escape(StringType s)
+inline StringType escape(const StringType& s)
 {
-    replace_substring(s, StringType{"~"}, StringType{"~0"});
-    replace_substring(s, StringType{"/"}, StringType{"~1"});
-    return s;
+    auto next_special = [&s](std::size_t from)
+    {
+        const auto tilde = s.find_first_of('~', from);
+        const auto slash = s.find_first_of('/', from);
+        return tilde < slash ? tilde : slash; // npos is the largest value
+    };
+
+    auto pos = next_special(0);
+    if (pos == StringType::npos)
+    {
+        return s;
+    }
+
+    StringType result;
+    result.reserve(s.size() + 2);
+
+    std::size_t run = 0;
+    while (pos != StringType::npos)
+    {
+        result.append(s.data() + run, pos - run);
+        result.append(s[pos] == '~' ? "~0" : "~1", 2);
+        run = pos + 1;
+        pos = next_special(run);
+    }
+    result.append(s.data() + run, s.size() - run);
+    return result;
 }
 
 /*!
@@ -60,12 +66,43 @@ inline StringType escape(StringType s)
  * @return    unescaped string
  *
  * Note the order of escaping "~1" to "/" and "~0" to "~" is important.
+ *
+ * Rebuilt in a single pass, see @ref escape. A "~" that is followed by
+ * neither "0" nor "1" is passed through unchanged; @ref json_pointer rejects
+ * such input before it gets here.
  */
 template<typename StringType>
 inline void unescape(StringType& s)
 {
-    replace_substring(s, StringType{"~1"}, StringType{"/"});
-    replace_substring(s, StringType{"~0"}, StringType{"~"});
+    auto pos = s.find_first_of('~', 0);
+    if (pos == StringType::npos)
+    {
+        return;
+    }
+
+    StringType result;
+    result.reserve(s.size());
+
+    std::size_t run = 0;
+    while (pos != StringType::npos)
+    {
+        result.append(s.data() + run, pos - run);
+
+        const auto next = pos + 1;
+        if (next < s.size() && (s[next] == '0' || s[next] == '1'))
+        {
+            result.append(s[next] == '0' ? "~" : "/", 1);
+            run = pos + 2;
+        }
+        else
+        {
+            result.append("~", 1);
+            run = pos + 1;
+        }
+        pos = s.find_first_of('~', run);
+    }
+    result.append(s.data() + run, s.size() - run);
+    s = result;
 }
 
 }  // namespace detail

include/nlohmann/json.hpp

@@ -783,21 +783,76 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         return it;
     }
 
-    reference set_parent(reference j, std::size_t old_capacity = detail::unknown_size())
+    /// @brief erase an element from the object and return the following one
+    /// Not every map returns an iterator from erase(iterator): some containers
+    /// (e.g., Abseil's hash maps) return void to avoid computing a successor
+    /// the caller may not need. Compute it before erasing for those.
+    template < typename It, detail::enable_if_t <
+                   !std::is_void<decltype(std::declval<object_t&>().erase(std::declval<It>()))>::value, int > = 0 >
+    typename object_t::iterator erase_from_object(It pos)
+    {
+        return m_data.m_value.object->erase(pos);
+    }
+
+    template < typename It, detail::enable_if_t <
+                   std::is_void<decltype(std::declval<object_t&>().erase(std::declval<It>()))>::value, int > = 0 >
+    typename object_t::iterator erase_from_object(It pos)
+    {
+        auto next = std::next(pos);
+        m_data.m_value.object->erase(pos);
+        return next;
+    }
+
+    /// @brief the capacity of the stored array, or unknown_size()
+    /// Only JSON_DIAGNOSTICS uses the value, to detect a reallocation that
+    /// would invalidate the parent pointers. Array types that do not have a
+    /// capacity() member function report unknown_size(), which is treated as
+    /// "the elements may have moved".
+#if JSON_DIAGNOSTICS
+    template < typename A = array_t, detail::enable_if_t < detail::has_capacity<A>::value, int > = 0 >
+    std::size_t array_capacity() const noexcept
+    {
+        return m_data.m_value.array->capacity();
+    }
+
+    template < typename A = array_t, detail::enable_if_t < !detail::has_capacity<A>::value, int > = 0 >
+    std::size_t array_capacity() const noexcept
+    {
+        return detail::unknown_size();
+    }
+#else
+    static constexpr std::size_t array_capacity() noexcept
+    {
+        return detail::unknown_size();
+    }
+#endif
+
+    /// @brief set the parent of a value that has just been added to an array
+    /// @param j the added value
+    /// @param old_capacity the value @ref array_capacity() returned before the
+    ///        insertion
+    reference set_parent_after_array_insert(reference j, std::size_t old_capacity)
     {
 #if JSON_DIAGNOSTICS
-        if (old_capacity != detail::unknown_size())
+        // see https://github.com/nlohmann/json/issues/2838
+        JSON_ASSERT(type() == value_t::array);
+        if (JSON_HEDLEY_UNLIKELY(old_capacity == detail::unknown_size()
+                                 || array_capacity() != old_capacity))
         {
-            // see https://github.com/nlohmann/json/issues/2838
-            JSON_ASSERT(type() == value_t::array);
-            if (JSON_HEDLEY_UNLIKELY(m_data.m_value.array->capacity() != old_capacity))
-            {
-                // capacity has changed: update all parents
-                set_parents();
-                return j;
-            }
+            // the capacity has changed, or the array type does not let us tell:
+            // the elements may have moved, so update all parents
+            set_parents();
+            return j;
         }
+#else
+        static_cast<void>(old_capacity);
+#endif
+        return set_parent(j);
+    }
 
+    reference set_parent(reference j)
+    {
+#if JSON_DIAGNOSTICS
         // ordered_json uses a vector internally, so pointers could have
         // been invalidated; see https://github.com/nlohmann/json/issues/2962
 #ifdef JSON_HEDLEY_MSVC_VERSION
@@ -816,7 +871,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         j.m_parent = this;
 #else
         static_cast<void>(j);
-        static_cast<void>(old_capacity);
 #endif
         return j;
     }
@@ -2009,22 +2063,17 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     reference at(size_type idx)
     {
         // at only works for arrays
-        if (JSON_HEDLEY_LIKELY(is_array()))
-        {
-            JSON_TRY
-            {
-                return set_parent(m_data.m_value.array->at(idx));
-            }
-            JSON_CATCH (std::out_of_range&)
-            {
-                // create a better exception explanation
-                JSON_THROW(out_of_range::create(401, detail::concat("array index ", std::to_string(idx), " is out of range"), this));
-            } // cppcheck-suppress[missingReturn]
-        }
-        else
+        if (JSON_HEDLEY_UNLIKELY(!is_array()))
         {
             JSON_THROW(type_error::create(304, detail::concat("cannot use at() with ", type_name()), this));
         }
+
+        if (JSON_HEDLEY_UNLIKELY(idx >= m_data.m_value.array->size()))
+        {
+            JSON_THROW(out_of_range::create(401, detail::concat("array index ", std::to_string(idx), " is out of range"), this));
+        }
+
+        return set_parent((*m_data.m_value.array)[idx]);
     }
 
     /// @brief access specified array element with bounds checking
@@ -2032,22 +2081,17 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     const_reference at(size_type idx) const
     {
         // at only works for arrays
-        if (JSON_HEDLEY_LIKELY(is_array()))
-        {
-            JSON_TRY
-            {
-                return m_data.m_value.array->at(idx);
-            }
-            JSON_CATCH (std::out_of_range&)
-            {
-                // create a better exception explanation
-                JSON_THROW(out_of_range::create(401, detail::concat("array index ", std::to_string(idx), " is out of range"), this));
-            } // cppcheck-suppress[missingReturn]
-        }
-        else
+        if (JSON_HEDLEY_UNLIKELY(!is_array()))
         {
             JSON_THROW(type_error::create(304, detail::concat("cannot use at() with ", type_name()), this));
         }
+
+        if (JSON_HEDLEY_UNLIKELY(idx >= m_data.m_value.array->size()))
+        {
+            JSON_THROW(out_of_range::create(401, detail::concat("array index ", std::to_string(idx), " is out of range"), this));
+        }
+
+        return (*m_data.m_value.array)[idx];
     }
 
     /// @brief access specified object element with bounds checking
@@ -2147,12 +2191,13 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 #if JSON_DIAGNOSTICS
                 // remember array size & capacity before resizing
                 const auto old_size = m_data.m_value.array->size();
-                const auto old_capacity = m_data.m_value.array->capacity();
+                const auto old_capacity = array_capacity();
 #endif
                 m_data.m_value.array->resize(idx + 1);
 
 #if JSON_DIAGNOSTICS
-                if (JSON_HEDLEY_UNLIKELY(m_data.m_value.array->capacity() != old_capacity))
+                if (JSON_HEDLEY_UNLIKELY(old_capacity == detail::unknown_size()
+                                         || array_capacity() != old_capacity))
                 {
                     // capacity has changed: update all parents
                     set_parents();
@@ -2543,7 +2588,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
             case value_t::object:
             {
-                result.m_it.object_iterator = m_data.m_value.object->erase(pos.m_it.object_iterator);
+                result.m_it.object_iterator = erase_from_object(pos.m_it.object_iterator);
                 break;
             }
 
@@ -3173,9 +3218,9 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         }
 
         // add the element to the array (move semantics)
-        const auto old_capacity = m_data.m_value.array->capacity();
+        const auto old_capacity = array_capacity();
         m_data.m_value.array->push_back(std::move(val));
-        set_parent(m_data.m_value.array->back(), old_capacity);
+        set_parent_after_array_insert(m_data.m_value.array->back(), old_capacity);
         // if val is moved from, basic_json move constructor marks it null, so we do not call the destructor
     }
 
@@ -3206,9 +3251,9 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         }
 
         // add the element to the array
-        const auto old_capacity = m_data.m_value.array->capacity();
+        const auto old_capacity = array_capacity();
         m_data.m_value.array->push_back(val);
-        set_parent(m_data.m_value.array->back(), old_capacity);
+        set_parent_after_array_insert(m_data.m_value.array->back(), old_capacity);
     }
 
     /// @brief add an object to an array
@@ -3294,9 +3339,9 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         }
 
         // add the element to the array (perfect forwarding)
-        const auto old_capacity = m_data.m_value.array->capacity();
+        const auto old_capacity = array_capacity();
         m_data.m_value.array->emplace_back(std::forward<Args>(args)...);
-        return set_parent(m_data.m_value.array->back(), old_capacity);
+        return set_parent_after_array_insert(m_data.m_value.array->back(), old_capacity);
     }
 
     /// @brief add an object to an object if key does not exist
@@ -3375,7 +3420,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     /// @sa https://json.nlohmann.me/api/basic_json/insert/
     iterator insert(const_iterator pos, basic_json&& val) // NOLINT(performance-unnecessary-value-param)
     {
-        return insert(pos, val);
+        return insert(std::move(pos), val);
     }
 
     /// @brief inserts copies of element into array

single_include/nlohmann/json.hpp

@@ -3297,6 +3297,8 @@ NLOHMANN_JSON_NAMESPACE_END
 
 
 
+#include <cstddef> // size_t
+
 // #include <nlohmann/detail/abi_macros.hpp>
 
 
@@ -3304,44 +3306,48 @@ NLOHMANN_JSON_NAMESPACE_BEGIN
 namespace detail
 {
 
-/*!
-@brief replace all occurrences of a substring by another string
-
-@param[in,out] s  the string to manipulate; changed so that all
-               occurrences of @a f are replaced with @a t
-@param[in]     f  the substring to replace with @a t
-@param[in]     t  the string to replace @a f
-
-@pre The search string @a f must not be empty. **This precondition is
-enforced with an assertion.**
-
-@since version 2.0.0
-*/
-template<typename StringType>
-inline void replace_substring(StringType& s, const StringType& f,
-                              const StringType& t)
-{
-    JSON_ASSERT(!f.empty());
-    for (auto pos = s.find(f);                // find the first occurrence of f
-            pos != StringType::npos;          // make sure f was found
-            s.replace(pos, f.size(), t),      // replace with t, and
-            pos = s.find(f, pos + t.size()))  // find the next occurrence of f
-    {}
-}
-
 /*!
  * @brief string escaping as described in RFC 6901 (Sect. 4)
  * @param[in] s string to escape
  * @return    escaped string
  *
  * Note the order of escaping "~" to "~0" and "/" to "~1" is important.
+ *
+ * The string is rebuilt in a single pass, appending whole runs between the
+ * characters that need escaping. Scanning with find_first_of() keeps the
+ * common case -- nothing to escape -- as fast as a single search, while
+ * repeated replace() calls would move the tail of the string once per
+ * escaped character.
  */
 template<typename StringType>
-inline StringType escape(StringType s)
+inline StringType escape(const StringType& s)
 {
-    replace_substring(s, StringType{"~"}, StringType{"~0"});
-    replace_substring(s, StringType{"/"}, StringType{"~1"});
-    return s;
+    auto next_special = [&s](std::size_t from)
+    {
+        const auto tilde = s.find_first_of('~', from);
+        const auto slash = s.find_first_of('/', from);
+        return tilde < slash ? tilde : slash; // npos is the largest value
+    };
+
+    auto pos = next_special(0);
+    if (pos == StringType::npos)
+    {
+        return s;
+    }
+
+    StringType result;
+    result.reserve(s.size() + 2);
+
+    std::size_t run = 0;
+    while (pos != StringType::npos)
+    {
+        result.append(s.data() + run, pos - run);
+        result.append(s[pos] == '~' ? "~0" : "~1", 2);
+        run = pos + 1;
+        pos = next_special(run);
+    }
+    result.append(s.data() + run, s.size() - run);
+    return result;
 }
 
 /*!
@@ -3350,12 +3356,43 @@ inline StringType escape(StringType s)
  * @return    unescaped string
  *
  * Note the order of escaping "~1" to "/" and "~0" to "~" is important.
+ *
+ * Rebuilt in a single pass, see @ref escape. A "~" that is followed by
+ * neither "0" nor "1" is passed through unchanged; @ref json_pointer rejects
+ * such input before it gets here.
  */
 template<typename StringType>
 inline void unescape(StringType& s)
 {
-    replace_substring(s, StringType{"~1"}, StringType{"/"});
-    replace_substring(s, StringType{"~0"}, StringType{"~"});
+    auto pos = s.find_first_of('~', 0);
+    if (pos == StringType::npos)
+    {
+        return;
+    }
+
+    StringType result;
+    result.reserve(s.size());
+
+    std::size_t run = 0;
+    while (pos != StringType::npos)
+    {
+        result.append(s.data() + run, pos - run);
+
+        const auto next = pos + 1;
+        if (next < s.size() && (s[next] == '0' || s[next] == '1'))
+        {
+            result.append(s[next] == '0' ? "~" : "/", 1);
+            run = pos + 2;
+        }
+        else
+        {
+            result.append("~", 1);
+            run = pos + 1;
+        }
+        pos = s.find_first_of('~', run);
+    }
+    result.append(s.data() + run, s.size() - run);
+    s = result;
 }
 
 }  // namespace detail
@@ -3935,17 +3972,18 @@ struct has_to_json < BasicJsonType, T, enable_if_t < !is_basic_json<T>::value >>
 template<typename T>
 using detect_key_compare = typename T::key_compare;
 
-template<typename T>
-struct has_key_compare : std::integral_constant<bool, is_detected<detect_key_compare, T>::value> {};
-
-// obtains the actual object key comparator
+// obtains the actual object key comparator: object_t::key_compare if the
+// object type defines it, and default_object_comparator_t otherwise
+//
+// note detected_or_t is used rather than std::conditional, because the latter
+// names both of its type arguments eagerly; object_t::key_compare would then
+// be a hard error for an object type that does not define it
 template<typename BasicJsonType>
 struct actual_object_comparator
 {
     using object_t = typename BasicJsonType::object_t;
     using object_comparator_t = typename BasicJsonType::default_object_comparator_t;
-    using type = typename std::conditional < has_key_compare<object_t>::value,
-          typename object_t::key_compare, object_comparator_t>::type;
+    using type = detected_or_t<object_comparator_t, detect_key_compare, object_t>;
 };
 
 template<typename BasicJsonType>
@@ -4541,6 +4579,13 @@ using has_erase_with_key_type = typename std::conditional <
                                 std::true_type,
                                 std::false_type >::type;
 
+template<typename T>
+using detect_capacity = decltype(std::declval<const T&>().capacity());
+
+// type trait to check if a type has a capacity() member function
+template<typename T>
+struct has_capacity : std::integral_constant<bool, is_detected<detect_capacity, T>::value> {};
+
 // a naive helper to check if a type is an ordered_map (exploits the fact that
 // ordered_map inherits capacity() from std::vector)
 template <typename T>
@@ -4948,7 +4993,10 @@ class exception : public std::exception
                     {
                         if (&element.second == current)
                         {
-                            tokens.emplace_back(element.first.c_str());
+                            // data() is null-terminated, so a key containing
+                            // a null byte is cut short here rather than
+                            // truncating the whole message at what()
+                            tokens.emplace_back(element.first.data());
                             break;
                         }
                     }
@@ -6953,7 +7001,9 @@ std::size_t hash(const BasicJsonType& j)
             seed = combine(seed, static_cast<std::size_t>(j.get_binary().subtype()));
             for (const auto byte : j.get_binary())
             {
-                seed = combine(seed, std::hash<std::uint8_t> {}(byte));
+                // the cast is needed for binary types whose value type is not
+                // an integer (e.g., std::byte)
+                seed = combine(seed, std::hash<std::uint8_t> {}(static_cast<std::uint8_t>(byte)));
             }
             return seed;
         }
@@ -13521,7 +13571,10 @@ class binary_reader
                                number_string,
                                out_of_range::create(406, concat("number overflow parsing '", number_string, '\''), nullptr));
                 }
-                return sax->number_float(parsed_float, std::move(number_string));
+                // number_string is a std::string, while the SAX interface takes a
+                // string_t; convert explicitly, as the two are only implicitly
+                // convertible for some string types
+                return sax->number_float(parsed_float, string_t(number_string.data(), number_string.size()));
             }
             case token_type::uninitialized:
             case token_type::literal_true:
@@ -14711,8 +14764,13 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
 
     iter_impl() = default;
     ~iter_impl() = default;
-    iter_impl(iter_impl&&) noexcept = default;
-    iter_impl& operator=(iter_impl&&) noexcept = default;
+    // the exception specification is left to be computed rather than declared:
+    // an array or object type whose iterator is not nothrow move constructible
+    // (std::deque's is not before libstdc++ 11) would make a declared noexcept
+    // differ from the implicit one, which deletes the function -- and is an
+    // error outright with older compilers
+    iter_impl(iter_impl&&) = default; // NOLINT(hicpp-noexcept-move,performance-noexcept-move-constructor,cppcoreguidelines-noexcept-move-operations)
+    iter_impl& operator=(iter_impl&&) = default; // NOLINT(hicpp-noexcept-move,performance-noexcept-move-constructor,cppcoreguidelines-noexcept-move-operations)
 
     /*!
     @brief constructor for a given JSON instance
@@ -15593,6 +15651,7 @@ NLOHMANN_JSON_NAMESPACE_END
 #endif  // JSON_NO_IO
 #include <limits> // max
 #include <numeric> // accumulate
+#include <set> // set
 #include <string> // string
 #include <utility> // move
 #include <vector> // vector
@@ -15652,7 +15711,7 @@ class json_pointer
                                string_t{},
                                [](const string_t& a, const string_t& b)
         {
-            return detail::concat(a, '/', detail::escape(b));
+            return detail::concat<string_t>(a, '/', detail::escape(b));
         });
     }
 
@@ -15846,7 +15905,7 @@ class json_pointer
             JSON_THROW(detail::parse_error::create(109, 0, detail::concat("array index '", s, "' is not a number"), nullptr));
         }
 
-        const char* p = s.c_str();
+        const char* p = s.data();
         char* p_end = nullptr; // NOLINT(misc-const-correctness)
         errno = 0; // strtoull doesn't reset errno
         const unsigned long long res = std::strtoull(p, &p_end, 10); // NOLINT(runtime/int)
@@ -15881,19 +15940,35 @@ class json_pointer
     }
 
   private:
+    /*!
+    @brief the reference token sequences that denote arrays
+
+    @ref unflatten collects the pointer prefixes that have a reference token 0
+    among their children; @ref get_and_create creates arrays exactly below
+    those prefixes and objects everywhere else. Deciding this up front keeps
+    the result independent of the order in which the flattened object is
+    iterated, which is unspecified for some object types.
+    */
+    using array_parents_t = std::set<std::vector<string_t>>;
+
     /*!
     @brief create and return a reference to the pointed to value
 
     @complexity Linear in the number of reference tokens.
 
+    @throw parse_error.106 if an array index begins with '0'
     @throw parse_error.109 if array index is not a number
     @throw type_error.313 if value cannot be unflattened
     */
     template<typename BasicJsonType>
-    BasicJsonType& get_and_create(BasicJsonType& j) const
+    BasicJsonType& get_and_create(BasicJsonType& j, const array_parents_t& array_parents) const
     {
         auto* result = &j;
 
+        // the reference tokens that have been consumed so far; used to look up
+        // whether the value to be created below is an array or an object
+        std::vector<string_t> prefix;
+
         // in case no reference tokens exist, return a reference to the JSON value
         // j which will be overwritten by a primitive value
         for (const auto& reference_token : reference_tokens)
@@ -15902,10 +15977,11 @@ class json_pointer
             {
                 case detail::value_t::null:
                 {
-                    if (reference_token == "0")
+                    if (array_parents.find(prefix) != array_parents.end())
                     {
-                        // start a new array if the reference token is 0
-                        result = &result->operator[](0);
+                        // some reference token below this position is 0, so the
+                        // value is an array
+                        result = &result->operator[](array_index<BasicJsonType>(reference_token));
                     }
                     else
                     {
@@ -15945,6 +16021,8 @@ class json_pointer
                 default:
                     JSON_THROW(detail::type_error::create(313, "invalid value to unflatten", &j));
             }
+
+            prefix.push_back(reference_token);
         }
 
         return *result;
@@ -16404,7 +16482,8 @@ class json_pointer
         {
             // use the text between the beginning of the reference token
             // (start) and the last slash (slash).
-            auto reference_token = reference_string.substr(start, slash - start);
+            const auto count = (slash == string_t::npos ? reference_string.size() : slash) - start;
+            auto reference_token = string_t(reference_string.data() + start, count);
 
             // check reference tokens are properly escaped
             for (std::size_t pos = reference_token.find_first_of('~');
@@ -16520,6 +16599,24 @@ class json_pointer
 
         BasicJsonType result;
 
+        // collect the pointer prefixes that have a reference token 0 among
+        // their children; the values below them are arrays, all others are
+        // objects (see array_parents_t)
+        array_parents_t array_parents;
+        for (const auto& element : *value.m_data.m_value.object)
+        {
+            json_pointer ptr(element.first);
+            std::vector<string_t> prefix;
+            for (auto& reference_token : ptr.reference_tokens)
+            {
+                if (reference_token == "0")
+                {
+                    array_parents.insert(prefix);
+                }
+                prefix.push_back(std::move(reference_token));
+            }
+        }
+
         // iterate the JSON object values
         for (const auto& element : *value.m_data.m_value.object)
         {
@@ -16532,7 +16629,7 @@ class json_pointer
             // that if the JSON pointer is "" (i.e., points to the whole value),
             // function get_and_create returns a reference to the result itself.
             // An assignment will then create a primitive value.
-            json_pointer(element.first).get_and_create(result) = element.second;
+            json_pointer(element.first).get_and_create(result, array_parents) = element.second;
         }
 
         return result;
@@ -17204,7 +17301,7 @@ class binary_writer
 
                 // step 2: write the string
                 oa->write_characters(
-                    reinterpret_cast<const CharType*>(j.m_data.m_value.string->c_str()),
+                    reinterpret_cast<const CharType*>(j.m_data.m_value.string->data()),
                     j.m_data.m_value.string->size());
                 break;
             }
@@ -17524,7 +17621,7 @@ class binary_writer
 
                 // step 2: write the string
                 oa->write_characters(
-                    reinterpret_cast<const CharType*>(j.m_data.m_value.string->c_str()),
+                    reinterpret_cast<const CharType*>(j.m_data.m_value.string->data()),
                     j.m_data.m_value.string->size());
                 break;
             }
@@ -17741,7 +17838,7 @@ class binary_writer
                 }
                 write_number_with_ubjson_prefix(j.m_data.m_value.string->size(), true, use_bjdata);
                 oa->write_characters(
-                    reinterpret_cast<const CharType*>(j.m_data.m_value.string->c_str()),
+                    reinterpret_cast<const CharType*>(j.m_data.m_value.string->data()),
                     j.m_data.m_value.string->size());
                 break;
             }
@@ -17830,7 +17927,9 @@ class binary_writer
                     for (size_t i = 0; i < j.m_data.m_value.binary->size(); ++i)
                     {
                         oa->write_character(to_char_type(bjdata_draft3 ? 'B' : 'U'));
-                        oa->write_character(to_char_type(j.m_data.m_value.binary->data()[i]));
+                        // the cast is needed for binary types whose value type
+                        // is not an integer (e.g., std::byte)
+                        oa->write_character(to_char_type(static_cast<std::uint8_t>(j.m_data.m_value.binary->data()[i])));
                     }
                 }
 
@@ -17891,7 +17990,7 @@ class binary_writer
                 {
                     write_number_with_ubjson_prefix(el.first.size(), true, use_bjdata);
                     oa->write_characters(
-                        reinterpret_cast<const CharType*>(el.first.c_str()),
+                        reinterpret_cast<const CharType*>(el.first.data()),
                         el.first.size());
                     write_ubjson(el.second, use_count, use_type, prefix_required, use_bjdata, bjdata_version);
                 }
@@ -17954,8 +18053,11 @@ class binary_writer
     {
         oa->write_character(to_char_type(element_type));
         oa->write_characters(
-            reinterpret_cast<const CharType*>(name.c_str()),
-            name.size() + 1u);
+            reinterpret_cast<const CharType*>(name.data()),
+            name.size());
+        // the terminating null byte is written explicitly rather than taken
+        // from the buffer, so that string_t::data() need not be null-terminated
+        oa->write_character(to_char_type(0x00));
     }
 
     /*!
@@ -17996,8 +18098,9 @@ class binary_writer
 
         write_number<std::int32_t>(to_bson_length(value.size() + 1ul), true);
         oa->write_characters(
-            reinterpret_cast<const CharType*>(value.c_str()),
-            value.size() + 1);
+            reinterpret_cast<const CharType*>(value.data()),
+            value.size());
+        oa->write_character(to_char_type(0x00));
     }
 
     /*!
@@ -18088,7 +18191,8 @@ class binary_writer
 
         const std::size_t embedded_document_size = std::accumulate(std::begin(value), std::end(value), static_cast<std::size_t>(0), [&array_index](std::size_t result, const typename BasicJsonType::array_t::value_type & el)
         {
-            return result + calc_bson_element_size(std::to_string(array_index++), el);
+            const auto key = std::to_string(array_index++);
+            return result + calc_bson_element_size(string_t(key.data(), key.size()), el);
         });
 
         return sizeof(std::int32_t) + embedded_document_size + 1ul;
@@ -18115,7 +18219,11 @@ class binary_writer
 
         for (const auto& el : value)
         {
-            write_bson_element(std::to_string(array_index++), el);
+            // the index is built as a std::string, while write_bson_element takes
+            // a string_t; convert explicitly, as the two are only implicitly
+            // convertible for some string types
+            const auto key = std::to_string(array_index++);
+            write_bson_element(string_t(key.data(), key.size()), el);
         }
 
         oa->write_character(to_char_type(0x00));
@@ -20140,7 +20248,7 @@ class serializer
                     auto i = val.m_data.m_value.object->cbegin();
                     for (std::size_t cnt = 0; cnt < val.m_data.m_value.object->size() - 1; ++cnt, ++i)
                     {
-                        o->write_characters(indent_string.c_str(), new_indent);
+                        o->write_characters(indent_string.data(), new_indent);
                         o->write_character('\"');
                         dump_escaped(i->first, ensure_ascii);
                         o->write_characters("\": ", 3);
@@ -20151,14 +20259,14 @@ class serializer
                     // last element
                     JSON_ASSERT(i != val.m_data.m_value.object->cend());
                     JSON_ASSERT(std::next(i) == val.m_data.m_value.object->cend());
-                    o->write_characters(indent_string.c_str(), new_indent);
+                    o->write_characters(indent_string.data(), new_indent);
                     o->write_character('\"');
                     dump_escaped(i->first, ensure_ascii);
                     o->write_characters("\": ", 3);
                     dump(i->second, true, ensure_ascii, indent_step, new_indent);
 
                     o->write_character('\n');
-                    o->write_characters(indent_string.c_str(), current_indent);
+                    o->write_characters(indent_string.data(), current_indent);
                     o->write_character('}');
                 }
                 else
@@ -20213,18 +20321,18 @@ class serializer
                     for (auto i = val.m_data.m_value.array->cbegin();
                             i != val.m_data.m_value.array->cend() - 1; ++i)
                     {
-                        o->write_characters(indent_string.c_str(), new_indent);
+                        o->write_characters(indent_string.data(), new_indent);
                         dump(*i, true, ensure_ascii, indent_step, new_indent);
                         o->write_characters(",\n", 2);
                     }
 
                     // last element
                     JSON_ASSERT(!val.m_data.m_value.array->empty());
-                    o->write_characters(indent_string.c_str(), new_indent);
+                    o->write_characters(indent_string.data(), new_indent);
                     dump(val.m_data.m_value.array->back(), true, ensure_ascii, indent_step, new_indent);
 
                     o->write_character('\n');
-                    o->write_characters(indent_string.c_str(), current_indent);
+                    o->write_characters(indent_string.data(), current_indent);
                     o->write_character(']');
                 }
                 else
@@ -20270,7 +20378,7 @@ class serializer
                         indent_string.resize(indent_string.size() * 2, ' ');
                     }
 
-                    o->write_characters(indent_string.c_str(), new_indent);
+                    o->write_characters(indent_string.data(), new_indent);
 
                     o->write_characters("\"bytes\": [", 10);
 
@@ -20279,14 +20387,14 @@ class serializer
                         for (auto i = val.m_data.m_value.binary->cbegin();
                                 i != val.m_data.m_value.binary->cend() - 1; ++i)
                         {
-                            dump_integer(*i);
+                            dump_integer(to_byte_value(*i));
                             o->write_characters(", ", 2);
                         }
-                        dump_integer(val.m_data.m_value.binary->back());
+                        dump_integer(to_byte_value(val.m_data.m_value.binary->back()));
                     }
 
                     o->write_characters("],\n", 3);
-                    o->write_characters(indent_string.c_str(), new_indent);
+                    o->write_characters(indent_string.data(), new_indent);
 
                     o->write_characters("\"subtype\": ", 11);
                     if (val.m_data.m_value.binary->has_subtype())
@@ -20298,7 +20406,7 @@ class serializer
                         o->write_characters("null", 4);
                     }
                     o->write_character('\n');
-                    o->write_characters(indent_string.c_str(), current_indent);
+                    o->write_characters(indent_string.data(), current_indent);
                     o->write_character('}');
                 }
                 else
@@ -20310,10 +20418,10 @@ class serializer
                         for (auto i = val.m_data.m_value.binary->cbegin();
                                 i != val.m_data.m_value.binary->cend() - 1; ++i)
                         {
-                            dump_integer(*i);
+                            dump_integer(to_byte_value(*i));
                             o->write_character(',');
                         }
-                        dump_integer(val.m_data.m_value.binary->back());
+                        dump_integer(to_byte_value(val.m_data.m_value.binary->back()));
                     }
 
                     o->write_characters("],\"subtype\":", 12);
@@ -20601,7 +20709,7 @@ class serializer
             {
                 case error_handler_t::strict:
                 {
-                    JSON_THROW(type_error::create(316, concat("incomplete UTF-8 string; last byte: 0x", hex_bytes(static_cast<std::uint8_t>(s.back() | 0))), nullptr));
+                    JSON_THROW(type_error::create(316, concat("incomplete UTF-8 string; last byte: 0x", hex_bytes(static_cast<std::uint8_t>(s[s.size() - 1] | 0))), nullptr));
                 }
 
                 case error_handler_t::ignore:
@@ -20708,6 +20816,19 @@ class serializer
         pos += 6;
     }
 
+    /*!
+    @brief convert a single element of a binary value to its byte value
+
+    The elements of a binary value are dumped as the numbers 0..255, regardless
+    of the value type of the configured BinaryType: that type may be signed
+    (`char`), unsigned (`std::uint8_t`), or not an integer at all
+    (`std::byte`), none of which @ref dump_integer can handle uniformly.
+    */
+    static std::uint8_t to_byte_value(binary_char_t x) noexcept
+    {
+        return static_cast<std::uint8_t>(x);
+    }
+
     // templates to avoid warnings about useless casts
     template <typename NumberType, enable_if_t<std::is_signed<NumberType>::value, int> = 0>
     bool is_negative_number(NumberType x)
@@ -20733,8 +20854,7 @@ class serializer
     template < typename NumberType, detail::enable_if_t <
                    std::is_integral<NumberType>::value ||
                    std::is_same<NumberType, number_unsigned_t>::value ||
-                   std::is_same<NumberType, number_integer_t>::value ||
-                   std::is_same<NumberType, binary_char_t>::value,
+                   std::is_same<NumberType, number_integer_t>::value,
                    int > = 0 >
     void dump_integer(NumberType x)
     {
@@ -22137,21 +22257,76 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         return it;
     }
 
-    reference set_parent(reference j, std::size_t old_capacity = detail::unknown_size())
+    /// @brief erase an element from the object and return the following one
+    /// Not every map returns an iterator from erase(iterator): some containers
+    /// (e.g., Abseil's hash maps) return void to avoid computing a successor
+    /// the caller may not need. Compute it before erasing for those.
+    template < typename It, detail::enable_if_t <
+                   !std::is_void<decltype(std::declval<object_t&>().erase(std::declval<It>()))>::value, int > = 0 >
+    typename object_t::iterator erase_from_object(It pos)
+    {
+        return m_data.m_value.object->erase(pos);
+    }
+
+    template < typename It, detail::enable_if_t <
+                   std::is_void<decltype(std::declval<object_t&>().erase(std::declval<It>()))>::value, int > = 0 >
+    typename object_t::iterator erase_from_object(It pos)
+    {
+        auto next = std::next(pos);
+        m_data.m_value.object->erase(pos);
+        return next;
+    }
+
+    /// @brief the capacity of the stored array, or unknown_size()
+    /// Only JSON_DIAGNOSTICS uses the value, to detect a reallocation that
+    /// would invalidate the parent pointers. Array types that do not have a
+    /// capacity() member function report unknown_size(), which is treated as
+    /// "the elements may have moved".
+#if JSON_DIAGNOSTICS
+    template < typename A = array_t, detail::enable_if_t < detail::has_capacity<A>::value, int > = 0 >
+    std::size_t array_capacity() const noexcept
+    {
+        return m_data.m_value.array->capacity();
+    }
+
+    template < typename A = array_t, detail::enable_if_t < !detail::has_capacity<A>::value, int > = 0 >
+    std::size_t array_capacity() const noexcept
+    {
+        return detail::unknown_size();
+    }
+#else
+    static constexpr std::size_t array_capacity() noexcept
+    {
+        return detail::unknown_size();
+    }
+#endif
+
+    /// @brief set the parent of a value that has just been added to an array
+    /// @param j the added value
+    /// @param old_capacity the value @ref array_capacity() returned before the
+    ///        insertion
+    reference set_parent_after_array_insert(reference j, std::size_t old_capacity)
     {
 #if JSON_DIAGNOSTICS
-        if (old_capacity != detail::unknown_size())
+        // see https://github.com/nlohmann/json/issues/2838
+        JSON_ASSERT(type() == value_t::array);
+        if (JSON_HEDLEY_UNLIKELY(old_capacity == detail::unknown_size()
+                                 || array_capacity() != old_capacity))
         {
-            // see https://github.com/nlohmann/json/issues/2838
-            JSON_ASSERT(type() == value_t::array);
-            if (JSON_HEDLEY_UNLIKELY(m_data.m_value.array->capacity() != old_capacity))
-            {
-                // capacity has changed: update all parents
-                set_parents();
-                return j;
-            }
+            // the capacity has changed, or the array type does not let us tell:
+            // the elements may have moved, so update all parents
+            set_parents();
+            return j;
         }
+#else
+        static_cast<void>(old_capacity);
+#endif
+        return set_parent(j);
+    }
 
+    reference set_parent(reference j)
+    {
+#if JSON_DIAGNOSTICS
         // ordered_json uses a vector internally, so pointers could have
         // been invalidated; see https://github.com/nlohmann/json/issues/2962
 #ifdef JSON_HEDLEY_MSVC_VERSION
@@ -22170,7 +22345,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         j.m_parent = this;
 #else
         static_cast<void>(j);
-        static_cast<void>(old_capacity);
 #endif
         return j;
     }
@@ -23363,22 +23537,17 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     reference at(size_type idx)
     {
         // at only works for arrays
-        if (JSON_HEDLEY_LIKELY(is_array()))
-        {
-            JSON_TRY
-            {
-                return set_parent(m_data.m_value.array->at(idx));
-            }
-            JSON_CATCH (std::out_of_range&)
-            {
-                // create a better exception explanation
-                JSON_THROW(out_of_range::create(401, detail::concat("array index ", std::to_string(idx), " is out of range"), this));
-            } // cppcheck-suppress[missingReturn]
-        }
-        else
+        if (JSON_HEDLEY_UNLIKELY(!is_array()))
         {
             JSON_THROW(type_error::create(304, detail::concat("cannot use at() with ", type_name()), this));
         }
+
+        if (JSON_HEDLEY_UNLIKELY(idx >= m_data.m_value.array->size()))
+        {
+            JSON_THROW(out_of_range::create(401, detail::concat("array index ", std::to_string(idx), " is out of range"), this));
+        }
+
+        return set_parent((*m_data.m_value.array)[idx]);
     }
 
     /// @brief access specified array element with bounds checking
@@ -23386,22 +23555,17 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     const_reference at(size_type idx) const
     {
         // at only works for arrays
-        if (JSON_HEDLEY_LIKELY(is_array()))
-        {
-            JSON_TRY
-            {
-                return m_data.m_value.array->at(idx);
-            }
-            JSON_CATCH (std::out_of_range&)
-            {
-                // create a better exception explanation
-                JSON_THROW(out_of_range::create(401, detail::concat("array index ", std::to_string(idx), " is out of range"), this));
-            } // cppcheck-suppress[missingReturn]
-        }
-        else
+        if (JSON_HEDLEY_UNLIKELY(!is_array()))
         {
             JSON_THROW(type_error::create(304, detail::concat("cannot use at() with ", type_name()), this));
         }
+
+        if (JSON_HEDLEY_UNLIKELY(idx >= m_data.m_value.array->size()))
+        {
+            JSON_THROW(out_of_range::create(401, detail::concat("array index ", std::to_string(idx), " is out of range"), this));
+        }
+
+        return (*m_data.m_value.array)[idx];
     }
 
     /// @brief access specified object element with bounds checking
@@ -23501,12 +23665,13 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 #if JSON_DIAGNOSTICS
                 // remember array size & capacity before resizing
                 const auto old_size = m_data.m_value.array->size();
-                const auto old_capacity = m_data.m_value.array->capacity();
+                const auto old_capacity = array_capacity();
 #endif
                 m_data.m_value.array->resize(idx + 1);
 
 #if JSON_DIAGNOSTICS
-                if (JSON_HEDLEY_UNLIKELY(m_data.m_value.array->capacity() != old_capacity))
+                if (JSON_HEDLEY_UNLIKELY(old_capacity == detail::unknown_size()
+                                         || array_capacity() != old_capacity))
                 {
                     // capacity has changed: update all parents
                     set_parents();
@@ -23897,7 +24062,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
             case value_t::object:
             {
-                result.m_it.object_iterator = m_data.m_value.object->erase(pos.m_it.object_iterator);
+                result.m_it.object_iterator = erase_from_object(pos.m_it.object_iterator);
                 break;
             }
 
@@ -24527,9 +24692,9 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         }
 
         // add the element to the array (move semantics)
-        const auto old_capacity = m_data.m_value.array->capacity();
+        const auto old_capacity = array_capacity();
         m_data.m_value.array->push_back(std::move(val));
-        set_parent(m_data.m_value.array->back(), old_capacity);
+        set_parent_after_array_insert(m_data.m_value.array->back(), old_capacity);
         // if val is moved from, basic_json move constructor marks it null, so we do not call the destructor
     }
 
@@ -24560,9 +24725,9 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         }
 
         // add the element to the array
-        const auto old_capacity = m_data.m_value.array->capacity();
+        const auto old_capacity = array_capacity();
         m_data.m_value.array->push_back(val);
-        set_parent(m_data.m_value.array->back(), old_capacity);
+        set_parent_after_array_insert(m_data.m_value.array->back(), old_capacity);
     }
 
     /// @brief add an object to an array
@@ -24648,9 +24813,9 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         }
 
         // add the element to the array (perfect forwarding)
-        const auto old_capacity = m_data.m_value.array->capacity();
+        const auto old_capacity = array_capacity();
         m_data.m_value.array->emplace_back(std::forward<Args>(args)...);
-        return set_parent(m_data.m_value.array->back(), old_capacity);
+        return set_parent_after_array_insert(m_data.m_value.array->back(), old_capacity);
     }
 
     /// @brief add an object to an object if key does not exist
@@ -24729,7 +24894,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     /// @sa https://json.nlohmann.me/api/basic_json/insert/
     iterator insert(const_iterator pos, basic_json&& val) // NOLINT(performance-unnecessary-value-param)
     {
-        return insert(pos, val);
+        return insert(std::move(pos), val);
     }
 
     /// @brief inserts copies of element into array

tests/src/unit-alt-string.cpp

@@ -11,8 +11,10 @@
 
 #include <nlohmann/json.hpp>
 
+#include <cstdint>
 #include <string>
 #include <utility>
+#include <vector>
 
 /* forward declarations */
 class alt_string;
@@ -22,6 +24,10 @@ void int_to_string(alt_string& target, std::size_t value); // NOLINT(misc-use-in
 /*
  * This is virtually a string class.
  * It covers std::string under the hood.
+ *
+ * It deliberately does not provide c_str(), back(), find(str, pos), replace(),
+ * or substr(): the library must not rely on them. Do not add members here
+ * without checking that the library actually needs them.
  */
 class alt_string
 {
@@ -106,11 +112,6 @@ class alt_string
         return str_impl < op.str_impl;
     }
 
-    const char* c_str() const
-    {
-        return str_impl.c_str();
-    }
-
     char& operator[](std::size_t index)
     {
         return str_impl[index];
@@ -121,16 +122,6 @@ class alt_string
         return str_impl[index];
     }
 
-    char& back()
-    {
-        return str_impl.back();
-    }
-
-    const char& back() const
-    {
-        return str_impl.back();
-    }
-
     void clear()
     {
         str_impl.clear();
@@ -146,28 +137,11 @@ class alt_string
         return str_impl.empty();
     }
 
-    std::size_t find(const alt_string& str, std::size_t pos = 0) const
-    {
-        return str_impl.find(str.str_impl, pos);
-    }
-
     std::size_t find_first_of(char c, std::size_t pos = 0) const
     {
         return str_impl.find_first_of(c, pos);
     }
 
-    alt_string substr(std::size_t pos = 0, std::size_t count = npos) const
-    {
-        const std::string s = str_impl.substr(pos, count);
-        return {s.data(), s.size()};
-    }
-
-    alt_string& replace(std::size_t pos, std::size_t count, const alt_string& str)
-    {
-        str_impl.replace(pos, count, str.str_impl);
-        return *this;
-    }
-
     void reserve( std::size_t new_cap = 0 )
     {
         str_impl.reserve(new_cap);
@@ -202,6 +176,31 @@ bool operator<(const char* op1, const alt_string& op2) noexcept
 
 TEST_CASE("alternative string type")
 {
+    SECTION("binary formats")
+    {
+        alt_json doc;
+        doc["pi"] = 3.141;
+        doc["happy"] = true;
+        doc["list"] = {1, 2, 3};
+
+        CHECK(alt_json::from_cbor(alt_json::to_cbor(doc)) == doc);
+        CHECK(alt_json::from_msgpack(alt_json::to_msgpack(doc)) == doc);
+        // BSON is not covered: it additionally needs string_t::find(value_type),
+        // which alt_string does not provide
+        CHECK(alt_json::from_ubjson(alt_json::to_ubjson(doc)) == doc);
+
+        // a UBJSON high-precision number is parsed into a std::string that the
+        // reader has to hand to the SAX interface as an alt_string
+        const std::vector<uint8_t> high_precision =
+        {
+            'H', 'i', 0x16, '3', '.', '1', '4', '1', '5', '9', '2', '6', '5', '3',
+            '5', '8', '9', '7', '9', '3', '2', '3', '8', '4', '6'
+        };
+        const auto number = alt_json::from_ubjson(high_precision);
+        CHECK(number.is_number_float());
+        CHECK(number.get<double>() == doctest::Approx(3.14159265358979323846));
+    }
+
     SECTION("dump")
     {
         {
@@ -332,6 +331,15 @@ TEST_CASE("alternative string type")
 
         CHECK(j.at(alt_json::json_pointer("/foo/0")) == j["foo"][0]);
         CHECK(j.at(alt_json::json_pointer("/foo/1")) == j["foo"][1]);
+
+        // RFC 6901 escaping works without string_t::find(str, pos), replace(),
+        // and substr()
+        auto j2 = alt_json::parse(R"({"a/b": 1, "m~n": 2, "~/~~//": 3})");
+        CHECK(j2.at(alt_json::json_pointer("/a~1b")) == 1);
+        CHECK(j2.at(alt_json::json_pointer("/m~0n")) == 2);
+        CHECK(j2.at(alt_json::json_pointer("/~0~1~0~0~1~1")) == 3);
+        CHECK(alt_json::json_pointer("/~0~1~0~0~1~1").to_string() == alt_string("/~0~1~0~0~1~1"));
+        CHECK(j2.flatten().unflatten() == j2);
     }
 
     SECTION("patch")

tests/src/unit-custom-array-type.cpp

@@ -0,0 +1,136 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+#include "doctest_compatibility.h"
+
+#include <nlohmann/json.hpp>
+
+#include <deque>
+#include <map>
+#include <memory>
+#include <string>
+#include <type_traits>
+#include <vector>
+
+namespace
+{
+
+// std::deque has no capacity() member function, which the library only needs
+// to detect a reallocation for JSON_DIAGNOSTICS
+using deque_json = nlohmann::basic_json<std::map, std::deque>;
+
+// a std::vector whose at() is hidden: the library performs its own bounds
+// check and must not fall back to the container's checked accessor
+template<class T, class Allocator = std::allocator<T>>
+class vector_without_at : public std::vector<T, Allocator>
+{
+  public:
+    using std::vector<T, Allocator>::vector;
+
+    void at() = delete;
+};
+
+using no_at_json = nlohmann::basic_json<std::map, vector_without_at>;
+
+} // namespace
+
+TEST_CASE("array type without capacity()")
+{
+    SECTION("the iterators of the default configuration stay nothrow movable")
+    {
+        // basic_json's iterators take their exception specification from the
+        // container iterators; std::deque's is not nothrow move constructible
+        // with older standard libraries, which must not cost the default
+        // configuration its noexcept
+        CHECK(std::is_nothrow_move_constructible<nlohmann::json::iterator>::value);
+        CHECK(std::is_nothrow_move_assignable<nlohmann::json::iterator>::value);
+        CHECK(std::is_nothrow_move_constructible<nlohmann::json::const_iterator>::value);
+        CHECK(std::is_move_constructible<deque_json::iterator>::value);
+        CHECK(std::is_move_assignable<deque_json::iterator>::value);
+    }
+
+    SECTION("adding elements")
+    {
+        deque_json j = deque_json::array();
+        j.push_back(1);
+        j.push_back("two");
+        j.emplace_back(3);
+        j += 4;
+
+        CHECK(j.size() == 4);
+        CHECK(j == deque_json({1, "two", 3, 4}));
+        CHECK(j.back() == 4);
+        CHECK(j.front() == 1);
+    }
+
+    SECTION("accessing and modifying elements")
+    {
+        auto j = deque_json::parse(R"([1,2,3])");
+
+        CHECK(j[1] == 2);
+        CHECK(j.at(2) == 3);
+
+        // growing through operator[] fills up with null values
+        j[5] = 6;
+        CHECK(j.size() == 6);
+        CHECK(j[4].is_null());
+        CHECK(j[5] == 6);
+
+        j.erase(0);
+        CHECK(j == deque_json({2, 3, nullptr, nullptr, 6}));
+
+        auto it = j.erase(j.begin());
+        CHECK(*it == 3);
+
+        j.insert(j.begin(), 1);
+        CHECK(j.front() == 1);
+    }
+
+    SECTION("serialization and deserialization")
+    {
+        const auto j = deque_json::parse(R"({"a":[1,[2,3]],"b":[]})");
+        CHECK(j.dump() == R"({"a":[1,[2,3]],"b":[]})");
+        CHECK(deque_json::parse(j.dump()) == j);
+        CHECK(deque_json::from_cbor(deque_json::to_cbor(j)) == j);
+
+        // empty containers are flattened to null and cannot be restored
+        const auto nested = deque_json::parse(R"({"a":[1,[2,3]]})");
+        CHECK(nested.flatten().unflatten() == nested);
+    }
+
+    SECTION("references stay valid while the array grows")
+    {
+        deque_json j = deque_json::array();
+        j.push_back(1);
+        auto& first = j[0];
+        for (int i = 0; i < 100; ++i)
+        {
+            j.push_back(i);
+        }
+        CHECK(&first == &j[0]);
+        CHECK(first == 1);
+    }
+}
+
+TEST_CASE("array type without at()")
+{
+    // built in memory rather than parsed, so that the exception message does
+    // not gain a byte range with JSON_DIAGNOSTIC_POSITIONS
+    no_at_json j = {1, 2, 3};
+    const auto& jc = j;
+
+    CHECK(j.at(0) == 1);
+    CHECK(j.at(2) == 3);
+    CHECK(jc.at(2) == 3);
+
+    CHECK_THROWS_WITH_AS(j.at(3), "[json.exception.out_of_range.401] array index 3 is out of range", no_at_json::out_of_range);
+    CHECK_THROWS_WITH_AS(jc.at(3), "[json.exception.out_of_range.401] array index 3 is out of range", no_at_json::out_of_range);
+
+    CHECK(j.at(no_at_json::json_pointer("/1")) == 2);
+    CHECK_THROWS_AS(j.at(no_at_json::json_pointer("/3")), no_at_json::out_of_range);
+}

tests/src/unit-custom-binary-type.cpp

@@ -0,0 +1,79 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+#include "doctest_compatibility.h"
+
+#include <nlohmann/json.hpp>
+
+#include <cstdint>
+#include <functional>
+#include <map>
+#include <memory>
+#include <string>
+#include <vector>
+
+#ifdef JSON_HAS_CPP_17
+    #include <cstddef>
+#endif
+
+namespace
+{
+
+// a BinaryType whose value type is signed: the elements must still be
+// processed as the numbers 0..255
+using char_binary_json = nlohmann::basic_json <
+                         std::map, std::vector, std::string, bool, std::int64_t, std::uint64_t,
+                         double, std::allocator, nlohmann::adl_serializer, std::vector<char>, void >;
+
+#ifdef JSON_HAS_CPP_17
+    // a BinaryType whose value type is not an integer type at all
+    using byte_binary_json = nlohmann::basic_json <
+    std::map, std::vector, std::string, bool, std::int64_t, std::uint64_t,
+    double, std::allocator, nlohmann::adl_serializer, std::vector<std::byte>, void >;
+#endif
+
+} // namespace
+
+TEST_CASE("binary type whose value type is not std::uint8_t")
+{
+    SECTION("a signed value type does not dump negative numbers")
+    {
+        const std::vector<char> chars{'\0', '\x01', '\xFF'};
+        CHECK(char_binary_json::binary(chars).dump() == R"({"bytes":[0,1,255],"subtype":null})");
+        CHECK(char_binary_json::binary(chars, 42).dump() == R"({"bytes":[0,1,255],"subtype":42})");
+        CHECK(char_binary_json::binary({}).dump() == R"({"bytes":[],"subtype":null})");
+    }
+
+    SECTION("the default binary type is unchanged")
+    {
+        CHECK(nlohmann::json::binary({0, 1, 255}, 42).dump() == R"({"bytes":[0,1,255],"subtype":42})");
+    }
+
+#ifdef JSON_HAS_CPP_17
+    SECTION("dumping a value type that is not an integer")
+    {
+        const std::vector<std::byte> bytes{std::byte{0}, std::byte{1}, std::byte{0xFF}};
+        CHECK(byte_binary_json::binary(bytes).dump() == R"({"bytes":[0,1,255],"subtype":null})");
+        CHECK(byte_binary_json::binary(bytes, 42).dump() == R"({"bytes":[0,1,255],"subtype":42})");
+        CHECK(byte_binary_json::binary({}).dump() == R"({"bytes":[],"subtype":null})");
+    }
+
+    SECTION("hashing and the binary formats")
+    {
+        const std::vector<std::byte> bytes{std::byte{0}, std::byte{1}, std::byte{0xFF}};
+        const auto j = byte_binary_json::binary(bytes);
+
+        CHECK(std::hash<byte_binary_json> {}(j) == std::hash<byte_binary_json> {}(j));
+        CHECK(byte_binary_json::from_cbor(byte_binary_json::to_cbor(j)) == j);
+        CHECK(byte_binary_json::from_msgpack(byte_binary_json::to_msgpack(j)) == j);
+
+        // UBJSON has no binary type, so binary values are written as an array
+        CHECK(byte_binary_json::from_ubjson(byte_binary_json::to_ubjson(j)) == byte_binary_json({0, 1, 255}));
+    }
+#endif
+}

tests/src/unit-custom-object-type.cpp

@@ -0,0 +1,187 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+#include "doctest_compatibility.h"
+
+#include <nlohmann/json.hpp>
+
+#include <cstdint>
+#include <map>
+#include <string>
+#include <type_traits>
+#include <utility>
+#include <vector>
+
+namespace
+{
+
+// An ObjectType that does *not* define a key_compare member type, which is
+// what every hash map looks like to the library.
+//
+// A hash map is deliberately not used here: object_t is probed for
+// key_compare inside the definition of basic_json, that is, while basic_json
+// is still an incomplete type, and whether a hash map can be instantiated
+// with an incomplete mapped type depends on the standard library (libstdc++ 9
+// needs the size of the mapped type for its node type and rejects it). So the
+// object type is built from std::map, and the inherited key_compare member
+// type is shadowed by an entity that is not a type -- the library's probe
+// then finds no type, exactly as for a hash map.
+template<class Key, class T, class Compare, class Allocator>
+struct no_key_compare_map : std::map<Key, T, Compare, Allocator>
+{
+    using base_t = std::map<Key, T, Compare, Allocator>;
+    using base_t::base_t;
+
+    // shadows base_t::key_compare, which is a type; never defined or called
+    void key_compare();
+};
+
+using no_key_compare_json = nlohmann::basic_json<no_key_compare_map>;
+
+// An ObjectType whose erase(iterator) returns void rather than the following
+// iterator, as for instance Abseil's hash maps do
+template<class Key, class T, class Compare, class Allocator>
+struct void_erase_map : std::map<Key, T, Compare, Allocator>
+{
+    using base_t = std::map<Key, T, Compare, Allocator>;
+    using base_t::base_t;
+    using iterator = typename base_t::iterator;
+    using base_t::erase;
+
+    void erase(iterator pos)
+    {
+        base_t::erase(pos);
+    }
+};
+
+using void_erase_json = nlohmann::basic_json<void_erase_map>;
+
+} // namespace
+
+TEST_CASE("object type whose erase() returns void")
+{
+    SECTION("erasing every element through the returned iterator")
+    {
+        void_erase_json j;
+        for (int i = 0; i < 8; ++i)
+        {
+            j["k" + std::to_string(i)] = i;
+        }
+
+        std::size_t erased = 0;
+        for (auto it = j.begin(); it != j.end(); ++erased)
+        {
+            it = j.erase(it);
+        }
+        CHECK(erased == 8);
+        CHECK(j.empty());
+    }
+
+    SECTION("erasing in the middle returns the following element")
+    {
+        void_erase_json j;
+        for (int i = 0; i < 4; ++i)
+        {
+            j["k" + std::to_string(i)] = i;
+        }
+
+        auto it = j.begin();
+        ++it;
+        const auto after = j.erase(it);
+        CHECK(j.size() == 3);
+        CHECK(after.key() == "k2");
+        CHECK(after.value() == 2);
+        CHECK(!j.contains("k1"));
+    }
+
+    SECTION("the other erase overloads are unaffected")
+    {
+        void_erase_json j;
+        j["a"] = 1;
+        j["b"] = 2;
+        j["c"] = 3;
+
+        CHECK(j.erase("a") == 1);
+        CHECK(j.erase("nope") == 0);
+        j.erase(j.begin(), j.end());
+        CHECK(j.empty());
+    }
+}
+
+TEST_CASE("object type without key_compare")
+{
+    SECTION("object_comparator_t falls back to default_object_comparator_t")
+    {
+        CHECK(std::is_same < no_key_compare_json::object_comparator_t,
+              no_key_compare_json::default_object_comparator_t >::value);
+    }
+
+    SECTION("object types defining key_compare are unaffected")
+    {
+        CHECK(std::is_same<nlohmann::json::object_comparator_t,
+              nlohmann::json::object_t::key_compare>::value);
+        CHECK(std::is_same<nlohmann::ordered_json::object_comparator_t,
+              nlohmann::ordered_json::object_t::key_compare>::value);
+    }
+
+    SECTION("creating and accessing values")
+    {
+        no_key_compare_json j;
+        j["one"] = 1;
+        j["two"] = "zwei";
+        j["three"]["nested"] = true;
+
+        CHECK(j.size() == 3);
+        CHECK(j.at("one") == 1);
+        CHECK(j["two"] == "zwei");
+        CHECK(j["three"]["nested"] == true);
+        CHECK(j.contains("one"));
+        CHECK(!j.contains("four"));
+        CHECK(j.find("one") != j.end());
+        CHECK(j.count("one") == 1);
+        CHECK(j.erase("one") == 1);
+        CHECK(j.size() == 2);
+    }
+
+    SECTION("serialization and deserialization")
+    {
+        const auto j = no_key_compare_json::parse(R"({"a":[1,2,3],"b":{"c":null}})");
+        CHECK(j["a"].size() == 3);
+        CHECK(j["a"][2] == 3);
+        CHECK(j["b"]["c"].is_null());
+        CHECK(no_key_compare_json::parse(j.dump()) == j);
+    }
+
+    SECTION("binary formats")
+    {
+        const auto j = no_key_compare_json::parse(R"({"a":[1,2,3],"b":"x"})");
+        CHECK(no_key_compare_json::from_cbor(no_key_compare_json::to_cbor(j)) == j);
+        CHECK(no_key_compare_json::from_msgpack(no_key_compare_json::to_msgpack(j)) == j);
+    }
+
+    SECTION("flatten and unflatten")
+    {
+        // "o" has a key that looks like an array index, so unflatten() must
+        // not turn it into an array
+        const auto j = no_key_compare_json::parse(
+                           R"({"c":[1,2,3],"d":{"e":"s"},"n":[[0,1],[2]],"o":{"2":"x"}})");
+        CHECK(j.flatten().unflatten() == j);
+    }
+
+    SECTION("conversion to and from nlohmann::json")
+    {
+        const auto j = no_key_compare_json::parse(R"({"a":1,"b":[true,null]})");
+        const nlohmann::json converted(j);
+
+        CHECK(converted.is_object());
+        CHECK(converted["a"] == 1);
+        CHECK(converted["b"][0] == true);
+        CHECK(converted["b"][1].is_null());
+        CHECK(no_key_compare_json(converted) == j);
+    }
+}

tests/src/unit-json_pointer.cpp

@@ -465,6 +465,16 @@ TEST_CASE("JSON pointers")
         // explicit roundtrip check
         CHECK(j.flatten().unflatten() == j);
 
+        // an object is only unflattened to an array if one of its keys is the
+        // reference token 0; this must not depend on which key is seen first
+        CHECK(json({{"/2", "x"}}).unflatten() == json({{"2", "x"}}));
+        CHECK(json({{"/10", "y"}, {"/2", "z"}}).unflatten() == json({{"10", "y"}, {"2", "z"}}));
+        CHECK(json({{"/0", 1}, {"/1", 2}}).unflatten() == json({1, 2}));
+        CHECK(json({{"/1", 2}, {"/0", 1}}).unflatten() == json({1, 2}));
+        CHECK(json({{"/0", 1}, {"/2", 3}}).unflatten() == json({1, nullptr, 3}));
+        CHECK(json({{"/a/1", 2}, {"/a/0", 1}}).unflatten() == json({{"a", {1, 2}}}));
+        CHECK(json({{"/a/1", 2}, {"/a/x", 1}}).unflatten() == json({{"a", {{"1", 2}, {"x", 1}}}}));
+
         // roundtrip for primitive values
         json j_null;
         CHECK(j_null.flatten().unflatten() == j_null);