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nlohmann
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+1
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@@ -51,7 +51,11 @@ If you do want to preserve the **insertion order**, you can use the type [`nlohm
|
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--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
|
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index, so every key access is a linear scan and building an object of `n` keys costs O(n²). This is unnoticeable at
|
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|
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@@ -68,7 +68,11 @@ 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`](https://json.nlohmann.me/api/ordered_map/index.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](https://json.nlohmann.me/features/types/template_parameters/#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`](https://json.nlohmann.me/api/ordered_map/index.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 typical object sizes but becomes significant for objects with many thousands of keys; see [`ordered_map` complexity](https://json.nlohmann.me/api/ordered_map/#complexity). The alternatives above keep a lookup index and do not have this cost.
|
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|
||||
|
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+6
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@@ -79,7 +79,8 @@ template<
|
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class NumberFloatType = double,
|
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template<typename U> class AllocatorType = std::allocator,
|
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template<typename T, typename SFINAE = void> class JSONSerializer = adl_serializer,
|
||||
class BinaryType = std::vector<std::uint8_t>
|
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class BinaryType = std::vector<std::uint8_t>,
|
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class CustomBaseClass = void
|
||||
>
|
||||
class basic_json;
|
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```
|
||||
@@ -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
|
||||
|
||||
|
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class NumberFloatType = double,
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template<typename U> class AllocatorType = std::allocator,
|
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template<typename T, typename SFINAE = void> class JSONSerializer = adl_serializer,
|
||||
class BinaryType = std::vector<std::uint8_t>
|
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class BinaryType = std::vector<std::uint8_t>,
|
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class CustomBaseClass = void
|
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>
|
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class basic_json;
|
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```
|
||||
@@ -106,6 +107,8 @@ 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](https://json.nlohmann.me/features/types/template_parameters/index.md).
|
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|
||||
## Objects
|
||||
|
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[RFC 8259](https://tools.ietf.org/html/rfc8259) describes JSON objects as follows:
|
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|
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|
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# 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"
|
||||
|
||||
Three requirements are checked with a `#!cpp static_assert`: the array iterator category, the width of
|
||||
[`BinaryType`](#binarytype)'s `value_type`, and [`NumberUnsignedType`](#numberintegertype-and-numberunsignedtype)
|
||||
being at least as wide as [`NumberIntegerType`](#numberintegertype-and-numberunsignedtype). The rest are not
|
||||
diagnosed with dedicated error messages, and violating most of them results in a compiler error somewhere inside
|
||||
the library. Four 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 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.
|
||||
- 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 at least as wide as `NumberIntegerType` |
|
||||
| [`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.
|
||||
|
||||
!!! tip "Reference implementation"
|
||||
|
||||
`docs/mkdocs/docs/examples/custom_object_type.hpp` wraps a private `#!cpp std::map` and satisfies every
|
||||
requirement above. It does not define `key_compare`, so `object_comparator_t` falls back to
|
||||
[`default_object_comparator_t`](../../api/basic_json/default_object_comparator_t.md) -- a good starting point for
|
||||
a custom `ObjectType`.
|
||||
|
||||
```cpp
|
||||
--8<-- "examples/custom_object_type.hpp"
|
||||
```
|
||||
|
||||
??? example "Compiling and using it"
|
||||
|
||||
```cpp
|
||||
--8<-- "examples/custom_object_type.cpp"
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```json
|
||||
--8<-- "examples/custom_object_type.output"
|
||||
```
|
||||
|
||||
### 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` | require a complete mapped type |
|
||||
| `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.
|
||||
|
||||
!!! tip "Reference implementation"
|
||||
|
||||
`docs/mkdocs/docs/examples/custom_array_type.hpp` wraps a private `#!cpp std::vector` and satisfies every
|
||||
requirement above -- a good starting point for a custom `ArrayType`.
|
||||
|
||||
```cpp
|
||||
--8<-- "examples/custom_array_type.hpp"
|
||||
```
|
||||
|
||||
??? example "Compiling and using it"
|
||||
|
||||
```cpp
|
||||
--8<-- "examples/custom_array_type.cpp"
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```json
|
||||
--8<-- "examples/custom_array_type.output"
|
||||
```
|
||||
|
||||
### 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. A non-`#!cpp const` `data()` would
|
||||
serve just as well, but `#!cpp std::string` has only had one since C++17, and the library still supports C++11.
|
||||
|
||||
### 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"
|
||||
|
||||
`docs/mkdocs/docs/examples/custom_string_type.hpp` wraps a private `#!cpp std::string` and satisfies every
|
||||
requirement above -- a good starting point for a custom `StringType`. The unit test
|
||||
`tests/src/unit-alt-string.cpp` contains a more thorough variant, `alt_string`, exercised against a larger part
|
||||
of the API.
|
||||
|
||||
```cpp
|
||||
--8<-- "examples/custom_string_type.hpp"
|
||||
```
|
||||
|
||||
??? example "Compiling and using it"
|
||||
|
||||
```cpp
|
||||
--8<-- "examples/custom_string_type.cpp"
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```json
|
||||
--8<-- "examples/custom_string_type.output"
|
||||
```
|
||||
|
||||
## `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`. A `#!cpp static_assert` requires it to be at least as
|
||||
wide as `NumberIntegerType`, which is what that amounts to for the standard integer types.
|
||||
- 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, so a wider `value_type` is
|
||||
rejected with a `#!cpp static_assert`.
|
||||
- Contiguous storage: the binary readers `#!cpp std::memcpy` into `#!cpp &binary[n]`, the writers `reinterpret_cast`
|
||||
`data()`. `#!cpp data() + n` would do for the readers too, but they share one helper with
|
||||
[`StringType`](#stringtype), whose non-`#!cpp const` `data()` is C++17 and later only.
|
||||
- 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.
|
||||
|
||||
!!! tip "Reference implementation"
|
||||
|
||||
`docs/mkdocs/docs/examples/custom_binary_type.hpp` wraps a private `#!cpp std::vector<std::uint8_t>` and satisfies
|
||||
every requirement above -- a good starting point for a custom `BinaryType`.
|
||||
|
||||
```cpp
|
||||
--8<-- "examples/custom_binary_type.hpp"
|
||||
```
|
||||
|
||||
??? example "Compiling and using it"
|
||||
|
||||
```cpp
|
||||
--8<-- "examples/custom_binary_type.cpp"
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```json
|
||||
--8<-- "examples/custom_binary_type.output"
|
||||
```
|
||||
|
||||
### 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
|
||||
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Reference in New Issue
Block a user