Fix with_t nav entry and document chaining of the with_*_t aliases

Indent the with_t entry in mkdocs.yml so it is listed under basic_json,
explain that the aliases can be chained and work on ordered_json, and
test both, including json::with_object_t<ordered_map> == ordered_json.

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

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

@@ -109,6 +109,9 @@ The class satisfies the following concept requirements:
 - **initializer_list_t** - type for initializer lists of `basic_json` values
 - [**input_format_t**](input_format_t.md) - type to choose the format to parse
 - [**json_sax_t**](../json_sax/index.md) - type for SAX events
+- [**with_object_t, with_array_t, with_string_t, with_boolean_t, with_integers_t, with_float_t, with_allocator_t,
+  with_json_serializer_t, with_binary_t, with_base_class_t**](with_t.md) - types to create a `basic_json` type with
+  one (or two) replaced template parameters
 
 ### Exceptions
 

docs/mkdocs/docs/api/basic_json/with_t.md

@@ -0,0 +1,136 @@
+# <small>nlohmann::basic_json::</small>with_t
+
+Member alias templates `with_object_t`, `with_array_t`, `with_string_t`, `with_boolean_t`, `with_integers_t`,
+`with_float_t`, `with_allocator_t`, `with_json_serializer_t`, `with_binary_t`, and `with_base_class_t`.
+
+```cpp
+template<template<typename, typename, typename...> class ObjectType2>
+using with_object_t = basic_json<ObjectType2, ArrayType, StringType, BooleanType,
+      NumberIntegerType, NumberUnsignedType, NumberFloatType,
+      AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;
+
+template<template<typename, typename...> class ArrayType2>
+using with_array_t = basic_json<ObjectType, ArrayType2, StringType, BooleanType,
+      NumberIntegerType, NumberUnsignedType, NumberFloatType,
+      AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;
+
+template<class StringType2>
+using with_string_t = basic_json<ObjectType, ArrayType, StringType2, BooleanType,
+      NumberIntegerType, NumberUnsignedType, NumberFloatType,
+      AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;
+
+template<class BooleanType2>
+using with_boolean_t = basic_json<ObjectType, ArrayType, StringType, BooleanType2,
+      NumberIntegerType, NumberUnsignedType, NumberFloatType,
+      AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;
+
+template<class NumberIntegerType2, class NumberUnsignedType2>
+using with_integers_t = basic_json<ObjectType, ArrayType, StringType, BooleanType,
+      NumberIntegerType2, NumberUnsignedType2, NumberFloatType,
+      AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;
+
+template<class NumberFloatType2>
+using with_float_t = basic_json<ObjectType, ArrayType, StringType, BooleanType,
+      NumberIntegerType, NumberUnsignedType, NumberFloatType2,
+      AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;
+
+template<template<typename> class AllocatorType2>
+using with_allocator_t = basic_json<ObjectType, ArrayType, StringType, BooleanType,
+      NumberIntegerType, NumberUnsignedType, NumberFloatType,
+      AllocatorType2, JSONSerializer, BinaryType, CustomBaseClass>;
+
+template<template<typename, typename = void> class JSONSerializer2>
+using with_json_serializer_t = basic_json<ObjectType, ArrayType, StringType, BooleanType,
+      NumberIntegerType, NumberUnsignedType, NumberFloatType,
+      AllocatorType, JSONSerializer2, BinaryType, CustomBaseClass>;
+
+template<class BinaryType2>
+using with_binary_t = basic_json<ObjectType, ArrayType, StringType, BooleanType,
+      NumberIntegerType, NumberUnsignedType, NumberFloatType,
+      AllocatorType, JSONSerializer, BinaryType2, CustomBaseClass>;
+
+template<class CustomBaseClass2>
+using with_base_class_t = basic_json<ObjectType, ArrayType, StringType, BooleanType,
+      NumberIntegerType, NumberUnsignedType, NumberFloatType,
+      AllocatorType, JSONSerializer, BinaryType, CustomBaseClass2>;
+```
+
+These member alias templates make it easier to create a `basic_json` type that is identical to the current type except
+for one (or, in the case of `with_integers_t`, two) of its [template parameters](index.md#template-parameters).
+Spelling out all 11 template parameters of `basic_json` just to change a single one is verbose and error-prone; these
+aliases only require the replacement type(s).
+
+with_object_t&lt;ObjectType2&gt;
+:   replaces `ObjectType`
+
+with_array_t&lt;ArrayType2&gt;
+:   replaces `ArrayType`
+
+with_string_t&lt;StringType2&gt;
+:   replaces `StringType`
+
+with_boolean_t&lt;BooleanType2&gt;
+:   replaces `BooleanType`
+
+with_integers_t&lt;NumberIntegerType2, NumberUnsignedType2&gt;
+:   replaces both `NumberIntegerType` and `NumberUnsignedType`; the two are combined into a single alias because they
+    are usually changed together (for instance, when switching to fixed-width integer types)
+
+with_float_t&lt;NumberFloatType2&gt;
+:   replaces `NumberFloatType`
+
+with_allocator_t&lt;AllocatorType2&gt;
+:   replaces `AllocatorType`
+
+with_json_serializer_t&lt;JSONSerializer2&gt;
+:   replaces `JSONSerializer`
+
+with_binary_t&lt;BinaryType2&gt;
+:   replaces `BinaryType`
+
+with_base_class_t&lt;CustomBaseClass2&gt;
+:   replaces `CustomBaseClass`; see also [`json_base_class_t`](json_base_class_t.md)
+
+## Notes
+
+All other template parameters are kept unchanged, so the resulting type still uses, for instance, the same
+`ObjectType` unless `with_object_t` itself is used.
+
+The aliases are members of every `basic_json` specialization, including [`ordered_json`](../ordered_json.md), and the
+type they produce is again a `basic_json` specialization. They can therefore be chained to replace several template
+parameters at once:
+
+```cpp
+using my_json = nlohmann::json::with_integers_t<int, unsigned int>::with_float_t<float>;
+using my_ordered_json = nlohmann::ordered_json::with_string_t<std::wstring>;
+```
+
+The result is the same type as spelling out all template parameters, so the order of the chained aliases does not
+matter. For instance, `nlohmann::json::with_object_t<nlohmann::ordered_map>` is `nlohmann::ordered_json`.
+
+## Examples
+
+??? example
+
+    The following code shows how `with_object_t` can be used to create a JSON type that stores object elements in a
+    `std::map` and therefore keeps them sorted by key, unlike the default type which preserves insertion order
+    only when `nlohmann::ordered_json` is used.
+
+    ```cpp
+    --8<-- "examples/with_t.cpp"
+    ```
+
+    Output:
+
+    ```json
+    --8<-- "examples/with_t.output"
+    ```
+
+## See also
+
+- [basic_json](index.md#template-parameters) - the template parameters that can be replaced
+- [json_base_class_t](json_base_class_t.md) - the type used for `CustomBaseClass`
+
+## Version history
+
+- Added in version 3.13.0.

docs/mkdocs/docs/examples/json_base_class_t.cpp

@@ -13,19 +13,7 @@ class visitor_adaptor_with_metadata
     void do_visit(const Ptr& ptr, const Fnc& fnc) const;
 };
 
-using json = nlohmann::basic_json <
+using json = nlohmann::json::with_base_class_t<visitor_adaptor_with_metadata>;
-             std::map,
-             std::vector,
-             std::string,
-             bool,
-             std::int64_t,
-             std::uint64_t,
-             double,
-             std::allocator,
-             nlohmann::adl_serializer,
-             std::vector<std::uint8_t>,
-             visitor_adaptor_with_metadata
-             >;
 
 template <class Fnc>
 void visitor_adaptor_with_metadata::visit(const Fnc& fnc) const

docs/mkdocs/docs/examples/with_t.cpp

@@ -0,0 +1,18 @@
+#include <iostream>
+#include <map>
+#include <nlohmann/json.hpp>
+
+// a JSON type that stores objects in a std::map (which keeps keys sorted)
+// instead of the default ordered associative container
+using sorted_json = nlohmann::json::with_object_t<std::map>;
+
+int main()
+{
+    sorted_json j;
+    j["c"] = 1;
+    j["a"] = 2;
+    j["b"] = 3;
+
+    // keys are sorted, because std::map is used to store the object
+    std::cout << j.dump() << std::endl;
+}

docs/mkdocs/docs/examples/with_t.output

@@ -0,0 +1 @@
+{"a":2,"b":3,"c":1}

docs/mkdocs/mkdocs.yml

@@ -232,6 +232,7 @@ nav:
           - 'update': api/basic_json/update.md
           - 'value': api/basic_json/value.md
           - 'value_t': api/basic_json/value_t.md
+          - 'with_t': api/basic_json/with_t.md
       - byte_container_with_subtype:
           - 'Overview': api/byte_container_with_subtype/index.md
           - '(constructor)': api/byte_container_with_subtype/byte_container_with_subtype.md

include/nlohmann/json.hpp

@@ -226,6 +226,70 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     /// SAX interface type, see @ref nlohmann::json_sax
     using json_sax_t = json_sax<basic_json>;
 
+    ////////////////////////////////////////////////////////////////////////////////
+    // utility templates to create a json type with different template parameters //
+    ////////////////////////////////////////////////////////////////////////////////
+
+    /// Json type using a different type for storing objects
+    /// @sa https://json.nlohmann.me/api/basic_json/with_t/
+    template<template<typename, typename, typename...> class ObjectType2>
+    using with_object_t = basic_json<ObjectType2, ArrayType, StringType, BooleanType,
+          NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;
+
+    /// Json type using a different type for storing arrays
+    /// @sa https://json.nlohmann.me/api/basic_json/with_t/
+    template<template<typename, typename...> class ArrayType2>
+    using with_array_t =  basic_json<ObjectType, ArrayType2, StringType, BooleanType,
+          NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;
+
+    /// Json type using a different type for storing strings
+    /// @sa https://json.nlohmann.me/api/basic_json/with_t/
+    template<class StringType2>
+    using with_string_t =  basic_json<ObjectType, ArrayType, StringType2, BooleanType,
+          NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;
+
+    /// Json type using a different type for storing booleans
+    /// @sa https://json.nlohmann.me/api/basic_json/with_t/
+    template<class BooleanType2>
+    using with_boolean_t =  basic_json<ObjectType, ArrayType, StringType, BooleanType2,
+          NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;
+
+    /// Json type using different types for storing signed and unsigned integers
+    /// @sa https://json.nlohmann.me/api/basic_json/with_t/
+    template<class NumberIntegerType2, class NumberUnsignedType2>
+    using with_integers_t =  basic_json<ObjectType, ArrayType, StringType, BooleanType,
+          NumberIntegerType2, NumberUnsignedType2, NumberFloatType, AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;
+
+    /// Json type using a different type for storing floating point numbers
+    /// @sa https://json.nlohmann.me/api/basic_json/with_t/
+    template<class NumberFloatType2>
+    using with_float_t =  basic_json<ObjectType, ArrayType, StringType, BooleanType,
+          NumberIntegerType, NumberUnsignedType, NumberFloatType2, AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;
+
+    /// Json type using a different type as base allocator
+    /// @sa https://json.nlohmann.me/api/basic_json/with_t/
+    template<template<typename> class AllocatorType2>
+    using with_allocator_t =  basic_json<ObjectType, ArrayType, StringType, BooleanType,
+          NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType2, JSONSerializer, BinaryType, CustomBaseClass>;
+
+    /// Json type using a different type as json serializer
+    /// @sa https://json.nlohmann.me/api/basic_json/with_t/
+    template<template<typename, typename = void> class JSONSerializer2>
+    using with_json_serializer_t =  basic_json<ObjectType, ArrayType, StringType, BooleanType,
+          NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer2, BinaryType, CustomBaseClass>;
+
+    /// Json type using a different type for storing binary data
+    /// @sa https://json.nlohmann.me/api/basic_json/with_t/
+    template<class BinaryType2>
+    using with_binary_t =  basic_json<ObjectType, ArrayType, StringType, BooleanType,
+          NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType2, CustomBaseClass>;
+
+    /// Json type using a different type as base class
+    /// @sa https://json.nlohmann.me/api/basic_json/with_t/
+    template<class CustomBaseClass2>
+    using with_base_class_t =  basic_json<ObjectType, ArrayType, StringType, BooleanType,
+          NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType, CustomBaseClass2>;
+
     ////////////////
     // exceptions //
     ////////////////
@@ -612,208 +676,100 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         /// constructor for rvalue binary arrays (internal type)
         json_value(binary_t&& value) : binary(create<binary_t>(std::move(value))) {}
 
-        // raw, allocation-free transfer of m_data from src to dst: no
+        void destroy(value_t t)
-        // set_parents()/assert_invariant() (the former is O(#children) per
-        // call under JSON_DIAGNOSTICS, which would make the walk below
-        // quadratic); dst takes ownership, src is left as value_t::null.
-        static void take(basic_json& dst, basic_json& src) noexcept
-        {
-            dst.m_data.m_type = src.m_data.m_type;
-            dst.m_data.m_value = src.m_data.m_value;
-            src.m_data.m_type = value_t::null;
-        }
-
-        // true if v is not an array/object, or is an already-empty one
-        static bool has_no_children(const basic_json& v) noexcept
-        {
-            switch (v.m_data.m_type)
-            {
-                case value_t::array:
-                    return v.m_data.m_value.array->empty();
-                case value_t::object:
-                    return v.m_data.m_value.object->empty();
-                default:
-                    return true;
-            }
-        }
-
-        static basic_json& last_child(basic_json& v)
-        {
-            if (v.m_data.m_type == value_t::array)
-            {
-                return v.m_data.m_value.array->back();
-            }
-            JSON_ASSERT(v.m_data.m_type == value_t::object);
-            return v.m_data.m_value.object->rbegin()->second;
-        }
-
-        // removes the last child of a non-empty array/object v; this never
-        // allocates, and since it is only ever called when that child is a
-        // scalar or an already-empty array/object, destroying it never
-        // recurses more than one level deep (see destroy() below)
-        static void pop_last_child(basic_json& v)
-        {
-            if (v.m_data.m_type == value_t::array)
-            {
-                v.m_data.m_value.array->pop_back();
-            }
-            else
-            {
-                JSON_ASSERT(v.m_data.m_type == value_t::object);
-                // erase() needs a forward iterator, so std::prev(end()) is
-                // used here rather than rbegin() (see last_child() above)
-                v.m_data.m_value.object->erase(std::prev(v.m_data.m_value.object->end()));
-            }
-        }
-
-        // deallocates the (already empty) array/object held by v; this is
-        // the same allocator-based free the old recursive implementation
-        // used, just factored out so every level of the walk in destroy()
-        // can share it
-        static void free_container(basic_json& v) noexcept
-        {
-            if (v.m_data.m_type == value_t::array)
-            {
-                JSON_ASSERT(v.m_data.m_value.array->empty());
-                AllocatorType<array_t> alloc;
-                std::allocator_traits<decltype(alloc)>::destroy(alloc, v.m_data.m_value.array);
-                std::allocator_traits<decltype(alloc)>::deallocate(alloc, v.m_data.m_value.array, 1);
-            }
-            else
-            {
-                JSON_ASSERT(v.m_data.m_type == value_t::object);
-                JSON_ASSERT(v.m_data.m_value.object->empty());
-                AllocatorType<object_t> alloc;
-                std::allocator_traits<decltype(alloc)>::destroy(alloc, v.m_data.m_value.object);
-                std::allocator_traits<decltype(alloc)>::deallocate(alloc, v.m_data.m_value.object, 1);
-            }
-            v.m_data.m_type = value_t::null; // avoid a double free if v is later destructed
-        }
-
-        void destroy_string() noexcept
-        {
-            if (string == nullptr)
-            {
-                // not initialized (e.g., due to exception in the ctor)
-                return;
-            }
-            AllocatorType<string_t> alloc;
-            std::allocator_traits<decltype(alloc)>::destroy(alloc, string);
-            std::allocator_traits<decltype(alloc)>::deallocate(alloc, string, 1);
-        }
-
-        void destroy_binary() noexcept
-        {
-            if (binary == nullptr)
-            {
-                // not initialized (e.g., due to exception in the ctor)
-                return;
-            }
-            AllocatorType<binary_t> alloc;
-            std::allocator_traits<decltype(alloc)>::destroy(alloc, binary);
-            std::allocator_traits<decltype(alloc)>::deallocate(alloc, binary, 1);
-        }
-
-        // t must be value_t::array or value_t::object
-        void destroy_container(value_t t) noexcept
         {
             if (
                 (t == value_t::object && object == nullptr) ||
-                (t == value_t::array && array == nullptr)
+                (t == value_t::array && array == nullptr) ||
+                (t == value_t::string && string == nullptr) ||
+                (t == value_t::binary && binary == nullptr)
             )
             {
                 // not initialized (e.g., due to exception in the ctor)
                 return;
             }
-
+            if (t == value_t::array || t == value_t::object)
-            // Destroy the tree without recursing per nesting level and
-            // without any heap allocation: a heap-allocated flattening
-            // stack (the previous implementation) can itself throw
-            // bad_alloc, which would escape this noexcept destructor and
-            // terminate the program (#5135).
-            //
-            // Instead, walk down the "last child" chain, reversing links
-            // as we go: cur is the container currently being emptied,
-            // and prev is its parent (value_t::null when there is none).
-            // Each parent's last child slot doubles as storage for that
-            // parent's own parent link while we are below it, so no
-            // extra memory is needed. We only ever remove a child once
-            // it is a scalar or an empty array/object, which neither
-            // allocates nor recurses more than one level deep.
-            //
-            // This json_value is not itself a basic_json, so the
-            // top-level container is first moved into a local stand-in
-            // ("cur"); a default-constructed basic_json has a null
-            // pointer in its m_value (see data::m_value's initializer),
-            // so swapping it with *this leaves this union's own pointer
-            // null, and it is never looked at or freed a second time.
-            basic_json cur;
-            cur.m_data.m_type = t;
-            using std::swap;
-            swap(cur.m_data.m_value, *this);
-
-            basic_json prev; // value_t::null: no parent
-
-            while (true)
             {
-                if (has_no_children(cur))
+                // flatten the current json_value to a heap-allocated stack
+                std::vector<basic_json> stack;
+
+                // move the top-level items to stack
+                if (t == value_t::array)
                 {
-                    if (prev.m_data.m_type == value_t::null)
+                    stack.reserve(array->size());
+                    std::move(array->begin(), array->end(), std::back_inserter(stack));
+                }
+                else
+                {
+                    stack.reserve(object->size());
+                    for (auto&& it : *object)
                     {
-                        free_container(cur);
+                        stack.push_back(std::move(it.second));
-                        return; // back at the top with nothing left to do
+                    }
+                }
+
+                while (!stack.empty())
+                {
+                    // move the last item to a local variable to be processed
+                    basic_json current_item(std::move(stack.back()));
+                    stack.pop_back();
+
+                    // if current_item is array/object, move
+                    // its children to the stack to be processed later
+                    if (current_item.is_array())
+                    {
+                        std::move(current_item.m_data.m_value.array->begin(), current_item.m_data.m_value.array->end(), std::back_inserter(stack));
+
+                        current_item.m_data.m_value.array->clear();
+                    }
+                    else if (current_item.is_object())
+                    {
+                        for (auto&& it : *current_item.m_data.m_value.object)
+                        {
+                            stack.push_back(std::move(it.second));
+                        }
+
+                        current_item.m_data.m_value.object->clear();
                     }
 
-                    // ascend: detach the grandparent link from prev's
+                    // it's now safe that current_item gets destructed
-                    // last slot, drop that (now null) slot, free cur
+                    // since it doesn't have any children
-                    // (it is empty), then move up one level
-                    basic_json gp;
-                    take(gp, last_child(prev));
-                    pop_last_child(prev);
-
-                    free_container(cur);
-
-                    take(cur, prev);
-                    take(prev, gp);
-                    continue;
                 }
-
-                basic_json& cur_last_ref = last_child(cur);
-
-                if (has_no_children(cur_last_ref))
-                {
-                    // scalar, or already-empty array/object
-                    pop_last_child(cur);
-                    continue;
-                }
-
-                // descend into the non-empty last child, reversing the
-                // link: its slot takes over prev, and the child becomes
-                // the new cur
-                basic_json tmp;
-                take(tmp, cur_last_ref);
-                take(cur_last_ref, prev);
-                take(prev, cur);
-                take(cur, tmp);
             }
-        }
 
-        void destroy(value_t t)
-        {
             switch (t)
             {
-                case value_t::string:
+                case value_t::object:
-                    destroy_string();
+                {
+                    AllocatorType<object_t> alloc;
+                    std::allocator_traits<decltype(alloc)>::destroy(alloc, object);
+                    std::allocator_traits<decltype(alloc)>::deallocate(alloc, object, 1);
                     break;
+                }
+
+                case value_t::array:
+                {
+                    AllocatorType<array_t> alloc;
+                    std::allocator_traits<decltype(alloc)>::destroy(alloc, array);
+                    std::allocator_traits<decltype(alloc)>::deallocate(alloc, array, 1);
+                    break;
+                }
+
+                case value_t::string:
+                {
+                    AllocatorType<string_t> alloc;
+                    std::allocator_traits<decltype(alloc)>::destroy(alloc, string);
+                    std::allocator_traits<decltype(alloc)>::deallocate(alloc, string, 1);
+                    break;
+                }
 
                 case value_t::binary:
-                    destroy_binary();
+                {
-                    break;
+                    AllocatorType<binary_t> alloc;
-
+                    std::allocator_traits<decltype(alloc)>::destroy(alloc, binary);
-                case value_t::object:
+                    std::allocator_traits<decltype(alloc)>::deallocate(alloc, binary, 1);
-                case value_t::array:
-                    destroy_container(t);
                     break;
+                }
 
                 case value_t::null:
                 case value_t::boolean:
@@ -822,7 +778,9 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                 case value_t::number_float:
                 case value_t::discarded:
                 default:
+                {
                     break;
+                }
             }
         }
     };

single_include/nlohmann/json.hpp

@@ -27348,6 +27348,70 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     /// SAX interface type, see @ref nlohmann::json_sax
     using json_sax_t = json_sax<basic_json>;
 
+    ////////////////////////////////////////////////////////////////////////////////
+    // utility templates to create a json type with different template parameters //
+    ////////////////////////////////////////////////////////////////////////////////
+
+    /// Json type using a different type for storing objects
+    /// @sa https://json.nlohmann.me/api/basic_json/with_t/
+    template<template<typename, typename, typename...> class ObjectType2>
+    using with_object_t = basic_json<ObjectType2, ArrayType, StringType, BooleanType,
+                                     NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;
+
+    /// Json type using a different type for storing arrays
+    /// @sa https://json.nlohmann.me/api/basic_json/with_t/
+    template<template<typename, typename...> class ArrayType2>
+    using with_array_t =  basic_json<ObjectType, ArrayType2, StringType, BooleanType,
+                                     NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;
+
+    /// Json type using a different type for storing strings
+    /// @sa https://json.nlohmann.me/api/basic_json/with_t/
+    template<class StringType2>
+    using with_string_t =  basic_json<ObjectType, ArrayType, StringType2, BooleanType,
+                                      NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;
+
+    /// Json type using a different type for storing booleans
+    /// @sa https://json.nlohmann.me/api/basic_json/with_t/
+    template<class BooleanType2>
+    using with_boolean_t =  basic_json<ObjectType, ArrayType, StringType, BooleanType2,
+                                       NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;
+
+    /// Json type using different types for storing signed and unsigned integers
+    /// @sa https://json.nlohmann.me/api/basic_json/with_t/
+    template<class NumberIntegerType2, class NumberUnsignedType2>
+    using with_integers_t =  basic_json<ObjectType, ArrayType, StringType, BooleanType,
+                                        NumberIntegerType2, NumberUnsignedType2, NumberFloatType, AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;
+
+    /// Json type using a different type for storing floating point numbers
+    /// @sa https://json.nlohmann.me/api/basic_json/with_t/
+    template<class NumberFloatType2>
+    using with_float_t =  basic_json<ObjectType, ArrayType, StringType, BooleanType,
+                                     NumberIntegerType, NumberUnsignedType, NumberFloatType2, AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;
+
+    /// Json type using a different type as base allocator
+    /// @sa https://json.nlohmann.me/api/basic_json/with_t/
+    template<template<typename> class AllocatorType2>
+    using with_allocator_t =  basic_json<ObjectType, ArrayType, StringType, BooleanType,
+                                         NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType2, JSONSerializer, BinaryType, CustomBaseClass>;
+
+    /// Json type using a different type as json serializer
+    /// @sa https://json.nlohmann.me/api/basic_json/with_t/
+    template<template<typename, typename = void> class JSONSerializer2>
+    using with_json_serializer_t =  basic_json<ObjectType, ArrayType, StringType, BooleanType,
+        NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer2, BinaryType, CustomBaseClass>;
+
+    /// Json type using a different type for storing binary data
+    /// @sa https://json.nlohmann.me/api/basic_json/with_t/
+    template<class BinaryType2>
+    using with_binary_t =  basic_json<ObjectType, ArrayType, StringType, BooleanType,
+                                      NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType2, CustomBaseClass>;
+
+    /// Json type using a different type as base class
+    /// @sa https://json.nlohmann.me/api/basic_json/with_t/
+    template<class CustomBaseClass2>
+    using with_base_class_t =  basic_json<ObjectType, ArrayType, StringType, BooleanType,
+                                          NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType, CustomBaseClass2>;
+
     ////////////////
     // exceptions //
     ////////////////
@@ -27734,208 +27798,100 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         /// constructor for rvalue binary arrays (internal type)
         json_value(binary_t&& value) : binary(create<binary_t>(std::move(value))) {}
 
-        // raw, allocation-free transfer of m_data from src to dst: no
+        void destroy(value_t t)
-        // set_parents()/assert_invariant() (the former is O(#children) per
-        // call under JSON_DIAGNOSTICS, which would make the walk below
-        // quadratic); dst takes ownership, src is left as value_t::null.
-        static void take(basic_json& dst, basic_json& src) noexcept
-        {
-            dst.m_data.m_type = src.m_data.m_type;
-            dst.m_data.m_value = src.m_data.m_value;
-            src.m_data.m_type = value_t::null;
-        }
-
-        // true if v is not an array/object, or is an already-empty one
-        static bool has_no_children(const basic_json& v) noexcept
-        {
-            switch (v.m_data.m_type)
-            {
-                case value_t::array:
-                    return v.m_data.m_value.array->empty();
-                case value_t::object:
-                    return v.m_data.m_value.object->empty();
-                default:
-                    return true;
-            }
-        }
-
-        static basic_json& last_child(basic_json& v)
-        {
-            if (v.m_data.m_type == value_t::array)
-            {
-                return v.m_data.m_value.array->back();
-            }
-            JSON_ASSERT(v.m_data.m_type == value_t::object);
-            return v.m_data.m_value.object->rbegin()->second;
-        }
-
-        // removes the last child of a non-empty array/object v; this never
-        // allocates, and since it is only ever called when that child is a
-        // scalar or an already-empty array/object, destroying it never
-        // recurses more than one level deep (see destroy() below)
-        static void pop_last_child(basic_json& v)
-        {
-            if (v.m_data.m_type == value_t::array)
-            {
-                v.m_data.m_value.array->pop_back();
-            }
-            else
-            {
-                JSON_ASSERT(v.m_data.m_type == value_t::object);
-                // erase() needs a forward iterator, so std::prev(end()) is
-                // used here rather than rbegin() (see last_child() above)
-                v.m_data.m_value.object->erase(std::prev(v.m_data.m_value.object->end()));
-            }
-        }
-
-        // deallocates the (already empty) array/object held by v; this is
-        // the same allocator-based free the old recursive implementation
-        // used, just factored out so every level of the walk in destroy()
-        // can share it
-        static void free_container(basic_json& v) noexcept
-        {
-            if (v.m_data.m_type == value_t::array)
-            {
-                JSON_ASSERT(v.m_data.m_value.array->empty());
-                AllocatorType<array_t> alloc;
-                std::allocator_traits<decltype(alloc)>::destroy(alloc, v.m_data.m_value.array);
-                std::allocator_traits<decltype(alloc)>::deallocate(alloc, v.m_data.m_value.array, 1);
-            }
-            else
-            {
-                JSON_ASSERT(v.m_data.m_type == value_t::object);
-                JSON_ASSERT(v.m_data.m_value.object->empty());
-                AllocatorType<object_t> alloc;
-                std::allocator_traits<decltype(alloc)>::destroy(alloc, v.m_data.m_value.object);
-                std::allocator_traits<decltype(alloc)>::deallocate(alloc, v.m_data.m_value.object, 1);
-            }
-            v.m_data.m_type = value_t::null; // avoid a double free if v is later destructed
-        }
-
-        void destroy_string() noexcept
-        {
-            if (string == nullptr)
-            {
-                // not initialized (e.g., due to exception in the ctor)
-                return;
-            }
-            AllocatorType<string_t> alloc;
-            std::allocator_traits<decltype(alloc)>::destroy(alloc, string);
-            std::allocator_traits<decltype(alloc)>::deallocate(alloc, string, 1);
-        }
-
-        void destroy_binary() noexcept
-        {
-            if (binary == nullptr)
-            {
-                // not initialized (e.g., due to exception in the ctor)
-                return;
-            }
-            AllocatorType<binary_t> alloc;
-            std::allocator_traits<decltype(alloc)>::destroy(alloc, binary);
-            std::allocator_traits<decltype(alloc)>::deallocate(alloc, binary, 1);
-        }
-
-        // t must be value_t::array or value_t::object
-        void destroy_container(value_t t) noexcept
         {
             if (
                 (t == value_t::object && object == nullptr) ||
-                (t == value_t::array && array == nullptr)
+                (t == value_t::array && array == nullptr) ||
+                (t == value_t::string && string == nullptr) ||
+                (t == value_t::binary && binary == nullptr)
             )
             {
                 // not initialized (e.g., due to exception in the ctor)
                 return;
             }
-
+            if (t == value_t::array || t == value_t::object)
-            // Destroy the tree without recursing per nesting level and
-            // without any heap allocation: a heap-allocated flattening
-            // stack (the previous implementation) can itself throw
-            // bad_alloc, which would escape this noexcept destructor and
-            // terminate the program (#5135).
-            //
-            // Instead, walk down the "last child" chain, reversing links
-            // as we go: cur is the container currently being emptied,
-            // and prev is its parent (value_t::null when there is none).
-            // Each parent's last child slot doubles as storage for that
-            // parent's own parent link while we are below it, so no
-            // extra memory is needed. We only ever remove a child once
-            // it is a scalar or an empty array/object, which neither
-            // allocates nor recurses more than one level deep.
-            //
-            // This json_value is not itself a basic_json, so the
-            // top-level container is first moved into a local stand-in
-            // ("cur"); a default-constructed basic_json has a null
-            // pointer in its m_value (see data::m_value's initializer),
-            // so swapping it with *this leaves this union's own pointer
-            // null, and it is never looked at or freed a second time.
-            basic_json cur;
-            cur.m_data.m_type = t;
-            using std::swap;
-            swap(cur.m_data.m_value, *this);
-
-            basic_json prev; // value_t::null: no parent
-
-            while (true)
             {
-                if (has_no_children(cur))
+                // flatten the current json_value to a heap-allocated stack
+                std::vector<basic_json> stack;
+
+                // move the top-level items to stack
+                if (t == value_t::array)
                 {
-                    if (prev.m_data.m_type == value_t::null)
+                    stack.reserve(array->size());
+                    std::move(array->begin(), array->end(), std::back_inserter(stack));
+                }
+                else
+                {
+                    stack.reserve(object->size());
+                    for (auto&& it : *object)
                     {
-                        free_container(cur);
+                        stack.push_back(std::move(it.second));
-                        return; // back at the top with nothing left to do
+                    }
+                }
+
+                while (!stack.empty())
+                {
+                    // move the last item to a local variable to be processed
+                    basic_json current_item(std::move(stack.back()));
+                    stack.pop_back();
+
+                    // if current_item is array/object, move
+                    // its children to the stack to be processed later
+                    if (current_item.is_array())
+                    {
+                        std::move(current_item.m_data.m_value.array->begin(), current_item.m_data.m_value.array->end(), std::back_inserter(stack));
+
+                        current_item.m_data.m_value.array->clear();
+                    }
+                    else if (current_item.is_object())
+                    {
+                        for (auto&& it : *current_item.m_data.m_value.object)
+                        {
+                            stack.push_back(std::move(it.second));
+                        }
+
+                        current_item.m_data.m_value.object->clear();
                     }
 
-                    // ascend: detach the grandparent link from prev's
+                    // it's now safe that current_item gets destructed
-                    // last slot, drop that (now null) slot, free cur
+                    // since it doesn't have any children
-                    // (it is empty), then move up one level
-                    basic_json gp;
-                    take(gp, last_child(prev));
-                    pop_last_child(prev);
-
-                    free_container(cur);
-
-                    take(cur, prev);
-                    take(prev, gp);
-                    continue;
                 }
-
-                basic_json& cur_last_ref = last_child(cur);
-
-                if (has_no_children(cur_last_ref))
-                {
-                    // scalar, or already-empty array/object
-                    pop_last_child(cur);
-                    continue;
-                }
-
-                // descend into the non-empty last child, reversing the
-                // link: its slot takes over prev, and the child becomes
-                // the new cur
-                basic_json tmp;
-                take(tmp, cur_last_ref);
-                take(cur_last_ref, prev);
-                take(prev, cur);
-                take(cur, tmp);
             }
-        }
 
-        void destroy(value_t t)
-        {
             switch (t)
             {
-                case value_t::string:
+                case value_t::object:
-                    destroy_string();
+                {
+                    AllocatorType<object_t> alloc;
+                    std::allocator_traits<decltype(alloc)>::destroy(alloc, object);
+                    std::allocator_traits<decltype(alloc)>::deallocate(alloc, object, 1);
                     break;
+                }
+
+                case value_t::array:
+                {
+                    AllocatorType<array_t> alloc;
+                    std::allocator_traits<decltype(alloc)>::destroy(alloc, array);
+                    std::allocator_traits<decltype(alloc)>::deallocate(alloc, array, 1);
+                    break;
+                }
+
+                case value_t::string:
+                {
+                    AllocatorType<string_t> alloc;
+                    std::allocator_traits<decltype(alloc)>::destroy(alloc, string);
+                    std::allocator_traits<decltype(alloc)>::deallocate(alloc, string, 1);
+                    break;
+                }
 
                 case value_t::binary:
-                    destroy_binary();
+                {
-                    break;
+                    AllocatorType<binary_t> alloc;
-
+                    std::allocator_traits<decltype(alloc)>::destroy(alloc, binary);
-                case value_t::object:
+                    std::allocator_traits<decltype(alloc)>::deallocate(alloc, binary, 1);
-                case value_t::array:
-                    destroy_container(t);
                     break;
+                }
 
                 case value_t::null:
                 case value_t::boolean:
@@ -27944,7 +27900,9 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                 case value_t::number_float:
                 case value_t::discarded:
                 default:
+                {
                     break;
+                }
             }
         }
     };

tests/src/unit-allocator.cpp

@@ -48,14 +48,7 @@ TEST_CASE("bad_alloc")
     SECTION("bad_alloc")
     {
         // create JSON type using the throwing allocator
-        using bad_json = nlohmann::basic_json<std::map,
+        using bad_json = nlohmann::json::with_allocator_t<bad_allocator>;
-              std::vector,
-              std::string,
-              bool,
-              std::int64_t,
-              std::uint64_t,
-              double,
-              bad_allocator>;
 
         // creating an object should throw
         CHECK_THROWS_AS(bad_json(bad_json::value_t::object), std::bad_alloc&);
@@ -129,14 +122,7 @@ void my_allocator_clean_up(T* p)
 TEST_CASE("controlled bad_alloc")
 {
     // create JSON type using the throwing allocator
-    using my_json = nlohmann::basic_json<std::map,
+    using my_json = nlohmann::json::with_allocator_t<my_allocator>;
-          std::vector,
-          std::string,
-          bool,
-          std::int64_t,
-          std::uint64_t,
-          double,
-          my_allocator>;
 
     SECTION("class json_value")
     {
@@ -588,102 +574,10 @@ TEST_CASE("bad my_allocator::construct")
 {
     SECTION("my_allocator::construct doesn't forward")
     {
-        using bad_alloc_json = nlohmann::basic_json<std::map,
+        using bad_alloc_json = nlohmann::json::with_allocator_t<allocator_no_forward>;
-              std::vector,
-              std::string,
-              bool,
-              std::int64_t,
-              std::uint64_t,
-              double,
-              allocator_no_forward>;
 
         bad_alloc_json j;
         j["test"] = bad_alloc_json::array_t();
         j["test"].push_back("should not leak");
     }
 }
-
-namespace
-{
-std::size_t counting_allocator_allocations = 0;
-std::size_t counting_allocator_deallocations = 0;
-
-template<class T>
-struct counting_allocator : std::allocator<T>
-{
-    using std::allocator<T>::allocator;
-
-    T* allocate(std::size_t n)
-    {
-        ++counting_allocator_allocations;
-        return std::allocator<T>::allocate(n);
-    }
-
-    void deallocate(T* p, std::size_t n)
-    {
-        ++counting_allocator_deallocations;
-        std::allocator<T>::deallocate(p, n);
-    }
-
-    template <class U>
-    struct rebind
-    {
-        using other = counting_allocator<U>;
-    };
-};
-} // namespace
-
-TEST_CASE("destructor performs no allocation, only deallocation")
-{
-    // see https://github.com/nlohmann/json/issues/4842 and
-    // https://github.com/nlohmann/json/issues/5135: destroying nested
-    // arrays/objects used to allocate a temporary stack (first with
-    // std::allocator, later - after #4842 - with the provided allocator).
-    // Since that stack could itself throw bad_alloc from inside the
-    // noexcept destructor (#5135), destroy() no longer allocates anything:
-    // it only ever frees what is already there.
-    using counting_json = nlohmann::basic_json<std::map,
-          std::vector,
-          std::string,
-          bool,
-          std::int64_t,
-          std::uint64_t,
-          double,
-          counting_allocator>;
-
-    SECTION("array")
-    {
-        auto* j = new counting_json({1, {2, {3, 4}}, 5}); // NOLINT(cppcoreguidelines-owning-memory)
-        const auto allocations_before = counting_allocator_allocations;
-        const auto deallocations_before = counting_allocator_deallocations;
-        delete j; // NOLINT(cppcoreguidelines-owning-memory)
-        CHECK(counting_allocator_allocations == allocations_before);
-        CHECK(counting_allocator_deallocations > deallocations_before);
-    }
-
-    SECTION("object")
-    {
-        auto* j = new counting_json({{"a", {{"b", {1, 2}}}}, {"c", 3}}); // NOLINT(cppcoreguidelines-owning-memory)
-        const auto allocations_before = counting_allocator_allocations;
-        const auto deallocations_before = counting_allocator_deallocations;
-        delete j; // NOLINT(cppcoreguidelines-owning-memory)
-        CHECK(counting_allocator_allocations == allocations_before);
-        CHECK(counting_allocator_deallocations > deallocations_before);
-    }
-
-    SECTION("mixed tree of empty/non-empty arrays and objects")
-    {
-        auto* j = new counting_json( // NOLINT(cppcoreguidelines-owning-memory)
-        {
-            {"empty_obj", counting_json::object()},
-            {"empty_arr", counting_json::array()},
-            {"nested", {{"a", counting_json::array({1, 2, counting_json::object()})}, {"b", 3}}},
-            {"tail", counting_json::array({counting_json::array({1}), 2, counting_json::array({3})})}
-        });
-        const auto allocations_before = counting_allocator_allocations;
-        const auto deallocations_before = counting_allocator_deallocations;
-        delete j; // NOLINT(cppcoreguidelines-owning-memory)
-        CHECK(counting_allocator_allocations == allocations_before);
-        CHECK(counting_allocator_deallocations > deallocations_before);
-    }
-}

tests/src/unit-alt-string.cpp

@@ -163,16 +163,7 @@ void int_to_string(alt_string& target, std::size_t value)
     target = std::to_string(value).c_str();
 }
 
-using alt_json = nlohmann::basic_json <
+using alt_json = nlohmann::json::with_string_t<alt_string>;
-                 std::map,
-                 std::vector,
-                 alt_string,
-                 bool,
-                 std::int64_t,
-                 std::uint64_t,
-                 double,
-                 std::allocator,
-                 nlohmann::adl_serializer >;
 
 bool operator<(const char* op1, const alt_string& op2) noexcept
 {

tests/src/unit-custom-base-class.cpp

@@ -38,20 +38,7 @@ class json_metadata
 };
 
 template<class T>
-using json_with_metadata =
+using json_with_metadata = nlohmann::json::with_base_class_t<json_metadata<T>>;
-    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::uint8_t>,
-    json_metadata<T>
-    >;
 
 TEST_CASE("JSON Node Metadata")
 {
@@ -268,19 +255,7 @@ class visitor_adaptor
     void do_visit(const Ptr& ptr, const Fnc& fnc) const;
 };
 
-using json_with_visitor_t = nlohmann::basic_json <
+using json_with_visitor_t = nlohmann::json::with_base_class_t<visitor_adaptor>;
-                            std::map,
-                            std::vector,
-                            std::string,
-                            bool,
-                            std::int64_t,
-                            std::uint64_t,
-                            double,
-                            std::allocator,
-                            nlohmann::adl_serializer,
-                            std::vector<std::uint8_t>,
-                            visitor_adaptor
-                            >;
 
 template <class Fnc>
 void visitor_adaptor::visit(const Fnc& fnc) const

tests/src/unit-regression2.cpp

@@ -40,9 +40,7 @@ using ordered_json = nlohmann::ordered_json;
 #endif
 
 #include <cstdio>
-#include <cstdlib>
 #include <list>
-#include <new>
 #include <tuple>
 #include <type_traits>
 #include <utility>
@@ -109,84 +107,6 @@ DOCTEST_CLANG_SUPPRESS_WARNING("-Wexit-time-destructors")
 
 using float_json = nlohmann::basic_json<std::map, std::vector, std::string, bool, std::int64_t, std::uint64_t, float>;
 
-#if (defined(__cpp_exceptions) || defined(__EXCEPTIONS) || defined(_CPPUNWIND)) && !defined(JSON_NOEXCEPTION)
-namespace
-{
-// An allocator whose allocate() can be told to fail on demand, so tests can
-// check that ~basic_json() tolerates - in fact, after #5135, never even
-// triggers - an allocation failure. This replaces an earlier version of
-// this test that overrode the process-wide ::operator new/::operator
-// delete, which affected every allocation in the whole unit-regression2
-// binary rather than just the values under test.
-std::size_t failing_allocator_allocations = 0;
-std::size_t failing_allocator_deallocations = 0;
-bool fail_next_allocation = false;
-
-template<class T>
-struct failing_allocator : std::allocator<T>
-{
-    using std::allocator<T>::allocator;
-
-    failing_allocator() noexcept = default;
-    template<class U>
-    failing_allocator(const failing_allocator<U>& /*unused*/) noexcept {} // NOLINT(google-explicit-constructor)
-
-    T* allocate(std::size_t n)
-    {
-        if (fail_next_allocation)
-        {
-            fail_next_allocation = false;
-            throw std::bad_alloc();
-        }
-        ++failing_allocator_allocations;
-        return std::allocator<T>::allocate(n);
-    }
-
-    void deallocate(T* p, std::size_t n)
-    {
-        ++failing_allocator_deallocations;
-        std::allocator<T>::deallocate(p, n);
-    }
-
-    template<class U>
-    struct rebind
-    {
-        using other = failing_allocator<U>;
-    };
-};
-
-using failing_json = nlohmann::basic_json<std::map, std::vector, std::string, bool,
-      std::int64_t, std::uint64_t, double, failing_allocator>;
-using failing_ordered_json = nlohmann::basic_json<nlohmann::ordered_map, std::vector, std::string, bool,
-      std::int64_t, std::uint64_t, double, failing_allocator>;
-
-// builds `depth` levels of nesting around a scalar, iteratively (never
-// recursing: each wrap only moves the previous, already-built value, which
-// is O(1)), each level an array or an object depending on `nest_objects`
-template<class BasicJsonType>
-BasicJsonType make_deep_nest(std::size_t depth, bool nest_objects)
-{
-    BasicJsonType v = 0;
-    for (std::size_t i = 0; i < depth; ++i)
-    {
-        if (nest_objects)
-        {
-            BasicJsonType wrapper = BasicJsonType::object();
-            wrapper["x"] = std::move(v);
-            v = std::move(wrapper);
-        }
-        else
-        {
-            BasicJsonType wrapper = BasicJsonType::array();
-            wrapper.push_back(std::move(v));
-            v = std::move(wrapper);
-        }
-    }
-    return v;
-}
-} // namespace
-#endif
-
 /////////////////////////////////////////////////////////////////////
 // for #1647
 /////////////////////////////////////////////////////////////////////
@@ -1020,211 +940,4 @@ TEST_CASE("regression test - excessive binary container size honors allow_except
     CHECK(json::from_cbor(std::vector<std::uint8_t> {0x9b, 0, 0, 0, 0, 0, 0, 0, 0x02}, true, false).is_discarded());
 }
 
-#if (defined(__cpp_exceptions) || defined(__EXCEPTIONS) || defined(_CPPUNWIND)) && !defined(JSON_NOEXCEPTION)
-TEST_CASE("regression test #5135 - destructor never allocates, even under memory pressure")
-{
-    // Before the fix, ~basic_json() flattened a nested array/object into a
-    // heap-allocated std::vector to avoid recursing; that allocation could
-    // itself throw bad_alloc, which escapes a noexcept destructor and
-    // terminates the program. destroy() no longer allocates anything, so
-    // none of the sections below ever observe fail_next_allocation being
-    // consumed: CHECK(fail_next_allocation) confirms it was never touched.
-
-    SECTION("the original report: a small, mixed array/object nest")
-    {
-        failing_allocator_allocations = 0;
-        failing_allocator_deallocations = 0;
-        {
-            failing_json j = failing_json::array(
-            {
-                failing_json::array({1, 2}),
-                failing_json::object({{"key", failing_json::array({3})}})
-            });
-            fail_next_allocation = true;
-        } // j is destroyed here, with every further allocation set to fail
-
-        CHECK(fail_next_allocation);
-        fail_next_allocation = false;
-        CHECK(failing_allocator_deallocations > 0);
-    }
-
-    SECTION("100000-deep nested array")
-    {
-        std::size_t allocations_before = 0;
-        {
-            failing_json j = make_deep_nest<failing_json>(100000, false);
-            allocations_before = failing_allocator_allocations;
-            fail_next_allocation = true;
-        }
-
-        CHECK(fail_next_allocation);
-        fail_next_allocation = false;
-        CHECK(failing_allocator_allocations == allocations_before);
-    }
-
-    SECTION("100000-deep nested object")
-    {
-        std::size_t allocations_before = 0;
-        {
-            failing_json j = make_deep_nest<failing_json>(100000, true);
-            allocations_before = failing_allocator_allocations;
-            fail_next_allocation = true;
-        }
-
-        CHECK(fail_next_allocation);
-        fail_next_allocation = false;
-        CHECK(failing_allocator_allocations == allocations_before);
-    }
-
-    SECTION("100000-deep nested ordered_json")
-    {
-        std::size_t allocations_before = 0;
-        {
-            failing_ordered_json j = make_deep_nest<failing_ordered_json>(100000, true);
-            allocations_before = failing_allocator_allocations;
-            fail_next_allocation = true;
-        }
-
-        CHECK(fail_next_allocation);
-        fail_next_allocation = false;
-        CHECK(failing_allocator_allocations == allocations_before);
-    }
-
-    SECTION("wide and deep: 1000 arrays of 1000 elements, each a small nested object")
-    {
-        std::size_t allocations_before = 0;
-        {
-            failing_json wide = failing_json::array();
-            for (std::size_t i = 0; i < 1000; ++i)
-            {
-                failing_json inner = failing_json::array();
-                for (std::size_t k = 0; k < 1000; ++k)
-                {
-                    inner.push_back(failing_json::object({{"a", 1}, {"b", failing_json::array({1, 2, 3})}}));
-                }
-                wide.push_back(std::move(inner));
-            }
-
-            allocations_before = failing_allocator_allocations;
-            fail_next_allocation = true;
-        }
-
-        CHECK(fail_next_allocation);
-        fail_next_allocation = false;
-        CHECK(failing_allocator_allocations == allocations_before);
-    }
-}
-#endif
-
-namespace
-{
-// a single-element chain of `depth` arrays, built iteratively (never
-// recursing: each wrap only moves the previous, already-built value)
-template<class BasicJsonType>
-BasicJsonType make_single_chain(std::size_t depth)
-{
-    BasicJsonType v = 1;
-    for (std::size_t i = 0; i < depth; ++i)
-    {
-        BasicJsonType wrapper = BasicJsonType::array();
-        wrapper.push_back(std::move(v));
-        v = std::move(wrapper);
-    }
-    return v;
-}
-
-// copies value first, to make sure nothing was corrupted by building it,
-// then lets both the copy and the original destruct via normal scope exit
-template<class BasicJsonType>
-void check_destroy_edge_case(const BasicJsonType& value)
-{
-    const BasicJsonType copy = value;
-    CHECK(copy == value);
-}
-} // namespace
-
-TEST_CASE_TEMPLATE("regression test #5135 - destroy() edge cases", BasicJsonType, json, ordered_json)
-{
-    using binary_t = typename BasicJsonType::binary_t;
-
-    SECTION("mix of empty objects, empty arrays, non-empty containers, and scalars")
-    {
-        BasicJsonType root = BasicJsonType::array();
-        root.push_back(BasicJsonType::object());
-        root.push_back(BasicJsonType::array());
-        root.push_back(BasicJsonType::object({{"k", 1}}));
-        root.push_back(BasicJsonType::array({1, 2, 3}));
-        root.push_back(nullptr);
-        root.push_back(true);
-        root.push_back(42);
-        root.push_back(3.14);
-        root.push_back("a string");
-        root.push_back(BasicJsonType(binary_t({1, 2, 3})));
-        check_destroy_edge_case(root);
-    }
-
-    SECTION("container child in first position only")
-    {
-        BasicJsonType root = BasicJsonType::array({BasicJsonType::array({1, 2}), 3, 4, 5});
-        check_destroy_edge_case(root);
-    }
-
-    SECTION("container child in last position only")
-    {
-        BasicJsonType root = BasicJsonType::array({1, 2, 3, BasicJsonType::array({4, 5})});
-        check_destroy_edge_case(root);
-    }
-
-    SECTION("container children in first and last position")
-    {
-        BasicJsonType root = BasicJsonType::array({BasicJsonType::array({1}), 2, 3, BasicJsonType::array({4})});
-        check_destroy_edge_case(root);
-    }
-
-    SECTION("single-element chain, 1000 levels deep")
-    {
-        BasicJsonType root = make_single_chain<BasicJsonType>(1000);
-        check_destroy_edge_case(root);
-    }
-
-    SECTION("top-level empty array")
-    {
-        BasicJsonType root = BasicJsonType::array();
-        check_destroy_edge_case(root);
-    }
-
-    SECTION("top-level empty object")
-    {
-        BasicJsonType root = BasicJsonType::object();
-        check_destroy_edge_case(root);
-    }
-
-    SECTION("object whose last child is a non-empty array whose last child is an empty object")
-    {
-        BasicJsonType inner_array = BasicJsonType::array({1, 2, BasicJsonType::object()});
-        BasicJsonType root = BasicJsonType::object({{"a", 1}, {"b", inner_array}});
-        check_destroy_edge_case(root);
-    }
-
-    SECTION("destruction via erase() on a deeply nested child")
-    {
-        BasicJsonType root = BasicJsonType::array();
-        root.push_back(make_single_chain<BasicJsonType>(500));
-        root.push_back(BasicJsonType::object({{"k", BasicJsonType::array({1, 2, 3})}}));
-        // erase() must destroy the removed subtree without recursing or
-        // allocating beyond what erase() itself needs
-        root.erase(0);
-        CAPTURE(root.size())
-        CHECK(root.size() == 1);
-    }
-
-    SECTION("destruction via assignment on a deep tree")
-    {
-        BasicJsonType root = make_single_chain<BasicJsonType>(2000);
-        // assigning a new value destroys the old one in place
-        root = nullptr;
-        CHECK(root.is_null());
-    }
-}
-
 DOCTEST_CLANG_SUPPRESS_WARNING_POP

tests/src/unit-udt.cpp

@@ -23,6 +23,7 @@ using nlohmann::json;
     using namespace nlohmann::literals; // NOLINT(google-build-using-namespace)
 #endif
 
+#include <deque>
 #include <map>
 #include <memory>
 #include <string>
@@ -684,8 +685,7 @@ static std::ostream& operator<<(std::ostream& os, small_pod l)
 TEST_CASE("custom serializer for pods" * doctest::test_suite("udt"))
 {
     using custom_json =
-        nlohmann::basic_json<std::map, std::vector, std::string, bool,
+        nlohmann::json::with_json_serializer_t<pod_serializer>;
-        std::int64_t, std::uint64_t, double, std::allocator, pod_serializer>;
 
     auto p = udt::small_pod{42, '/', 42};
     custom_json const j = p;
@@ -703,7 +703,7 @@ TEST_CASE("custom serializer for pods" * doctest::test_suite("udt"))
 template <typename T, typename>
 struct another_adl_serializer;
 
-using custom_json = nlohmann::basic_json<std::map, std::vector, std::string, bool, std::int64_t, std::uint64_t, double, std::allocator, another_adl_serializer>;
+using custom_json = nlohmann::json::with_json_serializer_t<another_adl_serializer>;
 
 template <typename T, typename>
 struct another_adl_serializer
@@ -734,6 +734,80 @@ TEST_CASE("custom serializer that does adl by default" * doctest::test_suite("ud
     CHECK(me == cj.get<udt::person>());
 }
 
+TEST_CASE("with_*_t aliases" * doctest::test_suite("udt"))
+{
+    // a custom base class used to check with_base_class_t
+    struct custom_base_class {};
+
+    CHECK(std::is_same<json::with_object_t<std::deque>,
+          nlohmann::basic_json<std::deque, std::vector, std::string, bool,
+          std::int64_t, std::uint64_t, double, std::allocator,
+          nlohmann::adl_serializer, std::vector<std::uint8_t>>>::value);
+
+    CHECK(std::is_same<json::with_array_t<std::deque>,
+          nlohmann::basic_json<std::map, std::deque, std::string, bool,
+          std::int64_t, std::uint64_t, double, std::allocator,
+          nlohmann::adl_serializer, std::vector<std::uint8_t>>>::value);
+
+    CHECK(std::is_same<json::with_string_t<std::wstring>,
+          nlohmann::basic_json<std::map, std::vector, std::wstring, bool,
+          std::int64_t, std::uint64_t, double, std::allocator,
+          nlohmann::adl_serializer, std::vector<std::uint8_t>>>::value);
+
+    CHECK(std::is_same<json::with_boolean_t<int>,
+          nlohmann::basic_json<std::map, std::vector, std::string, int,
+          std::int64_t, std::uint64_t, double, std::allocator,
+          nlohmann::adl_serializer, std::vector<std::uint8_t>>>::value);
+
+    CHECK(std::is_same<json::with_integers_t<std::int32_t, std::uint32_t>,
+          nlohmann::basic_json<std::map, std::vector, std::string, bool,
+          std::int32_t, std::uint32_t, double, std::allocator,
+          nlohmann::adl_serializer, std::vector<std::uint8_t>>>::value);
+
+    CHECK(std::is_same<json::with_float_t<float>,
+          nlohmann::basic_json<std::map, std::vector, std::string, bool,
+          std::int64_t, std::uint64_t, float, std::allocator,
+          nlohmann::adl_serializer, std::vector<std::uint8_t>>>::value);
+
+    CHECK(std::is_same<json::with_allocator_t<std::allocator>,
+          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::uint8_t>>>::value);
+
+    CHECK(std::is_same<json::with_json_serializer_t<nlohmann::adl_serializer>,
+          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::uint8_t>>>::value);
+
+    CHECK(std::is_same<json::with_binary_t<std::vector<char>>,
+          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>>>::value);
+
+    CHECK(std::is_same<json::with_base_class_t<custom_base_class>,
+          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::uint8_t>, custom_base_class>>::value);
+
+    // with_string_t on ordered_json must keep ordered_map as the object type
+    CHECK(std::is_same<nlohmann::ordered_json::with_string_t<std::wstring>,
+          nlohmann::basic_json<nlohmann::ordered_map, std::vector, std::wstring, bool,
+          std::int64_t, std::uint64_t, double, std::allocator,
+          nlohmann::adl_serializer, std::vector<std::uint8_t>>>::value);
+
+    // the aliases are members of the resulting type, so they can be chained
+    CHECK(std::is_same<json::with_integers_t<int, unsigned int>::with_float_t<float>,
+          nlohmann::basic_json<std::map, std::vector, std::string, bool,
+          int, unsigned int, float, std::allocator,
+          nlohmann::adl_serializer, std::vector<std::uint8_t>>>::value);
+    CHECK(std::is_same<json::with_float_t<float>::with_integers_t<int, unsigned int>,
+          json::with_integers_t<int, unsigned int>::with_float_t<float>>::value);
+
+    // replacing the object type of json with ordered_map yields ordered_json
+    CHECK(std::is_same<json::with_object_t<nlohmann::ordered_map>, nlohmann::ordered_json>::value);
+    CHECK(std::is_same<nlohmann::ordered_json::with_object_t<std::map>, json>::value);
+}
+
 TEST_CASE("different basic_json types conversions")
 {
     SECTION("null")