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json/single_include/nlohmann/json_fwd.hpp
Niels Lohmann 2e91641de2 Test JSON_BRACE_INIT_COPY_SEMANTICS for real, and fix one-element tuples under it (#5544) 
* Test JSON_BRACE_INIT_COPY_SEMANTICS for real, and fix one-element tuples under it

The opt-in JSON_BRACE_INIT_COPY_SEMANTICS was never exercised by CI:

- Its only test, in unit-regression3.cpp, was guarded by
  `#if defined(JSON_BRACE_INIT_COPY_SEMANTICS)` after the #include. The
  header #undefs the macro unconditionally in macro_unscope.hpp, so the
  guard was always false and the test compiled to nothing, whatever -D
  flag was passed.
- The ci_test_brace_init_copy_semantics target that passes the flag was
  not named by any workflow.

Move the test into its own translation unit that defines the macro before
including the header, as unit-diagnostics.cpp does for JSON_DIAGNOSTICS.
It now runs in every CI job and for every standard. Remove the unused
target: it ran the whole suite with the macro, and that suite deliberately
relies on default brace-init semantics in about 90 places
(e.g. `json({1})` meaning `[1]`), so it could never pass.

Running the whole suite with the macro did find one library bug:
to_json for std::tuple builds `j = { std::get<Idx>(t)... }`, so with copy
semantics a one-element tuple became its element. `json(std::tuple<int>{5})`
was `5` instead of `[5]`, and `get<std::tuple<int>>()` threw type_error.302
on the result. Under the macro, a one-element tuple now builds exactly what
the default deduction builds. Without the macro nothing changes.

The new tests also pin that the library's other conversions produce the
same values with and without the macro. The macro page now says that the
macro affects every single-element list (`json j = {1}` is `1`), and that
all translation units must agree on it, since it has no ABI tag.

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

* Make JSON_BRACE_INIT_COPY_SEMANTICS part of the ABI tag

The macro changes the body of the initializer-list constructor and adds a
to_json_tuple_impl overload, both with the same mangled names in either
mode, so mixing translation units silently picked one definition. Encode
it in the inline namespace as `_bics`, as JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON
does with `_ldvcmp`. The macro is new in the unreleased 3.13.0, so no
existing namespace name changes.

- Move the macro's default into abi_macros.hpp so json_fwd.hpp computes
  the same namespace, and keep it defined under JSON_TEST_KEEP_MACROS.
- Check the tag in the ABI config tests and in the unit test.
- List `_bics` (and the missing `_dp`) in the namespace docs and in the
  natvis generator; regenerate nlohmann_json.natvis.
- Replace the "define it consistently" warning with an ABI note.

Suggested by @gregmarr in the review of #5544.

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

* Fix the cppcheck, clang-tidy and legacy-comparison CI failures

- to_json_tuple_impl() moved the element in both branches of a ternary;
  only one runs, but cppcheck reported accessMoved. Use if/else.
- The ABI tag test looked for "json_abi_bics", which misses when another
  tag comes first, as in json_abi_ldvcmp_bics; look for "_bics".
- readability-qualified-auto in the items() test.

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

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
2026-09-24 17:05:37 +02:00

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// __ _____ _____ _____
// __| | __| | | | JSON for Modern C++
// | | |__ | | | | | | version 3.12.0
// |_____|_____|_____|_|___| https://github.com/nlohmann/json
//
// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
// SPDX-License-Identifier: MIT
#ifndef INCLUDE_NLOHMANN_JSON_FWD_HPP_
#define INCLUDE_NLOHMANN_JSON_FWD_HPP_
#include <cstdint> // int64_t, uint64_t
#include <map> // map
#include <memory> // allocator
#include <string> // string
#include <vector> // vector
// #include <nlohmann/detail/abi_macros.hpp>
// __ _____ _____ _____
// __| | __| | | | JSON for Modern C++
// | | |__ | | | | | | version 3.12.0
// |_____|_____|_____|_|___| https://github.com/nlohmann/json
//
// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
// SPDX-License-Identifier: MIT
// This file contains all macro definitions affecting or depending on the ABI
#ifndef JSON_SKIP_LIBRARY_VERSION_CHECK
#if defined(NLOHMANN_JSON_VERSION_MAJOR) && defined(NLOHMANN_JSON_VERSION_MINOR) && defined(NLOHMANN_JSON_VERSION_PATCH)
#if NLOHMANN_JSON_VERSION_MAJOR != 3 || NLOHMANN_JSON_VERSION_MINOR != 12 || NLOHMANN_JSON_VERSION_PATCH != 0
#warning "Already included a different version of the library!"
#endif
#endif
#endif
#define NLOHMANN_JSON_VERSION_MAJOR 3 // NOLINT(modernize-macro-to-enum)
#define NLOHMANN_JSON_VERSION_MINOR 12 // NOLINT(modernize-macro-to-enum)
#define NLOHMANN_JSON_VERSION_PATCH 0 // NOLINT(modernize-macro-to-enum)
#ifndef JSON_DIAGNOSTICS
#define JSON_DIAGNOSTICS 0
#endif
#ifndef JSON_DIAGNOSTIC_POSITIONS
#define JSON_DIAGNOSTIC_POSITIONS 0
#endif
#ifndef JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON
#define JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON 0
#endif
#ifndef JSON_BRACE_INIT_COPY_SEMANTICS
#define JSON_BRACE_INIT_COPY_SEMANTICS 0
#endif
#if JSON_DIAGNOSTICS
#define NLOHMANN_JSON_ABI_TAG_DIAGNOSTICS _diag
#else
#define NLOHMANN_JSON_ABI_TAG_DIAGNOSTICS
#endif
#if JSON_DIAGNOSTIC_POSITIONS
#define NLOHMANN_JSON_ABI_TAG_DIAGNOSTIC_POSITIONS _dp
#else
#define NLOHMANN_JSON_ABI_TAG_DIAGNOSTIC_POSITIONS
#endif
#if JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON
#define NLOHMANN_JSON_ABI_TAG_LEGACY_DISCARDED_VALUE_COMPARISON _ldvcmp
#else
#define NLOHMANN_JSON_ABI_TAG_LEGACY_DISCARDED_VALUE_COMPARISON
#endif
#if JSON_BRACE_INIT_COPY_SEMANTICS
#define NLOHMANN_JSON_ABI_TAG_BRACE_INIT_COPY_SEMANTICS _bics
#else
#define NLOHMANN_JSON_ABI_TAG_BRACE_INIT_COPY_SEMANTICS
#endif
#ifndef NLOHMANN_JSON_NAMESPACE_NO_VERSION
#define NLOHMANN_JSON_NAMESPACE_NO_VERSION 0
#endif
// Construct the namespace ABI tags component
#define NLOHMANN_JSON_ABI_TAGS_CONCAT_EX(a, b, c, d) json_abi ## a ## b ## c ## d
#define NLOHMANN_JSON_ABI_TAGS_CONCAT(a, b, c, d) \
NLOHMANN_JSON_ABI_TAGS_CONCAT_EX(a, b, c, d)
#define NLOHMANN_JSON_ABI_TAGS \
NLOHMANN_JSON_ABI_TAGS_CONCAT( \
NLOHMANN_JSON_ABI_TAG_DIAGNOSTICS, \
NLOHMANN_JSON_ABI_TAG_LEGACY_DISCARDED_VALUE_COMPARISON, \
NLOHMANN_JSON_ABI_TAG_DIAGNOSTIC_POSITIONS, \
NLOHMANN_JSON_ABI_TAG_BRACE_INIT_COPY_SEMANTICS)
// Construct the namespace version component
#define NLOHMANN_JSON_NAMESPACE_VERSION_CONCAT_EX(major, minor, patch) \
_v ## major ## _ ## minor ## _ ## patch
#define NLOHMANN_JSON_NAMESPACE_VERSION_CONCAT(major, minor, patch) \
NLOHMANN_JSON_NAMESPACE_VERSION_CONCAT_EX(major, minor, patch)
#if NLOHMANN_JSON_NAMESPACE_NO_VERSION
#define NLOHMANN_JSON_NAMESPACE_VERSION
#else
#define NLOHMANN_JSON_NAMESPACE_VERSION \
NLOHMANN_JSON_NAMESPACE_VERSION_CONCAT(NLOHMANN_JSON_VERSION_MAJOR, \
NLOHMANN_JSON_VERSION_MINOR, \
NLOHMANN_JSON_VERSION_PATCH)
#endif
// Combine namespace components
#define NLOHMANN_JSON_NAMESPACE_CONCAT_EX(a, b) a ## b
#define NLOHMANN_JSON_NAMESPACE_CONCAT(a, b) \
NLOHMANN_JSON_NAMESPACE_CONCAT_EX(a, b)
#ifndef NLOHMANN_JSON_NAMESPACE
#define NLOHMANN_JSON_NAMESPACE \
nlohmann::NLOHMANN_JSON_NAMESPACE_CONCAT( \
NLOHMANN_JSON_ABI_TAGS, \
NLOHMANN_JSON_NAMESPACE_VERSION)
#endif
#ifndef NLOHMANN_JSON_NAMESPACE_BEGIN
#define NLOHMANN_JSON_NAMESPACE_BEGIN \
namespace nlohmann \
{ \
inline namespace NLOHMANN_JSON_NAMESPACE_CONCAT( \
NLOHMANN_JSON_ABI_TAGS, \
NLOHMANN_JSON_NAMESPACE_VERSION) \
{
#endif
#ifndef NLOHMANN_JSON_NAMESPACE_END
#define NLOHMANN_JSON_NAMESPACE_END \
} /* namespace (inline namespace) NOLINT(readability/namespace) */ \
} // namespace nlohmann
#endif
/*!
@brief namespace for Niels Lohmann
@see https://github.com/nlohmann
@since version 1.0.0
*/
NLOHMANN_JSON_NAMESPACE_BEGIN
/*!
@brief default JSONSerializer template argument
This serializer ignores the template arguments and uses ADL
([argument-dependent lookup](https://en.cppreference.com/w/cpp/language/adl))
for serialization.
*/
template<typename T = void, typename SFINAE = void>
struct adl_serializer;
/// a class to store JSON values
/// @sa https://json.nlohmann.me/api/basic_json/
template<template<typename U, typename V, typename... Args> class ObjectType =
std::map,
template<typename U, typename... Args> class ArrayType = std::vector,
class StringType = std::string, class BooleanType = bool,
class NumberIntegerType = std::int64_t,
class NumberUnsignedType = std::uint64_t,
class NumberFloatType = double,
template<typename U> class AllocatorType = std::allocator,
template<typename T, typename SFINAE = void> class JSONSerializer =
adl_serializer,
class BinaryType = std::vector<std::uint8_t>, // cppcheck-suppress syntaxError
class CustomBaseClass = void>
class basic_json;
/// @brief JSON Pointer defines a string syntax for identifying a specific value within a JSON document
/// @sa https://json.nlohmann.me/api/json_pointer/
template<typename RefStringType>
class json_pointer;
/*!
@brief default specialization
@sa https://json.nlohmann.me/api/json/
*/
using json = basic_json<>;
/// @brief a minimal map-like container that preserves insertion order
/// @sa https://json.nlohmann.me/api/ordered_map/
template<class Key, class T, class IgnoredLess, class Allocator>
struct ordered_map;
/// @brief specialization that maintains the insertion order of object keys
/// @sa https://json.nlohmann.me/api/ordered_json/
using ordered_json = basic_json<nlohmann::ordered_map>;
NLOHMANN_JSON_NAMESPACE_END
#endif // INCLUDE_NLOHMANN_JSON_FWD_HPP_
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