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Make JSON_BRACE_INIT_COPY_SEMANTICS part of the ABI tag

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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>
This commit is contained in:
Niels Lohmann
2026-09-24 07:17:57 +02:00
parent 2534db3570
commit 574d98ad93
12 changed files with 793 additions and 28 deletions
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docs/mkdocs/docs/api/macros/json_brace_init_copy_semantics.md
+4 -3
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@@ -54,10 +54,11 @@ The default value is `0` (disabled — existing behavior is preserved).
affected. The library's own conversions are not affected either: for example, `std::tuple<int>{5}` still becomes
`[5]`.
!!! warning "Define it consistently"
!!! note "ABI compatibility"
The macro changes the behavior of a `basic_json` constructor but not the library's ABI tag, so all translation units
of a program must agree on its value. Mixing translation units compiled with and without it is an ODR violation.
The value of this macro is encoded in the [namespace](../../features/namespace.md) (tag `_bics`), resulting in
distinct symbol names. Translation units compiled with and without it can therefore be linked into the same program
without One Definition Rule (ODR) violations, but they cannot exchange instances of library types.
!!! tip "Workaround without the macro"
docs/mkdocs/docs/features/namespace.md
+3
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@@ -15,6 +15,9 @@ The complete default namespace name is derived as follows:
- [`JSON_DIAGNOSTICS`](../api/macros/json_diagnostics.md) defined non-zero appends `_diag`.
- [`JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON`](../api/macros/json_use_legacy_discarded_value_comparison.md)
defined non-zero appends `_ldvcmp`.
- [`JSON_DIAGNOSTIC_POSITIONS`](../api/macros/json_diagnostic_positions.md) defined non-zero appends `_dp`.
- [`JSON_BRACE_INIT_COPY_SEMANTICS`](../api/macros/json_brace_init_copy_semantics.md) defined non-zero appends
`_bics`.
- The inline namespace ends with the suffix `_v` followed by the 3 components of the version number separated by
underscores. To omit the version component, see [Disabling the version component](#disabling-the-version-component)
below.