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Only compile the simdutf backend from C++17 on

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simdutf.h rejects anything below C++17 with an #error, so defining
JSON_USE_SIMDUTF in a C++11 or C++14 translation unit did not fail with a
message about simdutf being unavailable - it failed to compile at all, taking
the library's C++11 support with it. Nothing caught this because no build ever
compiled that path.

Gate the include and both uses on JSON_HAS_CPP_17, the same way number_parse.hpp
gates std::from_chars. Below C++17 the macro now has no effect and the scalar
validator runs; it accepts and rejects exactly the same input, so the macro is
safe to set project-wide even when some translation units use an older standard.

Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MXDi7NTMKAmoArUZSKMc4T
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-08-28 14:59:30 +00:00
committed by Claude
co-authored by Claude
parent 78e2a87ab1
commit feed1a48a4
3 changed files with 40 additions and 21 deletions
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docs/mkdocs/docs/api/macros/json_use_simdutf.md
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@@ -18,6 +18,13 @@ scalar path.
When `JSON_USE_SIMDUTF` is defined you must make the `simdutf.h` header available on the include path and link the
simdutf library. When it is not defined, no simdutf header is included and there is no dependency.
!!! note "Requires C++17"
simdutf requires C++17 and its header rejects older standards with an `#!cpp #error`. The backend is therefore only
compiled in from C++17 on. In C++11 and C++14 the macro has no effect and the scalar validator is used, which
accepts and rejects exactly the same input -- only throughput differs. Setting the macro project-wide is therefore
safe even when some translation units are built with an older standard.
!!! warning "Define consistently"
The macro selects between two definitions of the same inline validation function. It must therefore be defined