Support zero-member types in NLOHMANN_DEFINE_TYPE_* macros (#4041)#5272
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Support zero-member types in NLOHMANN_DEFINE_TYPE_* macros (#4041)#5272nlohmann wants to merge 3 commits into
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NLOHMANN_DEFINE_TYPE_INTRUSIVE(Type) and its 11 sibling macros produced broken code for types with no members to serialize. Invoking a variadic macro so __VA_ARGS__ is empty is only standard-conforming since C++20, so a plain __VA_OPT__ fix (as tried in #5142) breaks every pre-C++20 build under -pedantic. Instead, make all 12 macros purely variadic and dispatch on argument count using a sentinel-padded extension of the existing NLOHMANN_JSON_GET_MACRO idiom, giving full C++11-C++26 support with no feature-test gate. Verified against real GCC 16 and Clang at -std=c++11/14/17/20 with -pedantic -Werror -Wvariadic-macros: zero regressions in the existing unit-udt_macro.cpp suite plus 12 new zero-member test cases. Signed-off-by: Niels Lohmann <mail@nlohmann.me>
Three issues surfaced on PR #5272's real CI that weren't caught by local testing against a narrower flag set: - GCC -Werror=noexcept: the four truly-empty from_json bodies (plain INTRUSIVE/NON_INTRUSIVE, with and without _WITH_DEFAULT) provably never throw but weren't declared noexcept; mark them noexcept explicitly. to_json and the derived-type from_json overloads are left alone since they genuinely can throw (object assignment / delegating to the base class's from_json). - clang-tidy bugprone-macro-parentheses: false positive on the same 8 zero-member bodies (Type/BaseType used purely as declarator types); suppressed with NOLINTNEXTLINE comments in the same style already used elsewhere in this file (see NLOHMANN_JSON_SERIALIZE_ENUM). - MSVC's traditional preprocessor doesn't fully expand NLOHMANN_JSON_CAT(prefix, NLOHMANN_JSON_TYPE_TAG(...))(...) in one pass, which broke a pre-existing one-member usage in unit-regression2.cpp with syntax errors. Wrap all 12 public dispatcher macros in an extra outer NLOHMANN_JSON_EXPAND(...), matching the pattern NLOHMANN_JSON_PASTE already uses for the same MSVC quirk. Re-verified against real GCC 16 and Clang at -std=c++11/14/17/20 with -pedantic -Werror -Wvariadic-macros -Wnoexcept, including the exact files that failed in CI (unit-udt_macro.cpp, unit-regression2.cpp), against both the modular headers and the re-amalgamated single header. Signed-off-by: Niels Lohmann <mail@nlohmann.me>
The four zero-member ONLY_SERIALIZE test objects are only ever read (via to_json), never mutated, so mark them const per clang-tidy. Signed-off-by: Niels Lohmann <mail@nlohmann.me>
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Summary
Fixes #4041:
NLOHMANN_DEFINE_TYPE_INTRUSIVE(Type)and its 11 sibling macros (_WITH_DEFAULT,_ONLY_SERIALIZE,NON_INTRUSIVEequivalents, and theDERIVED_TYPEfamily) produced broken generated code for types with no member variables to serialize.Three prior PRs attempted this (#5142, #5099, #4994); all are stale and fail CI. This is a fresh implementation, not based on any of them:
__VA_OPT__approach breaks every pre-C++20 build (error C3861: '__VA_OPT__': identifier not foundon MSVC's C++11 test project) because it isn't gated behind a C++20 check.__VA_ARGS__is empty is only standard-conforming since C++20 — GCC/Clang reject it under-pedanticpre-C++20 regardless of what the macro body does (verified directly against real GCC 16 / Clang with this repo's actual-pedantic -Werror -Wvariadic-macrosflags). So a__VA_OPT__-only fix can, at best, only ever cover C++20+.##__VA_ARGS__GNU-extension attempt doesn't avoid this either (also verified directly) and only special-cases GCC.Instead, this PR makes all 12 macros purely variadic (no fixed leading parameter) and dispatches on total argument count — 1 vs 2+ member arguments, or 2 vs 3+ for the
DERIVED_TYPEmacros whose fixed prefix isType, BaseType— using a sentinel-padded extension of the existingNLOHMANN_JSON_GET_MACROpositional-dispatch idiom already used byNLOHMANN_JSON_PASTE1..64in this file. This gives full C++11 through C++26 support with no feature-test gate at all.WITH_NAMESvariants are intentionally out of scope, matching issue #4041 and all three prior PRs.Changes
include/nlohmann/detail/macro_scope.hpp: newNLOHMANN_JSON_CAT/NLOHMANN_JSON_TYPE_TAG/NLOHMANN_JSON_DERIVED_TYPE_TAGdispatch helpers; each of the 12 macros renamed to an_N/_2(has-members) body plus a new_1/_2(zero-member) body, with the public macro name becoming a 1-line dispatcher.single_include/nlohmann/json.hpp: re-amalgamated (make amalgamate, astyle 3.4.13).tests/src/unit-udt_macro.cpp: 12 new zero-member test cases (all 6 base macro variants + all 6 derived variants, the latter using a real base class to confirm base-class delegation still works with zero own members).docs/mkdocs/docs/features/arbitrary_types.md: new "Zero-member types" note with an example.Test plan
unit-udt_macro.cppsuite (46 cases) passes unmodified — zero regressions from the macro rename/dispatch refactor.nlohmann::jsonandnlohmann::ordered_json) pass.g++-16andclang++at-std=c++11/14/17/20with-pedantic -Werror -Wvariadic-macros(matchingcmake/gcc_flags.cmake).make check-amalgamationclean.unit-udt.cpp) still compiles/passes to rule out macro-name collisions from the new helpers.This PR description was written by Claude Code.