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fix: support zero-member types in NLOHMANN_DEFINE_TYPE_* macros #5177

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fix: support zero-member types in NLOHMANN_DEFINE_TYPE_* macros #5177

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8 changes: 6 additions & 2 deletions include/nlohmann/detail/macro_scope.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -339,7 +339,9 @@
NLOHMANN_JSON_PASTE4, \
NLOHMANN_JSON_PASTE3, \
NLOHMANN_JSON_PASTE2, \
NLOHMANN_JSON_PASTE1)(__VA_ARGS__))
NLOHMANN_JSON_PASTE1, \
NLOHMANN_JSON_PASTE0)(__VA_ARGS__))
#define NLOHMANN_JSON_PASTE0(func)
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@chatgpt-codex-connector chatgpt-codex-connector Bot May 17, 2026

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P1 Badge Route empty member lists to the zero-arg paste branch

Adding NLOHMANN_JSON_PASTE0 here does not make zero-member NLOHMANN_DEFINE_TYPE_* calls work, because the call sites pass NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__); when __VA_ARGS__ is empty this still leaves an empty argument after the comma, so dispatch selects NLOHMANN_JSON_PASTE2 and expands to invalid code like nlohmann_json_t.. In practice, NLOHMANN_DEFINE_TYPE_INTRUSIVE(MyType) still fails to compile, so the new tests introduced in this commit cannot build.

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#define NLOHMANN_JSON_PASTE2(func, v1) func(v1)
#define NLOHMANN_JSON_PASTE3(func, v1, v2) NLOHMANN_JSON_PASTE2(func, v1) NLOHMANN_JSON_PASTE2(func, v2)
#define NLOHMANN_JSON_PASTE4(func, v1, v2, v3) NLOHMANN_JSON_PASTE2(func, v1) NLOHMANN_JSON_PASTE3(func, v2, v3)
Expand Down Expand Up @@ -467,7 +469,9 @@
NLOHMANN_JSON_DOUBLE_PASTE3, \
NLOHMANN_JSON_DOUBLE_PASTE3, \
NLOHMANN_JSON_DOUBLE_PASTE1, \
NLOHMANN_JSON_DOUBLE_PASTE1)(__VA_ARGS__))
NLOHMANN_JSON_DOUBLE_PASTE1, \
NLOHMANN_JSON_DOUBLE_PASTE0)(__VA_ARGS__))
#define NLOHMANN_JSON_DOUBLE_PASTE0(func)
#define NLOHMANN_JSON_DOUBLE_PASTE3(func, v1, v2) func(v1, v2)
#define NLOHMANN_JSON_DOUBLE_PASTE5(func, v1, v2, v3, v4) NLOHMANN_JSON_DOUBLE_PASTE3(func, v1, v2) NLOHMANN_JSON_DOUBLE_PASTE3(func, v3, v4)
#define NLOHMANN_JSON_DOUBLE_PASTE7(func, v1, v2, v3, v4, v5, v6) NLOHMANN_JSON_DOUBLE_PASTE3(func, v1, v2) NLOHMANN_JSON_DOUBLE_PASTE5(func, v3, v4, v5, v6)
Expand Down
33 changes: 33 additions & 0 deletions tests/src/unit-udt_macro.cpp
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Expand Up @@ -778,8 +778,41 @@ class derived_person_only_serialize_private_3 : person_without_default_construct
NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_ONLY_SERIALIZE_WITH_NAMES(derived_person_only_serialize_private_3, person_without_default_constructor_3, "json_hair_color", hair_color)
};

class empty_type
{
public:
NLOHMANN_DEFINE_TYPE_INTRUSIVE(empty_type)

bool operator==(const empty_type& /*unused*/) const
{
return true;
}
};

class empty_type_non_intrusive
{};

NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(empty_type_non_intrusive)

} // namespace persons

TEST_CASE("NLOHMANN_DEFINE_TYPE_* with zero members")
{
SECTION("intrusive")
{
persons::empty_type v;
CHECK(json(v).dump() == "{}");
CHECK(json::parse("{}").get<persons::empty_type>() == v);
}

SECTION("non-intrusive")
{
persons::empty_type_non_intrusive v;
CHECK(json(v).dump() == "{}");
CHECK(json::parse("{}").get<persons::empty_type_non_intrusive>() == v);
}
}

TEST_CASE_TEMPLATE("Serialization/deserialization via NLOHMANN_DEFINE_TYPE_INTRUSIVE and NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE", Pair, // NOLINT(readability-math-missing-parentheses, bugprone-throwing-static-initialization)
std::pair<nlohmann::json, persons::person_with_private_data>,
std::pair<nlohmann::json, persons::person_without_private_data_1>,
Expand Down
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