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Fix derived-type macro dispatch capping members at 62 instead of 63

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NLOHMANN_JSON_GET_MACRO resolves 64 positional arguments, with NAME at
position 65. NLOHMANN_JSON_TYPE_TAG dispatches on Type plus the member
list, so it resolves correctly up to the 63 members NLOHMANN_JSON_PASTE
supports. NLOHMANN_JSON_DERIVED_TYPE_TAG dispatched on the two-token
Type,BaseType prefix plus the member list, running out one slot early:
at 63 members, position 65 landed on the last member name instead of a
sentinel and NLOHMANN_JSON_CAT built an undefined identifier such as
NLOHMANN_JSON_DEFINE_DERIVED_TYPE_INTRUSIVE_m63, with the compiler
reporting "unknown type name 'm1'" once per member and nothing pointing
at an argument-count limit.

That silently reduced all six NLOHMANN_DEFINE_DERIVED_TYPE_* macros from
63 members to 62, contradicting the "up to 63 members" contract in
docs/mkdocs/docs/api/macros/nlohmann_define_derived_type.md.

Drop the leading Type and defer to NLOHMANN_JSON_TYPE_TAG so the tag is
computed from BaseType plus the member list, which fits the available
slots. The zero-own-member derived bodies are therefore selected by tag
1 rather than 2, and the sentinel table for the derived tag is no longer
needed.

Add a regression test at the documented maximum for both the plain and
the derived macros; it fails to compile against the previous dispatch.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-08-19 21:03:08 +02:00
parent 4dcd45b26d
commit 0b28c63bad
3 changed files with 97 additions and 36 deletions
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single_include/nlohmann/json.hpp
+17 -18
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@@ -2967,14 +2967,20 @@ void templated_json_throw(ExceptionType exception)
// (issue #4041, e.g. NLOHMANN_DEFINE_TYPE_INTRUSIVE(Type) with no further
// arguments). NLOHMANN_JSON_TYPE_TAG(...) expands to the token 1 when
// __VA_ARGS__ is a single token (Type alone) and to N for two or more (Type,
// member...); NLOHMANN_JSON_DERIVED_TYPE_TAG(...) is the same idea shifted by
// one slot for the two-token Type,BaseType prefix used by the derived-type
// macros. Both reuse the existing 64-slot NLOHMANN_JSON_GET_MACRO dispatch
// member...). It reuses the existing 64-slot NLOHMANN_JSON_GET_MACRO dispatch
// with one extra trailing sentinel token appended so its own trailing "..."
// is never left completely empty at the lowest supported argument count --
// invoking a variadic macro so that "..." matches nothing is only granted
// unconditionally by the standard since C++20, and pre-C++20 compilers may
// reject it under -pedantic regardless of what the macro body does.
//
// NLOHMANN_JSON_DERIVED_TYPE_TAG(...) answers the same question for the
// derived-type macros, whose fixed prefix is Type,BaseType. It drops the
// leading Type and defers to NLOHMANN_JSON_TYPE_TAG rather than shifting the
// slot table by one: NLOHMANN_JSON_GET_MACRO only resolves 64 positional
// arguments, so tagging Type,BaseType,member... directly would run out one
// slot early and cap the derived-type macros at 62 members instead of the 63
// that NLOHMANN_JSON_PASTE supports.
#define NLOHMANN_JSON_CAT_(x, y) x ## y
#define NLOHMANN_JSON_CAT(x, y) NLOHMANN_JSON_CAT_(x, y)
@@ -2988,15 +2994,8 @@ void templated_json_throw(ExceptionType exception)
N, N, N, 1, \
NLOHMANN_JSON_TYPE_TAG_SENTINEL))
#define NLOHMANN_JSON_DERIVED_TYPE_TAG(...) NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_GET_MACRO(__VA_ARGS__, \
N, N, N, N, N, N, N, N, N, N, \
N, N, N, N, N, N, N, N, N, N, \
N, N, N, N, N, N, N, N, N, N, \
N, N, N, N, N, N, N, N, N, N, \
N, N, N, N, N, N, N, N, N, N, \
N, N, N, N, N, N, N, N, N, N, \
N, N, 2, N, \
NLOHMANN_JSON_DERIVED_TYPE_TAG_SENTINEL))
#define NLOHMANN_JSON_DERIVED_TYPE_TAG_(Type, ...) NLOHMANN_JSON_TYPE_TAG(__VA_ARGS__)
#define NLOHMANN_JSON_DERIVED_TYPE_TAG(...) NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_DERIVED_TYPE_TAG_(__VA_ARGS__))
/*!
@brief macro
@@ -3126,7 +3125,7 @@ void templated_json_throw(ExceptionType exception)
NLOHMANN_JSON_BASIC_TYPE_TEMPLATE(friend void to_json(BasicJsonType& nlohmann_json_j, const Type& nlohmann_json_t) { nlohmann::to_json(nlohmann_json_j, static_cast<const BaseType &>(nlohmann_json_t)); NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__)) }) \
NLOHMANN_JSON_BASIC_TYPE_TEMPLATE(friend void from_json(const BasicJsonType& nlohmann_json_j, Type& nlohmann_json_t) { nlohmann::from_json(nlohmann_json_j, static_cast<BaseType&>(nlohmann_json_t)); NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_FROM, __VA_ARGS__)) })
#define NLOHMANN_JSON_DEFINE_DERIVED_TYPE_INTRUSIVE_2(Type, BaseType) \
#define NLOHMANN_JSON_DEFINE_DERIVED_TYPE_INTRUSIVE_1(Type, BaseType) \
NLOHMANN_JSON_BASIC_TYPE_TEMPLATE(friend void to_json(BasicJsonType& nlohmann_json_j, const Type& nlohmann_json_t) { nlohmann::to_json(nlohmann_json_j, static_cast<const BaseType &>(nlohmann_json_t)); }) \
/* NOLINTNEXTLINE(bugprone-macro-parentheses) Type/BaseType are used as declarator types, not in expressions */ \
NLOHMANN_JSON_BASIC_TYPE_TEMPLATE(friend void from_json(const BasicJsonType& nlohmann_json_j, Type& nlohmann_json_t) { nlohmann::from_json(nlohmann_json_j, static_cast<BaseType&>(nlohmann_json_t)); })
@@ -3147,7 +3146,7 @@ void templated_json_throw(ExceptionType exception)
NLOHMANN_JSON_BASIC_TYPE_TEMPLATE(friend void to_json(BasicJsonType& nlohmann_json_j, const Type& nlohmann_json_t) { nlohmann::to_json(nlohmann_json_j, static_cast<const BaseType&>(nlohmann_json_t)); NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__)) }) \
NLOHMANN_JSON_BASIC_TYPE_TEMPLATE(friend void from_json(const BasicJsonType& nlohmann_json_j, Type& nlohmann_json_t) { nlohmann::from_json(nlohmann_json_j, static_cast<BaseType&>(nlohmann_json_t)); const Type nlohmann_json_default_obj{}; NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_FROM_WITH_DEFAULT, __VA_ARGS__)) })
#define NLOHMANN_JSON_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_DEFAULT_2(Type, BaseType) \
#define NLOHMANN_JSON_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_DEFAULT_1(Type, BaseType) \
NLOHMANN_JSON_BASIC_TYPE_TEMPLATE(friend void to_json(BasicJsonType& nlohmann_json_j, const Type& nlohmann_json_t) { nlohmann::to_json(nlohmann_json_j, static_cast<const BaseType&>(nlohmann_json_t)); }) \
/* NOLINTNEXTLINE(bugprone-macro-parentheses) Type/BaseType are used as declarator types, not in expressions */ \
NLOHMANN_JSON_BASIC_TYPE_TEMPLATE(friend void from_json(const BasicJsonType& nlohmann_json_j, Type& nlohmann_json_t) { nlohmann::from_json(nlohmann_json_j, static_cast<BaseType&>(nlohmann_json_t)); })
@@ -3167,7 +3166,7 @@ void templated_json_throw(ExceptionType exception)
#define NLOHMANN_JSON_DEFINE_DERIVED_TYPE_INTRUSIVE_ONLY_SERIALIZE_N(Type, BaseType, ...) \
NLOHMANN_JSON_BASIC_TYPE_TEMPLATE(friend void to_json(BasicJsonType& nlohmann_json_j, const Type& nlohmann_json_t) { nlohmann::to_json(nlohmann_json_j, static_cast<const BaseType &>(nlohmann_json_t)); NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__)) })
#define NLOHMANN_JSON_DEFINE_DERIVED_TYPE_INTRUSIVE_ONLY_SERIALIZE_2(Type, BaseType) \
#define NLOHMANN_JSON_DEFINE_DERIVED_TYPE_INTRUSIVE_ONLY_SERIALIZE_1(Type, BaseType) \
NLOHMANN_JSON_BASIC_TYPE_TEMPLATE(friend void to_json(BasicJsonType& nlohmann_json_j, const Type& nlohmann_json_t) { nlohmann::to_json(nlohmann_json_j, static_cast<const BaseType &>(nlohmann_json_t)); })
#define NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_ONLY_SERIALIZE(...) NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_CAT(NLOHMANN_JSON_DEFINE_DERIVED_TYPE_INTRUSIVE_ONLY_SERIALIZE_, NLOHMANN_JSON_DERIVED_TYPE_TAG(__VA_ARGS__))(__VA_ARGS__))
@@ -3186,7 +3185,7 @@ void templated_json_throw(ExceptionType exception)
NLOHMANN_JSON_BASIC_TYPE_TEMPLATE(void to_json(BasicJsonType& nlohmann_json_j, const Type& nlohmann_json_t) { nlohmann::to_json(nlohmann_json_j, static_cast<const BaseType &>(nlohmann_json_t)); NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__)) }) \
NLOHMANN_JSON_BASIC_TYPE_TEMPLATE(void from_json(const BasicJsonType& nlohmann_json_j, Type& nlohmann_json_t) { nlohmann::from_json(nlohmann_json_j, static_cast<BaseType&>(nlohmann_json_t)); NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_FROM, __VA_ARGS__)) })
#define NLOHMANN_JSON_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_2(Type, BaseType) \
#define NLOHMANN_JSON_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_1(Type, BaseType) \
NLOHMANN_JSON_BASIC_TYPE_TEMPLATE(void to_json(BasicJsonType& nlohmann_json_j, const Type& nlohmann_json_t) { nlohmann::to_json(nlohmann_json_j, static_cast<const BaseType &>(nlohmann_json_t)); }) \
/* NOLINTNEXTLINE(bugprone-macro-parentheses) Type/BaseType are used as declarator types, not in expressions */ \
NLOHMANN_JSON_BASIC_TYPE_TEMPLATE(void from_json(const BasicJsonType& nlohmann_json_j, Type& nlohmann_json_t) { nlohmann::from_json(nlohmann_json_j, static_cast<BaseType&>(nlohmann_json_t)); })
@@ -3207,7 +3206,7 @@ void templated_json_throw(ExceptionType exception)
NLOHMANN_JSON_BASIC_TYPE_TEMPLATE(void to_json(BasicJsonType& nlohmann_json_j, const Type& nlohmann_json_t) { nlohmann::to_json(nlohmann_json_j, static_cast<const BaseType &>(nlohmann_json_t)); NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__)) }) \
NLOHMANN_JSON_BASIC_TYPE_TEMPLATE(void from_json(const BasicJsonType& nlohmann_json_j, Type& nlohmann_json_t) { nlohmann::from_json(nlohmann_json_j, static_cast<BaseType&>(nlohmann_json_t)); const Type nlohmann_json_default_obj{}; NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_FROM_WITH_DEFAULT, __VA_ARGS__)) })
#define NLOHMANN_JSON_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_DEFAULT_2(Type, BaseType) \
#define NLOHMANN_JSON_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_DEFAULT_1(Type, BaseType) \
NLOHMANN_JSON_BASIC_TYPE_TEMPLATE(void to_json(BasicJsonType& nlohmann_json_j, const Type& nlohmann_json_t) { nlohmann::to_json(nlohmann_json_j, static_cast<const BaseType &>(nlohmann_json_t)); }) \
/* NOLINTNEXTLINE(bugprone-macro-parentheses) Type/BaseType are used as declarator types, not in expressions */ \
NLOHMANN_JSON_BASIC_TYPE_TEMPLATE(void from_json(const BasicJsonType& nlohmann_json_j, Type& nlohmann_json_t) { nlohmann::from_json(nlohmann_json_j, static_cast<BaseType&>(nlohmann_json_t)); })
@@ -3227,7 +3226,7 @@ void templated_json_throw(ExceptionType exception)
#define NLOHMANN_JSON_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE_N(Type, BaseType, ...) \
NLOHMANN_JSON_BASIC_TYPE_TEMPLATE(void to_json(BasicJsonType& nlohmann_json_j, const Type& nlohmann_json_t) { nlohmann::to_json(nlohmann_json_j, static_cast<const BaseType &>(nlohmann_json_t)); NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__)) })
#define NLOHMANN_JSON_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE_2(Type, BaseType) \
#define NLOHMANN_JSON_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE_1(Type, BaseType) \
NLOHMANN_JSON_BASIC_TYPE_TEMPLATE(void to_json(BasicJsonType& nlohmann_json_j, const Type& nlohmann_json_t) { nlohmann::to_json(nlohmann_json_j, static_cast<const BaseType &>(nlohmann_json_t)); })
#define NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE(...) NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_CAT(NLOHMANN_JSON_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE_, NLOHMANN_JSON_DERIVED_TYPE_TAG(__VA_ARGS__))(__VA_ARGS__))