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Fix from_json() for enums with underlying type bool (#5679)

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get_arithmetic_value() rejects boolean_t, so the default from_json()
for enums failed to compile for an enum whose underlying type is
bool (e.g. enum class Flag : bool { off, on }), even though the
matching to_json() serializes such enums as an unsigned number.

Read the underlying value through number_unsigned_t in that case,
matching what to_json() writes, then cast back to the underlying
type before constructing the enum.

Fixes #5671.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-09-30 20:06:46 +02:00
committed by GitHub
parent 0050d0f7a4
commit 68beba727c
3 changed files with 20 additions and 4 deletions
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8
include/nlohmann/detail/conversions/from_json.hpp
View File

@@ -167,9 +167,13 @@ template<typename BasicJsonType, typename EnumType,
enable_if_t<std::is_enum<EnumType>::value, int> = 0>
inline void from_json(const BasicJsonType& j, EnumType& e)
{
typename std::underlying_type<EnumType>::type val;
using underlying_type = typename std::underlying_type<EnumType>::type;
// get_arithmetic_value() does not accept boolean_t; read the number that to_json() wrote instead
using value_type = typename std::conditional<std::is_same<underlying_type, typename BasicJsonType::boolean_t>::value,
typename BasicJsonType::number_unsigned_t, underlying_type>::type;
value_type val;
get_arithmetic_value(j, val);
e = static_cast<EnumType>(val);
e = static_cast<EnumType>(static_cast<underlying_type>(val));
}
#endif // JSON_DISABLE_ENUM_SERIALIZATION

8
single_include/nlohmann/json.hpp
View File

@@ -5664,9 +5664,13 @@ template<typename BasicJsonType, typename EnumType,
enable_if_t<std::is_enum<EnumType>::value, int> = 0>
inline void from_json(const BasicJsonType& j, EnumType& e)
{
typename std::underlying_type<EnumType>::type val;
using underlying_type = typename std::underlying_type<EnumType>::type;
// get_arithmetic_value() does not accept boolean_t; read the number that to_json() wrote instead
using value_type = typename std::conditional<std::is_same<underlying_type, typename BasicJsonType::boolean_t>::value,
typename BasicJsonType::number_unsigned_t, underlying_type>::type;
value_type val;
get_arithmetic_value(j, val);
e = static_cast<EnumType>(val);
e = static_cast<EnumType>(static_cast<underlying_type>(val));
}
#endif // JSON_DISABLE_ENUM_SERIALIZATION

8
tests/src/unit-conversions.cpp
View File

@@ -1358,6 +1358,14 @@ TEST_CASE("value conversion")
CHECK(json(value_1).get<c_enum>() == value_1);
CHECK(json(cpp_enum::value_1).get<cpp_enum>() == cpp_enum::value_1);
}
SECTION("get an enum with underlying type bool (#5671)")
{
enum class bool_enum : bool { off, on };
CHECK(json(bool_enum::off).get<bool_enum>() == bool_enum::off);
CHECK(json(bool_enum::on).get<bool_enum>() == bool_enum::on);
}
#endif
SECTION("more involved conversions")