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Make cross-string-type basic_json conversion explicit without implicit conversions

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The converting constructor from another basic_json specialization was
always implicit, so a value with a different string_t (std::wstring, a
string with a custom allocator, ...) silently converted into a temporary,
e.g. when passed to a function taking const nlohmann::json&. Such
conversions do not produce correct values (#3425), and
JSON_USE_IMPLICIT_CONVERSIONS=0 did not catch them.

When JSON_USE_IMPLICIT_CONVERSIONS is 0, the constructor is now explicit
if the string types differ. Specializations sharing a string type (json
and ordered_json, different serializers or object maps) stay implicitly
convertible, so the NLOHMANN_DEFINE_TYPE_* macros keep working with
nested json members. get<BasicJsonType>() constructs explicitly and
works in both modes.

Fixes #2649.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-09-27 16:32:26 +02:00
parent f682cd2ef1
commit 3ae4d7c45e
5 changed files with 131 additions and 6 deletions
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include/nlohmann/json.hpp
+33 -2
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@@ -1605,12 +1605,42 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
assert_invariant();
}
private:
/// whether a basic_json specialization can be converted implicitly into this one;
/// with JSON_USE_IMPLICIT_CONVERSIONS set to 0, this is only the case if both share
/// the same string type (see https://github.com/nlohmann/json/issues/2649)
template<typename BasicJsonType>
using is_implicitly_convertible_basic_json = std::integral_constant < bool,
static_cast<bool>(JSON_USE_IMPLICIT_CONVERSIONS)
|| std::is_same<typename BasicJsonType::string_t, string_t>::value >;
/// tag to select the constructor that performs the conversion from another basic_json specialization
struct convert_basic_json_tag {};
public:
/// @brief create a JSON value from an existing one
/// @sa https://json.nlohmann.me/api/basic_json/basic_json/
template < typename BasicJsonType,
detail::enable_if_t <
detail::is_basic_json<BasicJsonType>::value&& !std::is_same<basic_json, BasicJsonType>::value, int > = 0 >
detail::is_basic_json<BasicJsonType>::value&& !std::is_same<basic_json, BasicJsonType>::value
&& is_implicitly_convertible_basic_json<BasicJsonType>::value, int > = 0 >
basic_json(const BasicJsonType& val)
: basic_json(val, convert_basic_json_tag{})
{}
/// @brief create a JSON value from an existing one
/// @sa https://json.nlohmann.me/api/basic_json/basic_json/
template < typename BasicJsonType,
detail::enable_if_t <
detail::is_basic_json<BasicJsonType>::value&& !std::is_same<basic_json, BasicJsonType>::value
&& !is_implicitly_convertible_basic_json<BasicJsonType>::value, int > = 0 >
explicit basic_json(const BasicJsonType& val)
: basic_json(val, convert_basic_json_tag{})
{}
private:
template<typename BasicJsonType>
basic_json(const BasicJsonType& val, convert_basic_json_tag /*unused*/)
#if JSON_DIAGNOSTIC_POSITIONS
: start_position(val.start_pos()),
end_position(val.end_pos())
@@ -1666,6 +1696,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
assert_invariant();
}
public:
/// @brief create a container (array or object) from an initializer list
/// @sa https://json.nlohmann.me/api/basic_json/basic_json/
basic_json(initializer_list_t init,
@@ -2432,7 +2463,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
int > = 0 >
BasicJsonType get_impl(detail::priority_tag<2> /*unused*/) const
{
return *this;
return BasicJsonType(*this);
}
/*!