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Make comparisons with scalars noexcept only when the conversion is (#5751)

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The comparison operators taking a scalar (==, !=, <, <=, >, >=, and
C++20's <=>) convert the scalar to a basic_json and compare, but were
unconditionally noexcept. When that conversion throws, the program
called std::terminate instead of propagating the exception, e.g. when
comparing a json with a string literal under memory pressure
(std::bad_alloc) or with an enum value not mapped by
NLOHMANN_JSON_SERIALIZE_ENUM_STRICT (out_of_range.410). clang-tidy
22.1 reports the latter as bugprone-exception-escape.

Declare the 16 scalar overloads
noexcept(std::is_nothrow_constructible<basic_json, ScalarType>::value):
they stay noexcept for numbers, Booleans, nullptr, and plain enums,
and are noexcept(false) for strings and enums whose to_json may throw.
The comparisons of two basic_json values are unchanged.

Restore the strict-enum comparisons removed from unit-conversions.cpp
in the previous PR, check that comparing an unmapped strict enum now
throws, and pin the new exception specifications in unit-noexcept.cpp.
Document the exception safety of overload (2) on all seven operator
pages. Ran make amalgamate.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-10-02 17:25:17 +02:00
committed by GitHub
parent 63c10a51fc
commit 6d0867a85f
11 changed files with 125 additions and 53 deletions
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include/nlohmann/json.hpp
+16 -16
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@@ -4865,7 +4865,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
/// @sa https://json.nlohmann.me/api/basic_json/operator_eq/
template<typename ScalarType>
requires std::is_scalar_v<ScalarType>
bool operator==(ScalarType rhs) const noexcept
bool operator==(ScalarType rhs) const noexcept(std::is_nothrow_constructible<basic_json, ScalarType>::value)
{
return *this == basic_json(rhs);
}
@@ -4888,7 +4888,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
/// @sa https://json.nlohmann.me/api/basic_json/operator_spaceship/
template<typename ScalarType>
requires std::is_scalar_v<ScalarType>
std::partial_ordering operator<=>(ScalarType rhs) const noexcept // *NOPAD*
std::partial_ordering operator<=>(ScalarType rhs) const noexcept(std::is_nothrow_constructible<basic_json, ScalarType>::value) // *NOPAD*
{
return *this <=> basic_json(rhs); // *NOPAD*
}
@@ -4913,7 +4913,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
/// @sa https://json.nlohmann.me/api/basic_json/operator_le/
template<typename ScalarType>
requires std::is_scalar_v<ScalarType>
bool operator<=(ScalarType rhs) const noexcept
bool operator<=(ScalarType rhs) const noexcept(std::is_nothrow_constructible<basic_json, ScalarType>::value)
{
return *this <= basic_json(rhs);
}
@@ -4934,7 +4934,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
/// @sa https://json.nlohmann.me/api/basic_json/operator_ge/
template<typename ScalarType>
requires std::is_scalar_v<ScalarType>
bool operator>=(ScalarType rhs) const noexcept
bool operator>=(ScalarType rhs) const noexcept(std::is_nothrow_constructible<basic_json, ScalarType>::value)
{
return *this >= basic_json(rhs);
}
@@ -4959,7 +4959,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
/// @sa https://json.nlohmann.me/api/basic_json/operator_eq/
template<typename ScalarType, typename std::enable_if<
std::is_scalar<ScalarType>::value, int>::type = 0>
friend bool operator==(const_reference lhs, ScalarType rhs) noexcept
friend bool operator==(const_reference lhs, ScalarType rhs) noexcept(std::is_nothrow_constructible<basic_json, ScalarType>::value)
{
return lhs == basic_json(rhs);
}
@@ -4968,7 +4968,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
/// @sa https://json.nlohmann.me/api/basic_json/operator_eq/
template<typename ScalarType, typename std::enable_if<
std::is_scalar<ScalarType>::value, int>::type = 0>
friend bool operator==(ScalarType lhs, const_reference rhs) noexcept
friend bool operator==(ScalarType lhs, const_reference rhs) noexcept(std::is_nothrow_constructible<basic_json, ScalarType>::value)
{
return basic_json(lhs) == rhs;
}
@@ -4984,7 +4984,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
/// @sa https://json.nlohmann.me/api/basic_json/operator_ne/
template<typename ScalarType, typename std::enable_if<
std::is_scalar<ScalarType>::value, int>::type = 0>
friend bool operator!=(const_reference lhs, ScalarType rhs) noexcept
friend bool operator!=(const_reference lhs, ScalarType rhs) noexcept(std::is_nothrow_constructible<basic_json, ScalarType>::value)
{
return lhs != basic_json(rhs);
}
@@ -4993,7 +4993,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
/// @sa https://json.nlohmann.me/api/basic_json/operator_ne/
template<typename ScalarType, typename std::enable_if<
std::is_scalar<ScalarType>::value, int>::type = 0>
friend bool operator!=(ScalarType lhs, const_reference rhs) noexcept
friend bool operator!=(ScalarType lhs, const_reference rhs) noexcept(std::is_nothrow_constructible<basic_json, ScalarType>::value)
{
return basic_json(lhs) != rhs;
}
@@ -5013,7 +5013,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
/// @sa https://json.nlohmann.me/api/basic_json/operator_lt/
template<typename ScalarType, typename std::enable_if<
std::is_scalar<ScalarType>::value, int>::type = 0>
friend bool operator<(const_reference lhs, ScalarType rhs) noexcept
friend bool operator<(const_reference lhs, ScalarType rhs) noexcept(std::is_nothrow_constructible<basic_json, ScalarType>::value)
{
return lhs < basic_json(rhs);
}
@@ -5022,7 +5022,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
/// @sa https://json.nlohmann.me/api/basic_json/operator_lt/
template<typename ScalarType, typename std::enable_if<
std::is_scalar<ScalarType>::value, int>::type = 0>
friend bool operator<(ScalarType lhs, const_reference rhs) noexcept
friend bool operator<(ScalarType lhs, const_reference rhs) noexcept(std::is_nothrow_constructible<basic_json, ScalarType>::value)
{
return basic_json(lhs) < rhs;
}
@@ -5042,7 +5042,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
/// @sa https://json.nlohmann.me/api/basic_json/operator_le/
template<typename ScalarType, typename std::enable_if<
std::is_scalar<ScalarType>::value, int>::type = 0>
friend bool operator<=(const_reference lhs, ScalarType rhs) noexcept
friend bool operator<=(const_reference lhs, ScalarType rhs) noexcept(std::is_nothrow_constructible<basic_json, ScalarType>::value)
{
return lhs <= basic_json(rhs);
}
@@ -5051,7 +5051,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
/// @sa https://json.nlohmann.me/api/basic_json/operator_le/
template<typename ScalarType, typename std::enable_if<
std::is_scalar<ScalarType>::value, int>::type = 0>
friend bool operator<=(ScalarType lhs, const_reference rhs) noexcept
friend bool operator<=(ScalarType lhs, const_reference rhs) noexcept(std::is_nothrow_constructible<basic_json, ScalarType>::value)
{
return basic_json(lhs) <= rhs;
}
@@ -5072,7 +5072,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
/// @sa https://json.nlohmann.me/api/basic_json/operator_gt/
template<typename ScalarType, typename std::enable_if<
std::is_scalar<ScalarType>::value, int>::type = 0>
friend bool operator>(const_reference lhs, ScalarType rhs) noexcept
friend bool operator>(const_reference lhs, ScalarType rhs) noexcept(std::is_nothrow_constructible<basic_json, ScalarType>::value)
{
return lhs > basic_json(rhs);
}
@@ -5081,7 +5081,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
/// @sa https://json.nlohmann.me/api/basic_json/operator_gt/
template<typename ScalarType, typename std::enable_if<
std::is_scalar<ScalarType>::value, int>::type = 0>
friend bool operator>(ScalarType lhs, const_reference rhs) noexcept
friend bool operator>(ScalarType lhs, const_reference rhs) noexcept(std::is_nothrow_constructible<basic_json, ScalarType>::value)
{
return basic_json(lhs) > rhs;
}
@@ -5101,7 +5101,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
/// @sa https://json.nlohmann.me/api/basic_json/operator_ge/
template<typename ScalarType, typename std::enable_if<
std::is_scalar<ScalarType>::value, int>::type = 0>
friend bool operator>=(const_reference lhs, ScalarType rhs) noexcept
friend bool operator>=(const_reference lhs, ScalarType rhs) noexcept(std::is_nothrow_constructible<basic_json, ScalarType>::value)
{
return lhs >= basic_json(rhs);
}
@@ -5110,7 +5110,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
/// @sa https://json.nlohmann.me/api/basic_json/operator_ge/
template<typename ScalarType, typename std::enable_if<
std::is_scalar<ScalarType>::value, int>::type = 0>
friend bool operator>=(ScalarType lhs, const_reference rhs) noexcept
friend bool operator>=(ScalarType lhs, const_reference rhs) noexcept(std::is_nothrow_constructible<basic_json, ScalarType>::value)
{
return basic_json(lhs) >= rhs;
}