operator==, !=, <, <=, >, >=, <=>(std::optional)
Defined in header <optional>
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Compare two optional objects |
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template< class T, class U > constexpr bool operator==( const optional<T>& lhs, const optional<U>& rhs ); |
(1) | (since C++17) |
template< class T, class U > constexpr bool operator!=( const optional<T>& lhs, const optional<U>& rhs ); |
(2) | (since C++17) |
template< class T, class U > constexpr bool operator<( const optional<T>& lhs, const optional<U>& rhs ); |
(3) | (since C++17) |
template< class T, class U > constexpr bool operator<=( const optional<T>& lhs, const optional<U>& rhs ); |
(4) | (since C++17) |
template< class T, class U > constexpr bool operator>( const optional<T>& lhs, const optional<U>& rhs ); |
(5) | (since C++17) |
template< class T, class U > constexpr bool operator>=( const optional<T>& lhs, const optional<U>& rhs ); |
(6) | (since C++17) |
template< class T, std::three_way_comparable_with<T> U > constexpr std::compare_three_way_result_t<T, U> |
(7) | (since C++20) |
Compare an optional object with a nullopt |
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template< class T > constexpr bool operator==( const optional<T>& opt, std::nullopt_t ) noexcept; |
(8) | (since C++17) |
template< class T > constexpr bool operator==( std::nullopt_t, const optional<T>& opt ) noexcept; |
(9) | (since C++17) (until C++20) |
template< class T > constexpr bool operator!=( const optional<T>& opt, std::nullopt_t ) noexcept; |
(10) | (since C++17) (until C++20) |
template< class T > constexpr bool operator!=( std::nullopt_t, const optional<T>& opt ) noexcept; |
(11) | (since C++17) (until C++20) |
template< class T > constexpr bool operator<( const optional<T>& opt, std::nullopt_t ) noexcept; |
(12) | (since C++17) (until C++20) |
template< class T > constexpr bool operator<( std::nullopt_t, const optional<T>& opt ) noexcept; |
(13) | (since C++17) (until C++20) |
template< class T > constexpr bool operator<=( const optional<T>& opt, std::nullopt_t ) noexcept; |
(14) | (since C++17) (until C++20) |
template< class T > constexpr bool operator<=( std::nullopt_t, const optional<T>& opt ) noexcept; |
(15) | (since C++17) (until C++20) |
template< class T > constexpr bool operator>( const optional<T>& opt, std::nullopt_t ) noexcept; |
(16) | (since C++17) (until C++20) |
template< class T > constexpr bool operator>( std::nullopt_t, const optional<T>& opt ) noexcept; |
(17) | (since C++17) (until C++20) |
template< class T > constexpr bool operator>=( const optional<T>& opt, std::nullopt_t ) noexcept; |
(18) | (since C++17) (until C++20) |
template< class T > constexpr bool operator>=( std::nullopt_t, const optional<T>& opt ) noexcept; |
(19) | (since C++17) (until C++20) |
template< class T > constexpr std::strong_ordering |
(20) | (since C++20) |
Compare an optional object with a value |
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template< class T, class U > constexpr bool operator==( const optional<T>& opt, const U& value ); |
(21) | (since C++17) |
template< class U, class T > constexpr bool operator==( const U& value, const optional<T>& opt ); |
(22) | (since C++17) |
template< class T, class U > constexpr bool operator!=( const optional<T>& opt, const U& value ); |
(23) | (since C++17) |
template< class U, class T > constexpr bool operator!=( const U& value, const optional<T>& opt ); |
(24) | (since C++17) |
template< class T, class U > constexpr bool operator<( const optional<T>& opt, const U& value ); |
(25) | (since C++17) |
template< class U, class T > constexpr bool operator<( const U& value, const optional<T>& opt ); |
(26) | (since C++17) |
template< class T, class U > constexpr bool operator<=( const optional<T>& opt, const U& value ); |
(27) | (since C++17) |
template< class U, class T > constexpr bool operator<=( const U& value, const optional<T>& opt ); |
(28) | (since C++17) |
template< class T, class U > constexpr bool operator>( const optional<T>& opt, const U& value ); |
(29) | (since C++17) |
template< class U, class T > constexpr bool operator>( const U& value, const optional<T>& opt ); |
(30) | (since C++17) |
template< class T, class U > constexpr bool operator>=( const optional<T>& opt, const U& value ); |
(31) | (since C++17) |
template< class U, class T > constexpr bool operator>=( const U& value, const optional<T>& opt ); |
(32) | (since C++17) |
template< class T, std::three_way_comparable_with<T> U > constexpr std::compare_three_way_result_t<T, U> |
(33) | (since C++20) |
Performs comparison operations on optional
objects.
optional
objects, lhs and rhs. The contained values are compared (using the corresponding operator of T
) only if both lhs and rhs contain values. Otherwise,
- lhs is considered equal to rhs if, and only if, both lhs and rhs do not contain a value.
- lhs is considered less than rhs if, and only if, rhs contains a value and lhs does not.
If the corresponding expression *lhs @ *rhs is ill-formed or its result is not convertible to bool, the program is ill-formed. |
(until C++26) |
This overload participates in overload resolution only if the corresponding expression *lhs @ *rhs is well-formed and its result is convertible to bool. |
(since C++26) |
nullopt
. Equivalent to (1-6) when comparing to an optional
that does not contain a value.
The |
(since C++20) |
T
) only if opt contains a value. Otherwise, opt is considered less than value.
If the corresponding expression *opt @ value or value @ *opt (depending on the positions of the operands) is ill-formed or its result is not convertible to bool, the program is ill-formed. |
(until C++26) |
This overload participates in overload resolution only if all following conditions are satisfied:
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(since C++26) |
Contents |
[edit] Parameters
lhs, rhs, opt | - | an optional object to compare
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value | - | value to compare to the contained value |
[edit] Return value
(lhs.has_value() == false ? true : *lhs == *rhs)
(lhs.has_value() == false ? false : *lhs != *rhs)
[edit] Exceptions
[edit] Notes
Feature-test macro | Value | Std | Feature |
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__cpp_lib_constrained_equality |
202403L |
(C++26) | constrained comparison operators for std::optional |
[edit] Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR | Applied to | Behavior as published | Correct behavior |
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LWG 2945 | C++17 | order of template parameters inconsistent for compare-with-T cases | made consistent |