operator==, !=, <, <=, >, >=, <=>(std::optional) (original) (raw)
| Defined in header | ||
|---|---|---|
| Compare two optional objects | ||
| 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> operator<=>( const optional<T>& lhs, const optional<U>& rhs ); | (7) | (since C++20) |
| Compare an optional object with a nullopt | ||
| 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 operator<=>( const optional<T>& opt, std::nullopt_t ) noexcept; | (20) | (since C++20) |
| Compare an optional object with a value | ||
| 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> operator<=>( const optional<T>& opt, const U& value ); | (33) | (since C++20) |
Performs comparison operations on optional objects.
1-7) Compares two 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.
1-6) Let @ denote the corresponding comparison operator, for each of these functions:
| 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) |
8-20) Compares opt with a nullopt. Equivalent to (1-6) when comparing to an optional that does not contain a value.
| The <, <=, >, >=, and != operators are synthesized from operator<=> and operator== respectively. | (since C++20) |
|---|
21-33) Compares opt with a value. The values are compared (using the corresponding operator of T) only if opt contains a value. Otherwise, opt is considered less than value.
21-32) Let @ denote the corresponding comparison operator, for each of these functions:
| 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: U is not a specialization of std::optional. The corresponding expression *opt @ value or value @ *opt (depending on the positions of the operands) is well-formed and its result is convertible to bool. | (since C++26) |
[edit] Parameters
| lhs, rhs, opt | - | an optional object to compare |
|---|---|---|
| value | - | value to compare to the contained value |
[edit] Return value
lhs.has_value() != rhs.has_value() ? false :
(lhs.has_value() == false ? true : *lhs == *rhs)lhs.has_value() != rhs.has_value() ? true :
(lhs.has_value() == false ? false : *lhs != *rhs)!rhs ? false : (!lhs ? true : *lhs < *rhs)
!lhs ? true : (!rhs ? false : *lhs <= *rhs)
!lhs ? false : (!rhs ? true : *lhs > *rhs)
!rhs ? true : (!lhs ? false : *lhs >= *rhs)
lhs && rhs ? *lhs <=> *rhs : lhs.has_value() <=> rhs.has_value()
8,9) !opt
10,11) opt.has_value()
false
opt.has_value()
!opt
true
opt.has_value()
false
true
!opt
opt.has_value() <=> false
opt.has_value() ? *opt == value : false
opt.has_value() ? value == *opt : false
opt.has_value() ? *opt != value : true
opt.has_value() ? value != *opt : true
opt.has_value() ? *opt < value : true
opt.has_value() ? value < *opt : false
opt.has_value() ? *opt <= value : true
opt.has_value() ? value <= *opt : false
opt.has_value() ? *opt > value : false
opt.has_value() ? value > *opt : true
opt.has_value() ? *opt >= value : false
opt.has_value() ? value >= *opt : true
opt.has_value() ? *opt <=> value : std::strong_ordering::less
[edit] Exceptions
1-7) May throw implementation-defined exceptions.
21-33) Throws when and what the comparison throws.
[edit] Notes
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
| __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 |
|---|---|---|---|
| LWG 2945 | C++17 | order of template parameters inconsistent for compare-with-T cases | made consistent |