operator==, !=, <, <=, >, >=(std::optional) - cppreference.com

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# operator==, !=, <, <=, >, >=(std::optional)

From cppreference.com
< cpp‎ | utility‎ | optional

Defined in header `<optional>` |  |
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)
Compare an `optional` object with a `nullopt` |  |
template< class T > constexpr bool operator==( const optional<T>& opt, std::nullopt_t ); | (7) | (since C++17)
template< class T > constexpr bool operator==( std::nullopt_t, const optional<T>& opt ); | (8) | (since C++17)
template< class T > constexpr bool operator!=( const optional<T>& opt, std::nullopt_t ); | (9) | (since C++17)
template< class T > constexpr bool operator!=( std::nullopt_t, const optional<T>& opt ); | (10) | (since C++17)
template< class T > constexpr bool operator<( const optional<T>& opt, std::nullopt_t ); | (11) | (since C++17)
template< class T > constexpr bool operator<( std::nullopt_t, const optional<T>& opt ); | (12) | (since C++17)
template< class T > constexpr bool operator<=( const optional<T>& opt, std::nullopt_t ); | (13) | (since C++17)
template< class T > constexpr bool operator<=( std::nullopt_t, const optional<T>& opt); | (14) | (since C++17)
template< class T > constexpr bool operator>( const optional<T>& opt, std::nullopt_t ); | (15) | (since C++17)
template< class T > constexpr bool operator>( std::nullopt_t, const optional<T>& opt ); | (16) | (since C++17)
template< class T > constexpr bool operator>=( const optional<T>& opt, std::nullopt_t ); | (17) | (since C++17)
template< class T > constexpr bool operator>=( std::nullopt_t, const optional<T>& opt ); | (18) | (since C++17)
Compare an `optional` object with a `T` |  |
template< class T, class U > constexpr bool operator==( const optional<T>& opt, const U& value); | (19) | (since C++17)
template< class T, class U > constexpr bool operator==( const U& value, const optional<T>& opt ); | (20) | (since C++17)
template< class T, class U > constexpr bool operator!=( const optional<T>& opt, const U& value ); | (21) | (since C++17)
template< class T, class U > 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 T, class U > 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 T, class U > 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 T, class U > 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 T, class U > constexpr bool operator>=( const U& value, const optional<T>& opt ); | (30) | (since C++17)

Performs comparison operations on `optional` objects.

1-6) 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.

7-18) Compares `opt` with a `nullopt`. Equivalent to (1-6) when comparing to an `optional` that does not contain a value.
19-30) 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`. If the corresponding comparison expression between *opt and value is not well-formed, or if its result is not convertible to bool, the behavior is undefined.

### [edit] Parameters

lhs, rhs, opt | - | an `optional` object to compare
value | - | value to compare to the contained value

### [edit] Return value

1) If bool(lhs) != bool(rhs), returns false

Otherwise, if bool(lhs) == false (and so bool(rhs) == false as well), returns true

Otherwise, returns *lhs == *rhs.
2) If bool(lhs) != bool(rhs), returns true

Otherwise, if bool(lhs) == false (and so bool(rhs) == false as well), returns false

Otherwise, returns *lhs != *rhs.
3) If bool(rhs) == false returns false

Otherwise, if bool(lhs) == false, returns true

Otherwise returns *lhs < *rhs
4) If bool(lhs) == false returns true

Otherwise, if bool(rhs) == false, returns false

Otherwise returns *lhs <= *rhs
5) If bool(lhs) == false returns false

Otherwise, if bool(rhs) == false, returns true

Otherwise returns *lhs > *rhs
6) If bool(rhs) == false returns true

Otherwise, if bool(lhs) == false, returns false

Otherwise returns *lhs >= *rhs
7-8) Returns !opt.
9-10) Returns bool(opt).
11) Returns false.
12) Returns bool(opt).
13) Returns !opt.
14) Returns true.
15) Returns bool(opt).
16) Returns false.
17) Returns true.
18) Returns !opt.
19) Returns bool(opt) ? *opt == value : false.
20) Returns bool(opt) ? value == *opt : false.
21) Returns bool(opt) ? *opt != value : true.
22) Returns bool(opt) ? value != *opt : true.
23) Returns bool(opt) ? *opt < value : true.
24) Returns bool(opt) ? value < *opt : false.
25) Returns bool(opt) ? *opt <= value : true.
26) Returns bool(opt) ? value <= *opt : false.
27) Returns bool(opt) ? *opt > value : false.
28) Returns bool(opt) ? value > *opt : true.
29) Returns bool(opt) ? *opt >= value : false.
30) Returns bool(opt) ? value >= *opt : true.

### [edit] Exceptions

1-6) (none)
7-18)
`noexcept` specification:
noexcept
19-30) (none)
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