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operator==, !=, <, <=, >, >=, <=>(std::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) 
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 T, class U > 
constexpr bool operator==( const T& value, const optional<U>& 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 T& value, const optional<U>& 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 T& value, const optional<U>& 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 T& value, const optional<U>& 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 T& value, const optional<U>& 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 T, class U > 
constexpr bool operator>=( const T& value, const optional<U>& 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.
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. If the corresponding two-way comparison expression between *opt and value is not well-formed, or if its result is not convertible to bool, the program is ill-formed.

Parameters

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

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) If bool(lhs) && bool(rhs) is true returns *x <=> *y.
Otherwise, returns bool(lhs) <=> bool(rhs).
8,9) Returns !opt.
10,11) Returns bool(opt).
12) Returns false.
13) Returns bool(opt).
14) Returns !opt.
15) Returns true.
16) Returns bool(opt).
17) Returns false.
18) Returns true.
19) Returns !opt.
20) Returns bool(opt) <=> false.
21) Returns bool(opt) ? *opt == value : false.
22) Returns bool(opt) ? value == *opt : false.
23) Returns bool(opt) ? *opt != value : true.
24) Returns bool(opt) ? value != *opt : true.
25) Returns bool(opt) ? *opt < value : true.
26) Returns bool(opt) ? value < *opt : false.
27) Returns bool(opt) ? *opt <= value : true.
28) Returns bool(opt) ? value <= *opt : false.
29) Returns bool(opt) ? *opt > value : false.
30) Returns bool(opt) ? value > *opt : true.
31) Returns bool(opt) ? *opt >= value : false.
32) Returns bool(opt) ? value >= *opt : true.
33) Returns bool(opt) ? *opt <=> value : std::strong_ordering::less.

Exceptions

1-7) May throw implementation-defined exceptions.
21-33) Throws when and what the comparison throws.

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

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