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

< 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)
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