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Showing content from https://en.cppreference.com/w/cpp/algorithm/../ranges/../utility/optional/operator_cmp.html below:

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

Compare two optional objects

template< class T, class U > constexpr bool operator==( const optional<T>& lhs, const optional& rhs );

(1) (since C++17)

template< class T, class U > constexpr bool operator!=( const optional<T>& lhs, const optional& rhs );

(2) (since C++17)

template< class T, class U > constexpr bool operator<( const optional<T>& lhs, const optional& rhs );

(3) (since C++17)

template< class T, class U > constexpr bool operator<=( const optional<T>& lhs, const optional& rhs );

(4) (since C++17)

template< class T, class U > constexpr bool operator>( const optional<T>& lhs, const optional& rhs );

(5) (since C++17)

template< class T, class U > constexpr bool operator>=( const optional<T>& lhs, const optional& rhs );

(6) (since C++17) (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) (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) (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

1) lhs.has_value() != rhs.has_value() ? false : (lhs.has_value() == false ? true : *lhs == *rhs)

2) lhs.has_value() != rhs.has_value() ? true : (lhs.has_value() == false ? false : *lhs != *rhs)

3) !rhs ? false : (!lhs ? true : *lhs < *rhs)

4) !lhs ? true : (!rhs ? false : *lhs <= *rhs)

5) !lhs ? false : (!rhs ? true : *lhs > *rhs)

6) !rhs ? true : (!lhs ? false : *lhs >= *rhs)

7) lhs && rhs ? *lhs <=> *rhs : lhs.has_value() <=> rhs.has_value()

8,9) !opt

10,11) opt.has_value()

12) false

13) opt.has_value()

14) !opt

15) true

16) opt.has_value()

17) false

18) true

19) !opt

20) opt.has_value() <=> false

21) opt.has_value() ? *opt == value : false

22) opt.has_value() ? value == *opt : false

23) opt.has_value() ? *opt != value : true

24) opt.has_value() ? value != *opt : true

25) opt.has_value() ? *opt < value : true

26) opt.has_value() ? value < *opt : false

27) opt.has_value() ? *opt <= value : true

28) opt.has_value() ? value <= *opt : false

29) opt.has_value() ? *opt > value : false

30) opt.has_value() ? value > *opt : true

31) opt.has_value() ? *opt >= value : false

32) opt.has_value() ? value >= *opt : true

33) 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 [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

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