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

 

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Language support Type support (basic types, RTTI) Library feature-test macros (C++20) Dynamic memory management Program utilities Coroutine support (C++20) Variadic functions is_constant_evaluated (C++20) is_within_lifetime (C++26) initializer_list (C++11) source_location (C++20) Three-way comparison three_way_comparablethree_way_comparable_with (C++20)(C++20) strong_ordering (C++20) weak_ordering (C++20) partial_ordering (C++20) common_comparison_category (C++20) compare_three_way_result (C++20) compare_three_way (C++20) strong_order (C++20) weak_order (C++20) partial_order (C++20) compare_strong_order_fallback (C++20) compare_weak_order_fallback (C++20) compare_partial_order_fallback (C++20)   is_eqis_ltis_lteq (C++20)(C++20)(C++20)   is_neqis_gtis_gteq (C++20)(C++20)(C++20)

General utilities Date and time Function objects Formatting library (C++20)   bitset hash (C++11) Relational operators (deprecated in C++20) rel_ops::operator!=rel_ops::operator>    rel_ops::operator<=rel_ops::operator>= Integer comparison functions cmp_equalcmp_lesscmp_less_than (C++20)(C++20)(C++20)   cmp_not_equalcmp_greatercmp_greater_than (C++20)(C++20)(C++20) in_range (C++20) Swap and type operations swap ranges::swap (C++20) exchange (C++14) declval (C++11) to_underlying (C++23) forward (C++11) forward_like (C++23) move (C++11) move_if_noexcept (C++11) as_const (C++17) Common vocabulary types pair tuple (C++11) optional (C++17) any (C++17) variant (C++17) tuple_size (C++11) tuple_element (C++11) apply (C++17) make_from_tuple (C++17) expected (C++23) Elementary string conversions to_chars (C++17) from_chars (C++17) chars_format (C++17) to_chars_result (C++17) from_chars_result (C++17)

 

std::optional

Member functions

optional::optional

optional::~optional

optional::operator=

Observers

optional::operator->optional::operator*

optional::operator booloptional::has_value

optional::value

optional::value_or

Iterators

optional::begin (C++26)

optional::end (C++26)

Monadic operations

optional::and_then (C++23)

optional::transform (C++23)

optional::or_else (C++23)

Modifiers

optional::emplace

optional::swap

optional::reset

Non-member functions

operator==operator!=operator<operator<=operator>operator>=operator<=> (C++20)

make_optional

swap(std::optional)

Deduction guides

Helper classes

hash<std::optional>

nullopt_t

in_place_t

bad_optional_access

Helper objects

nullopt

in_place

 

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