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

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Diagnostics library

Memory management library

Metaprogramming library (C++11)

General utilities library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Strings library

Text processing library

Numerics library

Date and time library

Input/output library

Filesystem library (C++17)

Concurrency support library (C++11)

Execution control library (C++26)

Technical specifications

Symbols index

External libraries

 

Utilities library

Language support Type support (basic types, RTTI) Library feature-test macros (C++20) Program utilities Variadic functions initializer_list (C++11) is_constant_evaluated (C++20) is_within_lifetime (C++26) source_location (C++20) Coroutine support (C++20) Contract support (C++26) 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 Function objects Bit manipulation (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)

 

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 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)
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)

See More

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)

Parameters

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

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

Exceptions

1-7) May throw implementation-defined exceptions.

21-33) Throws when and what the comparison throws.

Notes

Feature-test macro	Value	Std	Feature
__cpp_lib_constrained_equality	202403L	(C++26)	constrained comparison operators for std::optional

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