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

From cppreference.net
C++
Utilities library
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 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)

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)

Contents

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