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

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C++
Utilities library
General utilities
Relational operators (deprecated in C++20)
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 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
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