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operator==, <=> (std::reference_wrapper)

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C++
Utilities library
Function objects
Function invocation
(C++17) (C++23)
Identity function object
(C++20)
Old binders and adaptors
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std::reference_wrapper
friend constexpr bool
operator == ( reference_wrapper lhs, reference_wrapper rhs ) ;
(1) (since C++26)
friend constexpr bool
operator == ( reference_wrapper lhs, reference_wrapper < const T > rhs ) ;
(2) (since C++26)
friend constexpr bool
operator == ( reference_wrapper lhs, const T & ref ) ;
(3) (since C++26)
friend constexpr auto
operator <=> ( reference_wrapper lhs, reference_wrapper rhs ) ;
(4) (since C++26)
friend constexpr auto
operator <=> ( reference_wrapper lhs, reference_wrapper < const T > rhs ) ;
(5) (since C++26)
friend constexpr auto
operator <=> ( reference_wrapper lhs, const T & ref ) ;
(6) (since C++26)

Performs comparison operations on reference_wrapper objects.

1,2) Compares two reference_wrapper objects. The objects compare equal if and only if lhs. get ( ) and rhs. get ( ) are equal.
1) This overload participates in overload resolution only if the expression lhs. get ( ) == rhs. get ( ) is well-formed and its result is convertible to bool .
2) This overload participates in overload resolution only if all following conditions are satisfied:
  • std:: is_const_v < T > is false .
  • The expression lhs. get ( ) == rhs. get ( ) is well-formed and its result is convertible to bool .
3) Compares reference_wrapper object with a reference. The parameters compare equal if and only if lhs. get ( ) is equal to ref .
This overload participates in overload resolution only if the expression lhs. get ( ) == ref is well-formed and its result is convertible to bool .
4,5) Compares two reference_wrapper objects using synth-three-way .
4) This overload participates in overload resolution only if the expression synth-three-way ( lhs. get ( ) , rhs. get ( ) ) is well-formed.
5) This overload participates in overload resolution only if all following conditions are satisfied:
6) Compares reference_wrapper object with a reference using synth-three-way .
This overload participates in overload resolution only if the expression synth-three-way ( lhs. get ( ) , ref ) is well-formed.

The < , <= , > , >= , and != operators are synthesized from operator <=> and operator == respectively.

Contents

Parameters

lhs, rhs - reference_wrapper object to compare
ref - reference to compare to the reference_wrapper object

Return value

1,2) lhs. get ( ) == rhs. get ( ) .
3) lhs. get ( ) == ref .
4,5) synth-three-way ( lhs. get ( ) , rhs. get ( ) ) .
6) synth-three-way ( lhs. get ( ) , ref ) .

Exceptions

Throws when and what the comparison throws.

Notes

The return types of operator <=> are deduced from return statements to avoid hard error when instantiating a std:: reference_wrapper < T > with synth-three-way-result <T> being ill-formed.

Feature-test macro Value Std Feature
__cpp_lib_reference_wrapper 202403L (C++26) Comparisons for std::reference_wrapper

Example

This section is incomplete
Reason: no example