operator==, <=>(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:

std::is_const_v<T> is false.
The expression synth-three-way(lhs.get(),rhs.get()) is well-formed.

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.

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

© cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.
https://en.cppreference.com/cpp/utility/functional/reference_wrapper/operator_cmp