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operator==, <=>(std::reference_wrapper) - cppreference.com

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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) 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.

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

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