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Showing content from https://en.cppreference.com/w/cpp/ranges/../utility/functional/reference_wrapper/operator_cmp.html below:

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

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.

[edit] Parameters lhs, rhs - reference_wrapper object to compare ref - reference to compare to the reference_wrapper object [edit] Return value

1,2) lhs.get() == rhs.get().

3) lhs.get() == ref.

[edit] Exceptions

Throws when and what the comparison throws.

[edit] 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.

[edit] Example

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