std::ranges::equal_to

 

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Negators not_fn (C++17) Searchers default_searcher (C++17) boyer_moore_searcher (C++17) boyer_moore_horspool_searcher (C++17)	Constrained comparators ranges::equal_to (C++20) ranges::not_equal_to (C++20) ranges::greater (C++20) ranges::less (C++20) ranges::greater_equal (C++20) ranges::less_equal (C++20) compare_three_way (C++20)
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Defined in header <functional>
struct equal_to;		(since C++20)

Function object for performing comparisons. The parameter types of the function call operator (but not the return type) are deduced from the arguments.

Implementation-defined strict total order over pointers

The function call operator yields the implementation-defined strict total order over pointers if the = operator between arguments invokes a built-in comparison operator for a pointer, even if the built-in = operator does not.

The implementation-defined strict total order is consistent with the partial order imposed by built-in comparison operators (<=>, <, >, <=, and >=), and consistent among following standard function objects:

• std::less, std::greater, std::less_equal, and std::greater_equal, when the template argument is a pointer type or void

• std::ranges::equal_to, std::ranges::not_equal_to, std::ranges::less, std::ranges::greater, std::ranges::less_equal, std::ranges::greater_equal, and std::compare_three_way

Member types

Member type	Definition
is_transparent	/* unspecified */

Member functions

operator()

checks if the arguments are equal (public member function)

std::ranges::equal_to::operator()

template< class T, class U >     requires std::equality_comparable_with<T, U> // with different semantic requirements constexpr bool operator()( T&& t, U&& u ) const;

Compares t and u, equivalent to return std::forward<T>(t) == std::forward<U>(u);, except when that expression resolves to a call to a built-in operator== comparing pointers.

When a call would not invoke a built-in operator comparing pointers, the behavior is undefined if std::equality_comparable_with<T, U> is not modeled.

When a call would invoke a built-in operator comparing pointers of type P, the result is instead determined as follows:

• Returns false if one of the (possibly converted) value of the first argument and the (possibly converted) value of the second argument precedes the other in the implementation-defined strict total ordering over all pointer values of type P. This strict total ordering is consistent with the partial order imposed by the built-in operators <, >, <=, and >=.
• Otherwise (neither precedes the other), returns true.

The behavior is undefined unless the conversion sequences from both T and U to P are equality-preserving.

Equality preservation

Expressions declared in requires-expressions of the standard library concepts are required to be equality-preserving (except where stated otherwise).

Notes

Compared to std::equal_to, std::ranges::equal_to additionally requires != to be valid, and that both argument types are required to be (homogeneously) comparable with themselves (via the equality_comparable_with constraint).

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 3530	C++20	syntactic checks were relaxed while comparing pointers	only semantic requirements relaxed

See also

equal_to

function object implementing x == y (class template)