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 (see below).

Equality preservation

An expression is equality preserving if it results in equal outputs given equal inputs.

• The inputs to an expression consist of its operands.
• The outputs of an expression consist of its result and all operands modified by the expression (if any).

In specification of standard concepts, operands are defined as the largest subexpressions that include only:

• an id-expression, and
• invocations of std::move, std::forward, and std::declval.

The cv-qualification and value category of each operand is determined by assuming that each template type parameter denotes a cv-unqualified complete non-array object type.

Every expression required to be equality preserving is further required to be stable: two evaluations of such an expression with the same input objects must have equal outputs absent any explicit intervening modification of those input objects.

Notes

Unlike std::equal_to, std::ranges::equal_to requires both == and != to be valid (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)