< cpp‎ | experimental‎ | ranges
Technical Specification
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tag specifiers
template< class T = void >

    requires EqualityComparable<T> ||
             Same<T, void> ||
             /* == on two const T lvalues invokes a built-in operator comparing pointers */

struct equal_to;
(ranges TS)
struct equal_to<void>;
(ranges TS)

Function object for performing comparisons. The primary template invokes operator == on const lvalues of type T. The specialization equal_to<void> deduces the parameter types of the function call operator from the arguments (but not the return type).

All specializations of equal_to are Semiregular.


[edit] Member types

Member type Definition
is_transparent (member only of equal_to<void> specialization) /* unspecified */

[edit] Member functions

checks if the arguments are equal
(public member function)


constexpr bool operator()(const T& x, const T& y) const;
(1) (member only of primary equal_to<T> template)
template< class T, class U >

    requires EqualityComparableWith<T, U> ||
             /* std::declval<T>() == std::declval<U>() resolves to
                a built-in operator comparing pointers */

constexpr bool operator()(T&& t, U&& u) const;
(2) (member only of equal_to<void> specialization)
1) Compares x and y. Equivalent to return ranges::equal_to<>{}(x, y);.
2) 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 builtin operator == comparing pointers.

When a call to (1) or (2) 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 builtin 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).

[edit] 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).

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.

[edit] Notes

Unlike std::equal_to, ranges::equal_to requires both == and != to be valid (via the EqualityComparable and EqualityComparableWith constraints).

[edit] Example

[edit] See also

function object implementing x == y
(class template) [edit]