# std::experimental::ranges::equal_to

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Defined in header `<experimental/ranges/functional>` |  |
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)
template<> 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`.

### Member types

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

### Member functions

operator() | checks if the arguments are equal (public member function)

## std::experimental::ranges::equal_to::operator()

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).

### 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.

### Notes

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

### Example

| This section is incomplete Reason: no example

### See also

equal_to | function object implementing x == y (class template)

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