NAME

std::experimental::ranges::WeaklyEqualityComparableWith - std::experimental::ranges::WeaklyEqualityComparableWith

Synopsis

Defined in header <experimental/ranges/concepts>
template <class T, class U>

concept bool WeaklyEqualityComparableWith =
requires(const std::remove_reference_t<T>& t,
const std::remove_reference_t<U>& u) {
{ t == u } -> Boolean&&; (ranges TS)
{ t != u } -> Boolean&&;
{ u == t } -> Boolean&&;
{ u != t } -> Boolean&&;

};

The concept WeaklyEqualityComparableWith<T, U> specifies that an object of type T
and an object of type U can be compared for equality with each other (in either
order) using both == and !=, and the results of the comparisons are consistent. More
formally, WeaklyEqualityComparableWith<T, U> is satisfied only if given

* t, an lvalue of type const std::remove_reference_t<T> and
* u, an lvalue of type const std::remove_reference_t<U>,

the following are true:

* t == u, u == t, t != u,u != t have the same domain;
* bool(u == t) == bool(t == u);
* bool(t != u) == !bool(t == u); and
* bool(u != t) == bool(t != u).

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.

Unless noted otherwise, every expression used in a requires-expression is required
to be equality preserving and stable, and the evaluation of the expression may only
modify its non-constant operands. Operands that are constant must not be modified.

Implicit expression variations

A requires-expression that uses an expression that is non-modifying for some
constant lvalue operand also implicitly requires additional variations of that
expression that accept a non-constant lvalue or (possibly constant) rvalue for the
given operand unless such an expression variation is explicitly required with
differing semantics. These implicit expression variations must meet the same
semantic requirements of the declared expression. The extent to which an
implementation validates the syntax of the variations is unspecified.

See also

EqualityComparable specifies that operator == is an equivalence relation
EqualityComparableWith (concept)