NAME¶

std::predicate - std::predicate

Synopsis¶

Defined in header <concepts>
template < class F, class... Args >

concept predicate = (since C++20)
std::regular_invocable<F, Args...> &&

boolean-testable<std::invoke_result_t<F, Args...>>;

The concept predicate<F, Args...> specifies that F is a predicate that accepts
arguments whose types and value categories are encoded by Args..., i.e., it can be
invoked with these arguments to produce a boolean-testable result.

Note that regular_invocable requires the invocation to not modify either the
callable object or the arguments and be equality-preserving.

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.