std:: predicate

The concept std :: 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

Expressions declared in requires expressions of the standard library concepts are required to be equality-preserving (except where stated otherwise).

References