The Predicate requirements describe a callable that returns a value testable as a
Predicate is typically used with algorithms that take input data (individual objects/containers) and a predicate, which is then called on input data to decide on further course of action. Some examples of predicate usage in C++ standard library are:
std::none_ofTake an array of elements and a predicate as an input. Call predicate on individual input elements, and return true if for all/any/none elements, predicate returns true.
std::find_ifTake sequence of elements, and a predicate. Return first element in the sequence, for which predicate returns value equal to
Description of algorithm facilities, given above, is crude and intended to explain Predicate in simple terms. For detailed info, refer to individual pages.
In other words, if an algorithm takes a Predicate
pred and an iterator
first, it should be able to test the object of the type pointed to by the iterator
first using the given predicate via a construct like
The function object
pred shall not apply any non-constant function through the dereferenced iterator and must accept a
const object argument, with the same behavior regardless of whether its argument is
const or non-
const (since C++20). This function object may be a pointer to function or an object of a type with an appropriate function call operator.
| specifies that a callable type is a Boolean predicate
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