Editing Standard Template Library (section)

=== Functions ===
The STL includes classes that [[Function overloading|overload]] the function call operator ({{Cpp|operator()}}). Instances of such classes are called functors or [[function objects]]. Functors allow the behavior of the associated function to be parameterized (e.g. through arguments passed to the functor's [[Constructor (object-oriented programming)|constructor]]) and can be used to keep associated per-functor state information along with the function. Since both functors and function pointers can be invoked using the syntax of a function call, they are interchangeable as arguments to templates when the corresponding parameter only appears in function call contexts.

A particularly common type of functor is the [[Predicate (computer programming)|predicate]]. For example, algorithms like {{Cpp|find_if}} take a [[unary function|unary]] predicate that operates on the elements of a sequence. Algorithms like sort, partial_sort, nth_element and all sorted [[Container (data structure)|containers]] use a [[binary function|binary]] predicate that must provide a [[strict weak ordering]], that is, it must behave like a [[set membership|membership]] test on a transitive, non-reflexive and asymmetric [[binary relation]]. If none is supplied, these algorithms and containers use [http://www.sgi.com/tech/stl/less.html less] by default, which in turn calls the less-than-operator <.