[function.objects.general]

A function object type is an object type ([basic.types.general]) that can be the type of the postfix-expression in a function call ([expr.call], [over.match.call]).185

A function object is an object of a function object type.

In the places where one would expect to pass a pointer to a function to an algorithmic template ([algorithms]), the interface is specified to accept a function object.

This not only makes algorithmic templates work with pointers to functions, but also enables them to work with arbitrary function objects.

185)185)

Such a type is a function pointer or a class type which has a member operator() or a class type which has a conversion to a pointer to function.