22 General utilities library [utilities]

22.10 Function objects [function.objects]

22.10.1 General [function.objects.general]

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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]).198
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.
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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.