Question

Are function pointers functors ? Is there a virtual functor in use that helps sibling functors compile silently ?

Answer 1

At least as the term is normally used in C++ (Warning: it’s used entirely differently relative to other languages such as Haskell), a functor is a class (or an instance of a class) that overloads operator() so it can be invoked like a function.

Since they use the same syntax, a template can be written to accept either a pointer to a function or an instance of a functor interchangeably. Not all algorithms will do so however — some expect (for example) that you supply something with typedefs for things like argument_type and result_type. The standard library provides a couple of classes (unary_function and binary_function) to use as base classes for your functors to supply these. You can supply them on your own if you prefer — these base classes are purely for convenience (and some people don’t find them particularly convenient).