When should you ever use functions over functors in C++?
c++, c++11
Solution
Functions support distributed overriding. Functors do not. You must define all of the overloads of a Functor within itself; you can add new overloads of a function anywhere.
Functions support ADL (argument dependent lookup), permitting overloading in the argument-type associated namespace. Functors do not.
Function pointers are (kind of) a type-erased stateless functor that is a POD, as evidenced by how stateless lambdas convert into it. Such features (POD, stateless, type erasure) are useful.
Problem
Functors are apparently more efficient since the compiler is more easily able to inline them, and they work much better with parametrization. When should you ever use a plain old function over a functor?