Functors are convenient for solving some common problems. The functor classes offer a consistent way to implement a callback interface. Suppose you have two separate software components, and component A calls component B. The functor encapsulates a function supplied by component B, and adapts that function to the signature expected by the calling function in component A. Functors solve the problem of calling unknown objects in C++. When you develop a component, all you need to know is what data it needs to send to the unknown object and what it needs to receive in return, and specify the appropriate functor type. In this way, functors can connect libraries from different sources.

Functors are required, in some cases, when you use other Threads Module packages. In multithreaded applications, for example, you can pass a functor to tell a new thread where to begin execution. You also use functors to create runnable classes whose instances can execute any function. See Using Threads.

Functors also provide a level of abstraction useful for functional programming and creating higher-order functions.