Type-safe Callbacks with Abstract Partners

Abstract

In C++, a caller must know the type of the object whose methods it is calling. The objects called are typically implemented previously, e.g., in object libraries, and therefore their types are known. However, in some cases, a library object or any other previously implemented object needs to call back a user whose type is not known. We should therefore not make restrictive assumptions about the types of the objects that are called back. This is not a trivial problem in C++.

This paper presents a simple framework for implementation of object libraries. It illustrates a type-safe solution, a design pattern if you will, for an object which will be used by another object of any unknown type to call its user back. The solution is called an abstract partner, which is an abstract class that declares requirements for the user object. The compiler checks that these requirements are met, which makes the solution safe.