8.9 Summary

Object factories are an important component of programs that use polymorphism. Object factories help in creating objects when their types are either not available or are available only in a form that's incompatible for use with language constructs.

Object factories are used mostly in object-oriented frameworks and libraries, as well as in various object persistence and streaming schemes. The latter case was analyzed in depth with a concrete example. The solution essentially distributes a switch of type across multiple implementation files, thus achieving low coupling. Although the factory remains a central authority that creates objects, it doesn't have to collect knowledge about all the static types in a hierarchy. Instead, it's the responsibility of each type to register itself with the factory. This marks a fundamental difference between the "wrong" and the "right" approach.

Type information cannot be easily transported at runtime in C++. This is a fundamental feature of the family of languages to which C++ belongs. Because of this, type identifiers that represent types have to be used instead. They are associated with creator objects that are callable entities, as described in Chapter 5 on generalized functors (pointers to functions or functors). A concrete object factory starting from these ideas was implemented and was then generalized into a class template.

Finally, we discussed clone factories (factories that can duplicate polymorphic objects).