Hands-On Design Patterns with C++

Polymorphic factory

Consider a slightly different problem—imagine that, in our game, each building produces a unit of some kind, and the type of the unit is uniquely associated with the type of the building. The Castle recruits knights, the Wizard Tower trains mages, and the Spider Mound produces giant spiders. Now, our generic code not only constructs a building of the type that is selected at runtime, but also creates new units whose types are also not known at compile time. We already have the building factory. We could implement the Unit factory in a similar way, where every building has a unique unit identifier associated with it. But this design exposes the correspondence between the units and the buildings to the rest of the program, ...