The Factory method pattern

The Factory method pattern is exactly the design pattern we need to solve our problem. The purpose of this pattern is to have a way of creating the derived class that we want without needed to specify the concreate class in our high-level module. This is done by defining an interface for creating objects, but letting subclasses decide which class to instantiate.

In our case, we will create a StageFactory interface with a method called Build that will return a Stage*. We can then create subclasses such as Level2Factory to instantiate our derived classes. Our StageManager class now only needs to know about the Stage and StageFactory abstractions:

Creating a Stage Factory

//StageManager.cpp 
void StageManager::Update() 
{ 
Stage* pStage = m_stageFactory->Build();

pStage->Init(); 

  while(currentStage == nextStage) 
    pStage->Update(); 

pStage->Shutdown(); 
m_StageFactory->Destroy(pStage);//stage must be destroyed
} 

Notice that we have moved the call...