Useful Design Patterns for Game Development - Learn C++ for Game Development - page 233

Easily Adding New Functionality with the Visitor Pattern
One of the main goals of writing reusable game engine code is to try to avoid including
game-specific functionality in your classes. This can be hard to achieve with a pure object-oriented
approach, as the aim of encapsulation is to hide the data in your classes behind interfaces. This
could mean that you are required to add methods to classes to work on data that are very specific to
a certain class.
We can get around this problem by loosening our encapsulation on classes that must interact with
game code, but we do so in a very structured manner. You can achieve this by using the visitor
pattern. A visitor is an object that knows how to carry out a specific task on a type of object. These
are incredibly useful when you need to carry out similar tasks on many objects that might inherit
from the same base class but have different parameters or types. Listing 23-15 shows an interface
class you can use to implement Visitor objects.
Listing 23-15. The Visitor Class
class Visitor
{
private:
friend class Visitable;
virtual void OnVisit(Visitable& visitable) = 0;
};
The Visitor class provides a pure virtual method OnVisit , which is passed an object that inherits
from a class named Visitable . Listing 23-16 lists the Visitable class.
Listing 23-16. The Visitable Class
class Visitable
{
public:
virtual ~Visitable() {}
void Visit(Visitor& visitor)
{
visitor.OnVisit(*this);
}
};
The Visitable class provides a Visit method that is passed the Visitor object. The Visit method
calls the OnVisit method on the Visitor . This allows us to make the OnVisit method private, which
ensures that only Visitable objects can be visited and that we are always passing a valid reference
to the OnVisit method.
The visitor pattern is very simple to set up. You can see a concrete example of how to use the
pattern in Listing 23-17, where the Option class from Text Adventure has been inherited from
Visitable .