Question

I’m in a class where we just learned about these design patterns. However I couldn’t see any difference between them. They sound just like the same, creating concrete classes over the abstract one. Could somebody help me kill this doubt? thanks (:

Answer 1

Both the visitor, the strategy, and the template pattern encompass the application of an algorithm. The biggest difference is in how they are evoked and how they are used in practice. While it may seem like they have the same use case, look at the construction of the objects to see the difference.

The strategy pattern is often used when we don’t have the ability to pass around functions as a first class object. It expects a very specific argument list and only that argument list in its call pattern. For instance:

struct MyStrat{
    void operator()(const Foo &_input){
        _input.up( 2 );
    }
};

std::for_each( myFooList.begin(), myFooList.end(), MyStrat() );

which is then applied to a list of objects of type “Foo.” We really have no other way of applying it to any other object.

The visitor pattern on the other hand is used when we want to apply an algorithm to a bunch of objects that might not share the same signature nor have the same member functions. We say visitor pattern because it’s often used when traversing a tree or another collection of “unrelated” objects (unrelated in an inheritance sense.)

struct MyVisitor{
    void visit(const Foo &_input){
         _input.up( 2 );
    }
    void visit(const Bar &_input){
         _input.raiseUp( 2 );
    }
    void visit(const Baz &_input){
         _input.setUp( 2 );
    }
 };

Here, the idea is that we’d like to “up” all these objects. They all don’t share the same member function signature but all are conceptually related. Hence, we can “visit” each of these classes but expect the algorithm to perform the same type of task.

By using a visitor pattern we avoid the need to wrap each class in a proxy pattern. Hence, for N classes we’d like to apply this algorithm to we don’t need to make N proxy classes. We only need to add N methods to a visitor class.

The template method is quite different from either the visitor and the strategy pattern. With the template what you’re trying to do is enforce the same type of algorithm but on different subclasses within a hierarchy. For instance:

class Duck{
public:
    int count() =0;
    void makeNoise(int times) =0;
    void quack(){ makeNoise( count() ); }//the template pattern is here
};

class Mallard : public Duck{
public:
    int count(){ return 4; }
    void makeNoise( cout << "quack" << endl; }
};

class Daffy{
public:
    int count(){ return 1; }
    void makeNoise( cout << "Why I ought to..." << endl; }
};

So the result of the algorithm varies within the heirarchy.