Design Patterns (1) - Strategy

book reference:

1. Gang-of-four: Design Patterns
2. Head first design patterns

resource reference:

1. wiki - design pattern
2. source-making
3. http://www.dofactory.com/Patterns/Patterns.aspx
4. http://blog.cumps.be/design-patterns-strategy-pattern/
5. http://www.patterndepot.com/put/8/JavaPatterns.htm
6. http://www.codeguru.com/forum/showthread.php?t=327982

Strategy Pattern:

Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it.

• The Context has-a behavior of Strategy, which can be implemented with different algorithms.
• We define a private member in Context: a pointer to the general superclass Strategy.
• We have a set_strategy method in Context class to change behavior at runtime.
• We also have a superclass: Strategy and a bunch of subclass : ConcreteStrategy to implement the behavior.
• In client code, we have one or multiple instances of Context.
• At any given time, only one ConcreteStrategy is availabe to be executed a single Context object.

Strategy Pattern example:   

#include <iostream>
 
using namespace std;
 
class StrategyInterface
{
    public:
        virtual void execute() = 0;
};
 
class ConcreteStrategyA: public StrategyInterface
{
    public:
        virtual void execute()
        {
            cout << "Called ConcreteStrategyA execute method" << endl;
        }
};
 
class ConcreteStrategyB: public StrategyInterface
{
    public:
        virtual void execute()
        {
            cout << "Called ConcreteStrategyB execute method" << endl;
        }
};
 
class ConcreteStrategyC: public StrategyInterface
{
    public:
        virtual void execute()
        {
            cout << "Called ConcreteStrategyC execute method" << endl;
        }
};
 
class Context
{
    private:
        StrategyInterface *_strategy;
 
    public:
        Context(StrategyInterface *strategy):_strategy(strategy)
        {
        }
        void set_strategy(StrategyInterface *strategy) 
        {
            _strategy = strategy;
        }
        void execute() 
        {
            _strategy->execute();
        }
};
 
int main(int argc, char *argv[])
{
    ConcreteStrategyA concreteStrategyA;
    ConcreteStrategyB concreteStrategyB;
    ConcreteStrategyC concreteStrategyC;

    Context contextA(&concreteStrategyA);
    Context contextB(&concreteStrategyB);
    Context contextC(&concreteStrategyC);
 
    contextA.execute();
    contextB.execute();
    contextC.execute();

    contextA.set_strategy(&concreteStrategyB);
    contextA.execute();
 
    return 0;
}