Strategy设计模式举例。

看图：

代码实现：

#include <iostream>

using namespace std;

class FlyBehavior {
public:virtual void fly() = 0;
};

class QuackBehavior {
public:virtual void quack() = 0;
};

class FlyWithWings :public FlyBehavior {
public:void fly() { cout << "fly with wings!" << endl; }
};

class FlyNoWay :public FlyBehavior {
public:void fly() { cout << "can not fly!" << endl; }
};

class Quack :public QuackBehavior {
public:void quack() { cout << "gaga!" << endl; }
};

class Squeak :public QuackBehavior {
public:void quack() { cout << "fricktion !" << endl; }
};

class QuackNoWay :public QuackBehavior {
public :void quack() { cout << "can not sound!" << endl; }
};

class Duck {
protected:
	FlyBehavior *flyBehavior;
	QuackBehavior *quackBehavior;
public:
	void fly() { flyBehavior->fly(); }
	void quack() { quackBehavior->quack(); }
	virtual void display() = 0;

};


class RubberDuck :public Duck {
public:
	RubberDuck() {
		flyBehavior = new FlyNoWay;
		quackBehavior = new Squeak;
	}
	void display() { cout <<"rubber duck" << endl; };
};

细节：

1. 在Duck类中，new的是虚类(virtual)或者虚方法类，而在RubberDuck中new的是实类。接下来，调用实类的方法时，会根据不同的实类调用不同的实方法。
2. new FlyNoWay 也可以写成new FlyNoWay()，但是最好写成new FlyNoWay()。
3. 结合本例，图中的聚合/集合该如何理解？