虚函数

函数体与函数调用相联系称为捆绑。晚捆绑又称为动态捆绑或运行时捆绑。为了引起晚捆绑，c++要求在基类中声明这个函数时使用virtual关键字。

不把析构函数设为虚函数是一个隐匿的错误，因为它常常不会对程序有直接的影响。但会不知不觉地引入存储器泄露。

#include <iostream>
#include <vector>

class Shape {
public:
    virtual void draw() = 0;
    virtual ~Shape() {
        std::cout<<"~Shape.\n";
    }
};

class Circle : public Shape {
public:
    void draw() {
        std::cout<<"Circle::draw\n";
    }
    ~Circle() {
        std::cout<<"~Circle\n";
    }
};

class Triangle : public Shape {
public:
    void draw() {
        std::cout<<"Triangle::draw.\n";
    }
    ~Triangle() {
        std::cout<<"~Triangle\n";
    }
};

class Square : public Shape {
public:
    void draw() {
        std::cout<<"Square::draw.\n";
    }
    ~Square() {
        std::cout<<"~Square\n";
    }
};

int main() {
    std::cout<<"sizeof(Shape): "<<sizeof(Shape)<<"\n";
    std::cout<<"sizeof(void *): "<<sizeof(void *)<<"\n";
    typedef std::vector<Shape*> Container;
    typedef Container::iterator Iter;

    Container shapes;
    shapes.push_back(new Circle);
    shapes.push_back(new Square);
    shapes.push_back(new Triangle);

    for(Iter i=shapes.begin(); i!=shapes.end(); i++)
        (*i)->draw();
    for(Iter j=shapes.begin(); j!=shapes.end(); j++)
        delete *j;

    return 0;
}