C++虚函数和纯虚函数

虚函数是C++实现多态的重要工具。

考虑一种情况，现在有形状这个基类，给基类有一个area（）函数，如下

class Shape {
	int area(){...}
}

在其派生类中重写这个方法

class A : public Shape{
	int area(){...}
}
class B : public Shape{
	int area(){...}
}

int main(){
    Shape* shape;
    A a;
    B b;
    
    shape = &a;
    shape.area();
    
    shape = &b;
    shape.area();
    
    return 0;
}

此时两个shape.area（）到底调用了哪一个函数？

结果是shape.area() 调用了基类的area方法。

如果要调用派生类的函数，需要使用虚函数，如下：

#include <iostream>
using namespace std;

class Shape {
protected:
    int width, height;
public:
    Shape(int a = 0, int b = 0)
    {
        width = a;
        height = b;
    }

    // 虚函数
    virtual int area()
    {
        cout << "Parent class area :" << endl;
        return 0;
    }
    // 纯虚函数
    virtual int Perimeter() = 0;

    void test() {
        cout << "test" << endl;
    }
};


class  A :public Shape
{
public:
    A(int a = 0, int b = 0) :Shape(a, b) {}
    // 虚函数
    int area()
    {
        cout << "A class area :" << endl;
        return 0;
    }

    // 纯虚函数
    int Perimeter() {
        return 1;
    }

    void test() {
        cout << "A" << endl;
    }
};


class  B :public Shape
{
public:
    B(int a = 0, int b = 0) :Shape(a, b) {}

    // 虚函数
    int area()
    {
        cout << "B class area :" << endl;
        return 0;
    }

    // 纯虚函数
    int Perimeter() {
        return 2;
    }

    void test() {
        cout << "B" << endl;
    }
};


int main() {

    A a(1, 2);
    B b(1, 2);

    a.area();
    a.Perimeter();


    b.area();
    b.Perimeter();

    Shape* shape;
    shape = &a;
    shape->area();
    shape->Perimeter();
    shape->test();

    shape = &b;
    shape->area();
    shape->Perimeter();
    shape->test();

    return 0;
}