C++ 学习之旅（16）——虚函数与纯虚函数virtual

关于虚函数的概念讲解，可以看这篇文章：

https://blog.csdn.net/siwuxie095/article/details/71159414

以下用例子进行实际说明：

#include <iostream> 
using namespace std;class Shape {
public:int width, height;Shape(int a = 0, int b = 0){width = a;height = b;}void area(){cout << "Parent class has no area" << endl;}
};class Rectangle : public Shape {
public:Rectangle(int a = 0, int b = 0) :Shape(a, b) { }void area(){cout << "Rectangle class area :" << width * height << endl;}
};class Triangle : public Shape {
public:Triangle(int a = 0, int b = 0) :Shape(a, b) { }void area(){cout << "Triangle class area :" << width * height / 2 << endl;}
};int main()
{Shape* sha1 = new Shape(10, 5);Rectangle* rec = new Rectangle(10, 5);Triangle*  tri = new Triangle(10, 5);sha1->area();rec->area();tri->area();cin.get();return 0;
}

我们有形状类Shape，它有两个子类：Rectangle矩形类和Triangle三角形类。

利用多态的思想，我们希望在对不同对象下达相同指令（计算面积）的时候，不同对象可以根据自身情况完成该指令。即如果是Shape类则没有面积，而矩形和三角形各自都有计算面积的公式，以上代码看似是可以实现的，运行结果如下：

在这里插入图片描述
但是，这种情况实际上只是根据实例化时所属的类选择不同的area方法，属于静态多态，如果出现以下情况：

Shape* sha2 = rec;
sha2->area();

输出却为：“Parent class has no area”

明明sha2指向的地址是矩形类的地址，但是却因为实例化时用了Shape类，而导致无法正确计算面积，此时就有必要引入动态多态了。只需在Shape类中的area函数前加上virtual关键字即可：

#include <iostream> 
using namespace std;class Shape {
public:int width, height;Shape(int a = 0, int b = 0){width = a;height = b;}virtual void area(){cout << "Parent class has no area" << endl;}
};class Rectangle : public Shape {
public:Rectangle(int a = 0, int b = 0) :Shape(a, b) { }void area(){cout << "Rectangle class area :" << width * height << endl;}
};class Triangle : public Shape {
public:Triangle(int a = 0, int b = 0) :Shape(a, b) { }void area(){cout << "Triangle class area :" << width * height / 2 << endl;}
};int main()
{Shape* sha1 = new Shape(10, 5);Rectangle* rec = new Rectangle(10, 5);Triangle*  tri = new Triangle(10, 5);sha1->area();rec->area();tri->area();Shape* sha2 = rec;sha2->area();cin.get();return 0;
}

输出结果如下：

在这里插入图片描述
而如果我们在基类中不能对虚函数给出有意义的实现，这个时候就会用到纯虚函数。只需把函数体改为= 0即可：

#include <iostream> 
using namespace std;class Shape {
public:int width, height;Shape(int a = 0, int b = 0){width = a;height = b;}virtual void area() = 0;
};class Rectangle : public Shape {
public:Rectangle(int a = 0, int b = 0) :Shape(a, b) { }void area(){cout << "Rectangle class area :" << width * height << endl;}
};class Triangle : public Shape {
public:Triangle(int a = 0, int b = 0) :Shape(a, b) { }void area(){cout << "Triangle class area :" << width * height / 2 << endl;}
};int main()
{Rectangle* rec = new Rectangle(10, 5);Triangle*  tri = new Triangle(10, 5);rec->area();tri->area();Shape* sha2 = rec;sha2->area();cin.get();return 0;
}