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：
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：
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; }注意此时我们无法实例化Shape类，因为它已经变成了抽象类，是作为一种规范，或者说接口而存在的。基类中定义了纯虚函数，子类必须对它进行定义，否则也是无法实例化的。
输出结果如下：
