5.2向上类型转换【C++】

5.2向上类型转换

根据赋值兼容原则，可以使用派生类的对象代替基类对象。向上类型转换就是把一个派生类的对象作为基类的对象来使用。

下面通过一个程序来加深对它的理解

#include <iostream>
using namespace std;
class Point
{public:
    Point(double a = 0, double b = 0) {  x = a; y = b;  }
    double Area( ) 
    {   cout << "Call Point’s Area function." << endl;
        return 0.0; 
    }
protected:
    double x, y;	// 点的坐标值
};

class Rectangle: public Point
{public:
    Rectangle(double a = 0, double b = 0, double c = 0, double d = 0): Point(a, b)
    {  x1 = c; y1 = d;   }
    double Area( )
    {  
		cout << "Call Rectangle’s Area function." << endl;
	    return (x - x1)*(y - y1); 
    }
protected:
    double x1, y1;//长方形右下角点的坐标值，基类中x, y为左上角坐标点的值
};

class Circle: public Point
{public:
    Circle(double a = 0, double b = 0, double c = 0): Point(a, b){  r = c;  }
    double Area( ) 
    { 
		cout << "Call Circle’s Area function." << endl;
        return 3.14*r*r; 
    }
protected:
    double r; //圆半径，基类中x, y为圆心坐标点的坐标值
};
double CalcArea(Point &ref){   return ( ref. Area( ) );  } 
int main( )
{
	Point p(0, 0);
    Rectangle r(0, 0, 1, 1);
    Circle c(0, 0, 1);
    cout << CalcArea(p) << endl;
    cout << CalcArea(r) << endl;
    cout << CalcArea(c) << endl;
    return 0;
}

从运行结果来看，3调用的都是point::area（），这不是我们想要的结果。为了实现灵活的调用，我们需要知道“绑定”这个概念。