#include usingnamespace std;
class Shape
{
public:
//若类中含有虚函数
//则会自动生成一张虚函数表
//该表存放的是类中虚函数的入口地址
//该类中会自动增加一个指针(虚函数表指针)
//该指针指向该虚函数表
virtual float area()
{
cout << "Shape::area()" << endl;
return 0;
}
};
#define PI 3.1415926
class Circle: public Shape
{
public:
Circle(float r): m_fR(r)
{
}
//若派生类中存在和基类虚函数A
//函数原型相同的函数B，
//则该派生类函数B默认为虚函数
//并且会使用该派生类函数B的函数地址
//覆盖掉虚函数表中原基类虚函数A的地址
float area()
{
cout << "Circle::area()" << endl;
return PI * m_fR * m_fR;
}
void info()
{
cout << "画的一首好圆" << endl;
}
private:
float m_fR;
};
class Triangle: public Shape
{
public:
Triangle(float h, float b):m_fH(h)
, m_fBottom(b){}
float area()
{
return m_fH * m_fBottom / 2;
}
private:
float m_fH;
float m_fBottom;
};
void showShapeArea(Shape &shape)
{
//对象的访问范围受限于其类型
//通过基类的指针或者引用
//来调用虚函数时
//会到虚函数表中的相应位置
//取得该虚函数的入口地址
//从而到该地址去执行函数代码
cout << shape.area() << endl;
}
int main()
{
cout << "shape:" << sizeof(Shape) << endl;
Shape shape;
Circle c(2);
Triangle t(3, 4);
showShapeArea(shape);
showShapeArea(c);
showShapeArea(t);
// Shape *pShape = &c;
// pShape->info(); //error
#if 0
char a = '\0';
int b = 1257;
a = b;
b = a;
//派生类对象可以赋值给基类的对象
// shape = c;
//基类的对象不能赋值给派生类对象
//c = shape;//error
Shape *pShape = &c;
Shape &refShape = c;
// Circle *pCircle = &shape; //error
// Circle &refCircle = shape; //error
#endif
cout << "Hello World!" << endl;
return 0;
}
