virtual实现C++运行时多态

#include<stdio.h>
class Shape{
public:
    Shape(int width,int height):mWidth(width),mHeight(height){
        printf("shape constructor\n");
    }
    long getArea(){
        printf("shape getArea,width=%d,height=%d\n",mWidth,mHeight);
        return 0;
    }

int mWidth,mHeight;
};
class Rectangle:public Shape{
public:
     Rectangle(int width,int height):Shape(width,height){
        printf("rectangle constructor\n");
    }
    long getArea(){
        printf("rectangle getArea,width=%d,height=%d\n",mWidth,mHeight);
        return 0;
    }

};
void function(Shape *shape){
    shape->getArea();
}
int main(){
    Shape shape(1,2);
    Rectangle rectangle(3,4);
    function(&shape);
    function(&rectangle);
}

log打印：

shape constructor

shape constructor

rectangle constructor

shape getArea,width=1,height=2

shape getArea,width=3,height=4

virtual关键字作用于函数动态绑定，可以实现C++的运行时多态

#include<stdio.h>
class Shape{
public:
    Shape(int width,int height):mWidth(width),mHeight(height){
        printf("shape constructor\n");
    }
    virtual long getArea();

    int mWidth,mHeight;
};
long Shape::getArea(){
        printf("shape getArea,width=%d,height=%d\n",mWidth,mHeight);
        return 0;
}

class Rectangle:public Shape{
public:
     Rectangle(int width,int height,int count):Shape(width,height),mCount(count){
        printf("rectangle constructor,mcount=%d\n",mCount);
    }
    virtual long getArea();
private:
    int mCount;
};
long Rectangle::getArea(){
    printf("rectangle getArea,width=%d,height=%d\n",mWidth,mHeight);
    return 0;
}

void function(Shape *shape){
    shape->getArea();
}
int main(){
    Shape shape(1,2);
    Rectangle rectangle(3,4,1);
    function(&shape);
    function(&rectangle);
}

gcc virtual_method.cpp -fno-rtti编译后运行log打印：

shape constructor

shape constructor

rectangle constructor,mcount=1

shape getArea,width=1,height=2

rectangle getArea,width=3,height=4

注意：

一般成员函数可以是虚函数
构造函数不能是虚函数
析构函数可以是虚函数（多态基类应该声明virtual析构函数，否则通过基类去析构派生类成员会导致析构不彻底，引起内存泄漏）

virtual实现C++运行时多态

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