[C++]虚函数实现多态

Shape.h:

#ifndef SHAPE_H
#define SHAPE_H

#include <iostream>
using namespace std;

class Shape {
public:
    Shape();
    ~Shape();
    double calcArea();
};

#endif 

Shape.cpp

#include "Shape.h"

Shape::Shape()
{
    cout << "Shape()" << endl;
}

Shape::~Shape() {
    cout << "~Shape()" << endl;
}

double Shape::calcArea()
{
    cout << "Shape->calcArea()" << endl;
    return 0;
}

Circle.h

#ifndef  CIRCLE_H
#define CIRCLE_H

#include "Shape.h"
class Circle : public Shape{
public:
    Circle(double r);
    ~Circle();
    double calcArea();
protected:
    double m_dR;
};

#endif 

Circle.cpp

#include "Circle.h"

Circle::Circle(double r)
{
    cout << "Circle()" << endl;
    m_dR = r;
}

Circle::~Circle()
{
    cout << "~Circle()" << endl;
}
 
double Circle::calcArea() {
    cout << "Ciecle->calcArea()" << endl;
    return 3.14*m_dR*m_dR;
}

Rect.h

#ifndef RECT_H
#define RECT_H

#include "Shape.h"

class Rect :public Shape
{
public:
    Rect(double witdth, double height);
    ~Rect();
    double calcArea();
protected:
    double m_dWith;
    double m_dHeight;
};
#endif 

Rect.cpp:

#include "Rect.h"

Rect::Rect(double witdth, double height) {
    cout << "Rect()" << endl;
    m_dWith = witdth;
    m_dHeight = height;
}

Rect::~Rect() {
    cout << "~Rect()" << endl;
}

double Rect::calcArea()
{
    cout<<"Rect->calcArea()"<<endl;
    return m_dWith*m_dHeight;

}

运行

Paste_Image.png

添加virtual:

#ifndef SHAPE_H
#define SHAPE_H

#include <iostream>
using namespace std;

class Shape {
public:
    Shape();
    ~Shape();
    virtual double calcArea();
};

#endif 

运行

Paste_Image.png