C++多态复习（看这一篇就够了，然后多码代码，多验证）

柔情只为你懂 2023-07-24 13:13 16阅读 0赞

### C++多态复习 ###

*   *  一.
     *  二.
     *  关于virtual的限制：
     *  纯虚函数：
     *  接口类
     *  RTTI:运行时类型识别
     *  异常处理

## 一. ##

#include <iostream>
    
    using namespace std;
    const double PI = 3.14;
    
    class Shape
    {
        
    public:
        Shape();
        ~Shape();
        double calcArea();
    };
    
    class Circle:public Shape{
        
    public:
        Circle(double r);
        ~Circle();
        double calcArea();
    protected:
        double m_dR;
    };
    
    class Rect:public Shape{
        
    public:
        Rect(double width,double height);
        ~Rect();
        double calcArea();
    protected:
        double m_dWidth;
        double m_dHeight;
    };
    Shape::Shape()
    {
        
        cout<<"Shape()"<<endl;
    }
    
    Shape::~Shape()
    {
        
        cout<<"~Shape()"<<endl;
    }
    
    double Shape::calcArea()
    {
        
        cout<<"Shape-->calcArea()"<<endl;
        return 0;
    }
    
    Circle::Circle(double r)
    {
        
        cout<<"Circle()"<<endl;
        m_dR = r;
    }
    
    Circle::~Circle(){
        
        cout<<"~Circle()"<<endl;
    }
    
    double Circle::calcArea()
    {
        
        cout<<"Circle-->calcArea()"<<endl;
        return PI*m_dR*m_dR;
    }
    
    Rect::Rect(double width,double height){
        
        cout<<"Rect()"<<endl;
        m_dHeight = height;
        m_dWidth = width;
    }
    
    Rect::~Rect(){
        
        cout<<"~Rect()"<<endl;
    }
    
    double Rect::calcArea()
    {
        
        cout<<"Rect-->clacArea()"<<endl;
        return m_dHeight*m_dWidth;
    }
    
    int main()
    {
        
        Shape *shape1 = new Rect(3,6);
        Shape *shape2 = new Circle(5);
    
        shape1->calcArea();
        shape2->calcArea();
    
        delete shape1;
        shape1 = NULL;
        delete shape2;
        shape2 = NULL;
        return 0;
    }

运行结果：  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzE2NDk3NDIz_size_16_color_FFFFFF_t_70]  
在父类的方法前加上virtual关键字后：

#include <iostream>
    
    using namespace std;
    const double PI = 3.14;
    
    class Shape
    {
        
    public:
        Shape();
        ~Shape();
        virtual double calcArea();
    };
    
    class Circle:public Shape{
        
    public:
        Circle(double r);
        ~Circle();
        double calcArea();
    protected:
        double m_dR;
    };
    
    class Rect:public Shape{
        
    public:
        Rect(double width,double height);
        ~Rect();
        double calcArea();
    protected:
        double m_dWidth;
        double m_dHeight;
    };
    Shape::Shape()
    {
        
        cout<<"Shape()"<<endl;
    }
    
    Shape::~Shape()
    {
        
        cout<<"~Shape()"<<endl;
    }
    
    double Shape::calcArea()
    {
        
        cout<<"Shape-->calcArea()"<<endl;
        return 0;
    }
    
    Circle::Circle(double r)
    {
        
        cout<<"Circle()"<<endl;
        m_dR = r;
    }
    
    Circle::~Circle(){
        
        cout<<"~Circle()"<<endl;
    }
    
    double Circle::calcArea()
    {
        
        cout<<"Circle-->calcArea()"<<endl;
        return PI*m_dR*m_dR;
    }
    
    Rect::Rect(double width,double height){
        
        cout<<"Rect()"<<endl;
        m_dHeight = height;
        m_dWidth = width;
    }
    
    Rect::~Rect(){
        
        cout<<"~Rect()"<<endl;
    }
    
    double Rect::calcArea()
    {
        
        cout<<"Rect-->clacArea()"<<endl;
        return m_dHeight*m_dWidth;
    }
    
    int main()
    {
        
        Shape *shape1 = new Rect(3,6);
        Shape *shape2 = new Circle(5);
    
        shape1->calcArea();
        shape2->calcArea();
    
        delete shape1;
        shape1 = NULL;
        delete shape2;
        shape2 = NULL;
        return 0;
    }

运行结果：  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzE2NDk3NDIz_size_16_color_FFFFFF_t_70 1]  
多态的语法核心是virtual关键字，必须使用virtual才能使多个类间建立多态关系。

## 二. ##

虚析构函数：防止内存泄漏

#include <iostream>
    
    using namespace std;
    const double PI = 3.14;
    
    class Shape
    {
        
    public:
        Shape();
        virtual double calcArea();
        virtual ~Shape();
    };
    class Coordinate
    {
        
    public:
        Coordinate(int x, int y);
        ~Coordinate();
    private:
        int m_iX;
        int m_iY;
    };
    Coordinate::Coordinate(int x, int y)
    {
        
        cout<<"Coordinate"<<endl;
        m_iX = x;
        m_iY = y;
    }
    Coordinate::~Coordinate()
    {
        
        cout<<"~Coordinate"<<endl;
    }
    
    class Rect:public Shape{
        
    public:
        Rect(double width,double height);
        ~Rect();
        double calcArea();
    protected:
        double m_dWidth;
        double m_dHeight;
    };
    
    class Circle:public Shape
    {
        
    public:
        Circle(int x, int y, double r);
        ~Circle();
        virtual double calcArea();
    private:
        double m_dR;
        Coordinate *m_pCenter;
    };
    
    Shape::Shape()
    {
        
        cout<<"Shape()"<<endl;
    }
    
    Shape::~Shape()
    {
        
        cout<<"~Shape()"<<endl;
    }
    
    double Shape::calcArea()
    {
        
        cout<<"Shape-->calcArea()"<<endl;
        return 0;
    }
    
    Circle::Circle(int x, int y, double r)
    {
        
        cout<<"Circle()"<<endl;
        m_dR = r;
        m_pCenter = new Coordinate(x,y);
    }
    
    Circle::~Circle(){
        
        cout<<"~Circle()"<<endl;
        delete m_pCenter;
        m_pCenter = NULL;
    }
    
    double Circle::calcArea()
    {
        
        cout<<"Circle-->calcArea()"<<endl;
        return PI*m_dR*m_dR;
    }
    
    Rect::Rect(double width,double height){
        
        cout<<"Rect()"<<endl;
        m_dHeight = height;
        m_dWidth = width;
    }
    
    Rect::~Rect(){
        
        cout<<"~Rect()"<<endl;
    }
    
    double Rect::calcArea()
    {
        
        cout<<"Rect-->clacArea()"<<endl;
        return m_dHeight*m_dWidth;
    }
    int main()
    {
        
        Shape *shape1 = new Rect(3,6);
        Shape *shape2 = new Circle(3,4,5.0);
    
        shape1->calcArea();
        shape2->calcArea();
    
        delete shape1;
        shape1 = NULL;
        delete shape2;
        shape2 = NULL;
        return 0;
    }

运行结果：  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzE2NDk3NDIz_size_16_color_FFFFFF_t_70 2]

## 关于virtual的限制： ##

普通函数不能是虚函数，virtual只能修饰某个类的成员函数。  
静态成员函数不能是虚函数，因为static函数是与类同生共死的，不依赖于对象。  
内联函数不能是虚函数，如果修饰inline函数，计算机会忽略inline而把它变成虚函数。  
构造函数不能为虚函数。

## 纯虚函数： ##

virtual double calaPerimeter() = 0;

含有纯虚函数的类叫作抽象类。  
抽象类无法实例化对象。

## 接口类 ##

仅含有纯虚函数的类称为接口类。  
只有成员函数，而且成员函数都是纯虚函数。

## RTTI:运行时类型识别 ##

cout<<typeid(*obj).name()<<endl;
        if(typeid(*obj)==typeid(Bird)){
        
            Bird *Bird = dynamic_cast<Bird *>(obj);
        }

dynamic\_cast只能应用于指针和引用的转换  
要转换的类型中必须包含虚函数  
转换成功返回子类的地址，失败返回NULL。

typeid返回一个type\_info对象的引用。只能获取对象的实际类型，而不能是指针的。  
如果想通过基类的指针获得派生类的数据类型，基类必须带有虚函数。

## 异常处理 ##

try…catch…尝试正常逻辑，捕获异常  
throw… 抛出异常，让catch捕获处理

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