C++之多态和虚函数

多态是指具有不同功能的函数可以用同一个函数名。

主要探索：

1 虚函数(纯虚函数)

2 虚基类

3  抽象类

4 虚析构函数

虚函数：

 用相同形式能同时调用派生类和基类一样函数。

第一个实例，非虚函数，基类和派生类具有相同函数：

#include <iostream>#include <string>using namespace std;//声明基类class student{protected:int _num;string _name;float _score;public :student(int ,string,float);    void display();};//定义基类构造函数student::student(int num,string name,float score):_num(num),_name(name),_score(score){}void student::display(){cout<<"num :"<<_num<<endl<<"name:"<<_name<<endl<<"score:"<<_score<<endl<<endl;}//派生类class graduate:public student{private:float _pay;public:graduate(int,string,float,float);//声明和基类相同函数void display();};graduate::graduate(int num,string name,float score,float pay):_pay(pay),student(num,name,score){}void graduate::display(){cout<<"num :"<<_num<<endl<<"name :"<<_name<<endl<<"score:"<<_score<<endl<<"pay  :"<<_pay<<endl<<endl;}int main(){student stu(100,"buyingfei",100);graduate gra(1000,"buyafei",120,10000);student *pt=&stu;pt->display();pt=&gra;pt->display();  //输出结果是基类一部分stu.display();return 0;}

运行结果：

第二个实例,使用虚函数

#include <iostream>#include <string>using namespace std;//声明基类class student{protected:int _num;string _name;float _score;public :student(int ,string,float);   virtual void display();};//定义基类构造函数student::student(int num,string name,float score):_num(num),_name(name),_score(score){} void student::display(){cout<<"num :"<<_num<<endl<<"name:"<<_name<<endl<<"score:"<<_score<<endl<<endl;}//派生类class graduate:public student{private:float _pay;public:graduate(int,string,float,float);//声明和基类相同函数       virtualvoid display();};graduate::graduate(int num,string name,float score,float pay):_pay(pay),student(num,name,score){}void graduate::display(){cout<<"num :"<<_num<<endl<<"name :"<<_name<<endl<<"score:"<<_score<<endl<<"pay  :"<<_pay<<endl<<endl;}int main(){student stu(100,"buyingfei",100);graduate gra(1000,"buyafei",120,10000);student *pt=&stu;pt->display();pt=&gra;pt->display();  stu.display();return 0;}

运行结果：

抽象类：

不用它生成对象，作为基类去建立派生类，凡是包含纯虚函数的类都是抽象类。

#include <iostream>#include <string>using namespace std;//定义抽象类class people{public :virtual void getMessage()const=0; //定义纯虚构函数};class student:public people{protected:int _num;string _name;float _score;public :student(int ,string,float);    virtual void getMessage() const;};student::student(int num,string name,float score):_num(num),_name(name),_score(score){} void student::getMessage() const{cout<<"num :"<<_num<<endl<<"name:"<<_name<<endl<<"score:"<<_score<<endl<<endl;}class graduate:public student{private:float _pay;public:graduate(int,string,float,float);//声明和基类相同函数virtualvoid getMessage() const{cout<<"num :"<<_num<<endl<<"name :"<<_name<<endl<<"score:"<<_score<<endl<<"pay  :"<<_pay<<endl<<endl;}};graduate::graduate(int num,string name,float score,float pay):_pay(pay),student(num,name,score){}int main(){student stu(100,"buyingfei",100);graduate gra(1000,"buyafei",120,10000);student *pt=&stu;pt->getMessage();pt=&gra;pt->getMessage();  stu.getMessage(); return 0;}

运行结果：

  

虚基类：

如果一个派生类有多个直接基类，而这些基类又有一个共同基类，则派生类简介保存共同基类的多份数据成员，虚基类则解决这问题。不是在声明基类时说明的，而是在声明派生类继承方式时指出的。

如class  派生类名 :virtual 继承方式  基类名。

虚析构函数：

派生类的对象从内存中撤销，一般先调用派生类析构函数，然后在调用基类析构函数，

但是通过new 运算符建立临时对象，若基类存在析构函数，并且定义指向基类的指针变量，通过delete 撤销对象时，不会执行派生类的析构函数。

这时建立虚析构函数。

普通对象析构函数调用

#include <iostream>#include <string>using namespace std;class student{protected:int _num;string _name;float _score;public :student(int ,string,float);    virtual void getMessage() const;~student(){  cout<<"destructor student"<<endl;}};student::student(int num,string name,float score):_num(num),_name(name),_score(score){} void student::getMessage() const{cout<<"num :"<<_num<<endl<<"name:"<<_name<<endl<<"score:"<<_score<<endl<<endl;}class graduate:public student{private:float _pay;public:graduate(int,string,float,float);//声明和基类相同函数virtualvoid getMessage() const{cout<<"num :"<<_num<<endl<<"name :"<<_name<<endl<<"score:"<<_score<<endl<<"pay  :"<<_pay<<endl<<endl;}~graduate(){ cout<<"destory graduate"<<endl;}};graduate::graduate(int num,string name,float score,float pay):_pay(pay),student(num,name,score){}int main(){student stu(100,"buyingfei",100);graduate gra(1000,"buyafei",120,10000);//student *pt=&stu;//pt->getMessage();//pt=&gra;//pt->getMessage();  gra.getMessage();stu.getMessage(); return 0;}

运行结果：

析构函数不是虚函数：

#include <iostream>#include <string>using namespace std; class point { public : point(){}      ~point(){  cout<<"destory point"<<endl; } }; class circle:public point { public: circle(){} ~circle(){ cout<<"destory circle"<<endl; } };int main(){   point *p=new circle;   delete p;  //这时并没有调用派生类析构函数return 0;}

运行结果：

基类析构函数是虚构函数：

#include <iostream>#include <string>using namespace std; class point { public : point(){}     virtual  ~point(){  cout<<"destory point"<<endl; } }; class circle:public point { public: circle(){} virtual ~circle(){ cout<<"destory circle"<<endl; } };int main(){   point *p=new circle;   delete p;return 0;}

运行结果：