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
virtual 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();
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
virtual void 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
virtual void 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：
