C++语言基础 例程 虚函数

贺老师的教学链接  本课讲解

指向基类的指针，为何只能访问来自基类成员？！

#include <iostream>
#include <string>
using namespace std;
//声明基类Student
class Student
{
public:
    Student(int, string,float);//声明构造函数
    void display( );                                             //声明输出函数
protected:                                      //受保护成员，派生类可以访问
    int num;
    string name;
    float score;
};
//Student类成员函数的实现
Student::Student(int n, string nam,float s)
{
    num=n;    //定义构造函数
    name=nam;
    score=s;
}

void Student::display( )                                        //定义输出函数
{
    cout<<"num:"<<num<<"\nname:"<<name<<"\nscore:"<<score<<"\n\n";
}
//声明公用派生类Graduate
class Graduate:public Student
{
public:
    Graduate(int, string, float, float);                          //声明构造函数
    void display( );                                             //声明输出函数
private:
    float pay;
};
// Graduate类成员函数的实现
void Graduate::display( )                                       //定义输出函数
{
    cout<<"num:"<<num<<"\nname:"<<name<<"\nscore:"<<score<<"\npay="<<pay<<endl;
}

Graduate::Graduate(int n, string nam,float s,float p):Student(n,nam,s),pay(p) { }

//主函数
int main()
{
    Student stud1(1001,"Li",87.5);                   //定义Student类对象stud1
    Graduate grad1(2001,"Wang",98.5,563.5);          //定义Graduate类对象grad1
    Student *pt=&stud1;                              //定义指向基类对象的指针变量pt
    pt->display( );
    pt=&grad1;
    pt->display( );
    return 0;
}

一招虚函数，从此出樊笼

#include <iostream>
#include <string>
using namespace std;
//声明基类Student
class Student
{
public:
    Student(int, string,float);//声明构造函数
    virtual void display( );                                             //声明输出函数
protected:                                      //受保护成员，派生类可以访问
    int num;
    string name;
    float score;
};
//Student类成员函数的实现
Student::Student(int n, string nam,float s)
{
    num=n;    //定义构造函数
    name=nam;
    score=s;
}

void Student::display( )                                        //定义输出函数
{
    cout<<"num:"<<num<<"\nname:"<<name<<"\nscore:"<<score<<"\n\n";
}
//声明公用派生类Graduate
class Graduate:public Student
{
public:
    Graduate(int, string, float, float);                          //声明构造函数
    void display( );                                             //声明输出函数
private:
    float pay;
};
// Graduate类成员函数的实现
void Graduate::display( )                                       //定义输出函数
{
    cout<<"num:"<<num<<"\nname:"<<name<<"\nscore:"<<score<<"\npay="<<pay<<endl;
}

Graduate::Graduate(int n, string nam,float s,float p):Student(n,nam,s),pay(p) { }

//主函数
int main()
{
    Student stud1(1001,"Li",87.5);                   //定义Student类对象stud1
    Graduate grad1(2001,"Wang",98.5,563.5);          //定义Graduate类对象grad1
    Student *pt=&stud1;                              //定义指向基类对象的指针变量pt
    pt->display( );
    pt=&grad1;
    pt->display( );
    return 0;
}

对比：未用虚函数

#include <iostream>
using namespace std;

class Circle
{
public:
    Circle(double r=0):radius(r) { }
    double area ( ) const
    {
        return 3.14159*radius*radius;
    }
protected:
    double radius;
};

class Cylinder:public Circle
{
public:
    Cylinder (double r=0,double h=0):Circle(r),height(h) {}
    double area() const
    {
        return 2*Circle::area( )+2*3.14159*Circle::radius*height;
    };
protected:
    double height;
};

int main( )
{
    Circle c1(5.2);
    Cylinder cy1(5.2, 10);

    Circle *pc=&c1;
    cout<<pc->area()<<endl;
    pc = &cy1;
    cout<<pc->area()<<endl;
    return 0;
}

对比：用了虚函数

#include <iostream>
using namespace std;

class Circle
{
public:
    Circle(double r=0):radius(r) { }
    double area ( ) const
    {
        return 3.14159*radius*radius;
    }
protected:
    double radius;
};

class Cylinder:public Circle
{
public:
    Cylinder (double r=0,double h=0):Circle(r),height(h) {}
    double area() const
    {
        return 2*Circle::area( )+2*3.14159*Circle::radius*height;
    };
protected:
    double height;
};

int main( )
{
    Circle c1(5.2);
    Cylinder cy1(5.2, 10);

    Circle *pc=&c1;
    cout<<pc->area()<<endl;
    pc = &cy1;
    cout<<pc->area()<<endl;
    return 0;
}

事情本就该这样——抽象的动物不会叫

#include <iostream>
using namespace std;
class Animal
{
public:
  virtual void cry()
    {
      cout<<"咋叫？"<<endl;
    }
};

class Dog: Animal
{
public:
    void cry()
    {
        cout<<"汪！"<<endl;
    }
};
class Cat: Animal{...};
class Mouse: Animal{...};
...
int main( )
{
    Animal *p, a;
    p = &a;
    p->cry();
    Dog d;
    p = &d;
    p->cry();
    Cat c;
    p = &c;
    p->cry();
    Mouse m;
    p = &m;
    p->cry();
    return 0;
}

wxWidgets中利用虚函数

class MyApp : public wxApp
{
public:
    // Called on application startup
    virtual bool OnInit();
};

class WXDLLIMPEXP_CORE wxApp : public wxAppBase
{
public:
    wxApp();
    virtual ~wxApp();

    // override base class (pure) virtuals
    virtual bool Initialize(int& argc, wxChar **argv);
    virtual void CleanUp();
    ……
};