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Studentclass 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Graduateclass 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Studentclass 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Graduateclass 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();    ……};