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();
……
};
