虚函数

# include <iostream>
using namespace std;
class A
{
public:
A (){cout<<"构造函数A"<<endl;}
virtual void a(){cout<<"AA"<<endl;}
};
class B : public A
{
public:B (){cout<<"构造函数B"<<endl;}
void a(){cout<<"BB"<<endl;}
};
class C : public A{public:
C (){cout<<"构造函数C"<<endl;}
void a(){cout<<"CC"<<endl;}
};
int main (void)
{
A * a = new A;a->a();
A * b = new B;b->a();
A * c = new C;c->a();
return 0;
}

# include <iostream>
using namespace std;
class A
{
public:
virtual void tada (){cout<<"AA"<<endl;}
};
class B: public A
{
public:virtual void tada (){cout<<"BB"<<endl;}
};
int main (void)
{
while (1)
{
int i;A * q;
cout <<"请输入1.2.3"<<endl;cin>>i;
bool b = false;
switch (i)
{
case 1 : q = new A;break;
case 2 : q = new B;break;
case 3 : cout<<"请输入123"<<endl;break;
default : b = true;break;
}
if(b ==true)
break;
q ->tada ();
}return 0;
}

# include <iostream>
using namespace std;
class A
{
public:
A (){cout<<"构造函数A"<<endl;};
virtual ~A (){cout<<"析构函数A"<<endl;};
A (A&){cout<<"复制构造函数A"<<endl;};
virtual void a(){cout<<"A函数"<<endl;};
};
class B : public A
{
public:B (){cout<<"构造函数B"<<endl;};
~B (){cout<<"析构函数B"<<endl;};
B (B&){cout<<"复制构造函数B"<<endl;};
void a(){cout<<"B函数"<<endl;};};
int main (void)
{
A *a ,* b;
a = new A;
a -> a();
b = new B;
b -> a();
delete a;
cout<<"*************"<<endl;
delete b;
return 0;
}