(C++)virtual與pure virtual

virtual function
如果子class 有override virtual function，就會跳到下面的class做，base class不做事
如果子class 沒有override virtual function，base class就要做事

Ex:

class B{
public:
virtual void f(){
cout<< "BBB";
}
};
class A:public B{
public:
void f(){
cout <<"AAA";
}
};
int main(){
B *b = new A();
b->f(); // 印出AAA
return 0;
}

Ex:

class B{
public:
virtual void f(){
cout<< "BBB";
}
};
class A:public B{
};
int main(){
B *b = new A();
b->f(); // 印出BBB
return 0;
}

如果Base Class的function不是virtual，則子class的function無法覆蓋。如下例：
Ex:
class B{
public:
     void f(){ // non-virtual
        cout<< "BBB";
    }
};
class A:public B{
public:
   
    void f(){ // cannot override a non-virtual function
       cout <<"AAA";
    }
};
int main(){
    B *b = new A();
    b->f(); // 印出BBB
    return 0;
}
 

Pure virtual function
base class的Pure virtual function不實做。
子class一定要override pure virtual function

Ex:
class B{
public:
virtual void f() = 0;
};
class A:public B{
public:
void f(){
cout <<"AAA";
}
};
int main(){
B *b = new A();
b->f(); // 印出AAA
return 0;
}

Virtual function: Differences between C++ and Java

	C++	Java
Pure Virtual Function	vitual void func() = 0;	abstarct void func();
Impure Virtual Function	vitual void func();	void func();
Non-Virtual Function	void func();	final void func();
Hiding Inherited Names	Yes	No
Prohibiting Inheritance	No	final class