从基类指针调用子类方法？

Question

I didn't know what to give as a title, I expect more experienced stackoverflow.com users to improve it.

Let's say we have the class A

class A {
  void hello(){  cout << "i'm an A" << endl; }
}

and its sub-class B

class B: public A {
  void hello(){  cout << "i'm a B" << endl; }
}

Then we did somewhere in our program

A* array[2];
array[0] = new A;
array[1] = new B;
array[0]->hello(); // output: "i'm an A"
array[1]->hello(); // output: "i'm a B"

why doesn't the array[1].hello(); output I'm a B since we instanciated a B object for that base-class pointer ? and how to make it happen ?

Answer 1

You have to make hello a virtual function:

class A {
    virtual void hello() { cout << "i'm an A" << endl; }
};

class B : public A {
    virtual void hello() override { cout << "i'm a B" << endl; } // 1) 
};

This tells the compiler that the actual function should not be determined by the static type (the type of the pointer or reference) but by the dynamic (run-time) type of the object.

1) The override keyword tells the compiler to check, if the function actually overrides a hello function in the base class (helps eg to catch spelling mistakes or differences in the parameter types).

Answer 2

Couple of changes here:

make function hello in class A, a virtual and public: because by default it is private

class A {
public: 
virtual void hello(){ cout << "i'm an A" << endl; }
};

Similarly make hello in class B virtual

class B: public A {
virtual void hello(){  cout << "i'm a B" << endl; }
};

Answer 3

Since you have inherited class A in class B, derived class B will call the function from the Base class. You need to override the function in class B, as you want to change the Base class functionality.

You can do this with the override and virtual keywords.

http://en.cppreference.com/w/cpp/language/override