pointers to object and descendants in C++

Question

I am working on a simple C++ (under Linux) project, which will have pointers to objects. I have class A, class B which extends A, and class C which extends B. Class C has a method (test) that does not exist in A or B.

Is is possible to have a single pointer 'p' that can point to an object of type A, B, and C ? How would I define that pointer?

Secondly, since a.test() doesn't exist, but c.test() does, can my generic pointer 'p' call p->test() ? Will this compile? What if at run time p points to an object of class A and I call p->test()? Is that a runtime error?

Answer 1

Is is possible to have a single pointer that can point to an object of type A, B, and C ?

I assume you mean "that can either point to an A object or to B object or to a C object", right?

Yes, you can have such a pointer.

How would I define that pointer?

A*

A base-class pointer can point to objects of derived classes.

Secondly, how would I call methods of the object if the pointer can point to A/B/C classes?

You define a virtual function in A and override it in B and C. Then, when you call the method through your A*, the language will perform dynamic dispatch, i.e. it will automatically call the right method depending on whether your A* points to an A, to a B or to a C.

Do I need to cast them before calling the methods?

No. That would pretty much defeat the purpose of virtual functions.

Here is a complete example:

#include <iostream>

class A
{
public:
    virtual ~A() {} // not really needed in this program,
                    // but almost always needed in real code
                    // when a class has a virtual function
    virtual void method() { std::cout << "A::method\n"; }
};

class B : public A
{
public:
    virtual void method() override { std::cout << "B::method\n"; }
};

class C : public A
{
public:
    virtual void method() override { std::cout << "C::method\n"; }
};

int main()
{
    A a;
    B b;
    C c;
    A* ptr = &a;
    ptr->method();
    ptr = &b;
    ptr->method();
    ptr = &c;
    ptr->method();
}

Output:

A::method
B::method
C::method