c++: access derived class method when passing a pointer to the base class?

Question

I apologize for the nooby question, but I've just started programming in obj-oriented paradigm.

I have a function that takes a pointer to a class void foo(myClass* C) and this class has its own method

      class myClass{
      public:
          double fooMethod();
          /* ... */
      }

Now I would like to pass to foo an object with an overloaded method fooMethod so I thought about creating a derived class

      class myDerivedClass : public myClass{
      public:
          double fooMethod(); // overloaded method
          /* ... */
      }

The problem is that if I instantiate an object myDerivedClass* D = new myDerivedClass and pass it foo(D), this function will use the method of the base class, i.e., myClass::fooMethod() and instead I would like it to use myDerivedClass::fooMethod().

How do I make this happen? I have this problem since I don't want to re-write already working code, like foo or myClass, but I would like to adapt my new features to the current structure of the code.

Thank you very much! L.

Answer 1

You can do like this

class myClass{
  public:
      virtual double fooMethod();
      /* ... */
  }

class myDerivedClass : public myClass{
  public:
      double fooMethod() override; // override  method, not overload
      /* ... */
  }

Then if you pass a myDerivedClass object pointer/reference, and use it to call fooMethod(), the overriden method in myDerivedClass will be called.

The concept is you can override base class method, not overload.

Answer 2

In object-oriented programming in order to derived class' method be executed when you call base class' method you need that method to be virtual. In some languages like Java all methods by default are virtual, but in C++ you have to manually add keyword virtual in method definition in base class, like this:

class myClass{
public:
    virtual double fooMethod();
    /* ... */
}