c++ - inheritance- the same method name in two different classes

Question

I'm new in C++ and I have maybe easy for you question.

class circle {
  protected:
     int r;
  public:
     circle(int re) { r=re; }

     double surface() {
        return 3.14*r*r;
     }
 };

 class sphere : public circle {
    public:
      sphere(int r) : circle(r) {}
      double surface(){
        return 4*3.14*r*r;
      }
 };

And now, my problem is how I can do something like that: create a sphere object and using it to get surface not of a sphere but of a circle. Can I use somehow the same methods names in two classes, when one is inherited by the second one?

Careful with those decimal commas. C++ uses . and your commas will compile, but do the wrong thing (most likely with a compiler warning). — Qaz
– Qaz, Commented Apr 10, 2015 at 13:56
ok, ok, that's has been written quickly, my fault, sorry. Any ideas for my question? — bingo157
– bingo157, Commented Apr 10, 2015 at 13:58
You're looking for something, that is impossible to model using classic OOP approach. It's the same problem like square and rectangle. Square is a rectange and yet it wouldn't have much profit in deriving from rectangle (rectangle needs to hold a and b, while square is required to have all borders equally long). Perhaps circle and sphere deriving both from RoundObject would be much more correct, but still pointless to me. — Mateusz Grzejek
– Mateusz Grzejek, Commented Apr 10, 2015 at 14:07
An obvious problem with class sphere : public circle is that a sphere is a generalisation of a circle into three dimensions, but derivation is actually specialisation. In an inheritance relationship the more general concept is represented by the base class, not by the derived class. — R. Martinho Fernandes
– R. Martinho Fernandes, Commented Apr 10, 2015 at 14:08

rems4e · Accepted Answer · 2015-04-10 13:59:42Z

5

You can have access to the base class' surface method by appending circle:: before its name :

sphere sph(1);
double s = sph.circle::surface();

answered Apr 10, 2015 at 13:59

rems4e

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Comments

user2030052 · Accepted Answer · 2015-04-10 14:07:04Z

3

Your design is initially wrong. Public inheritance in C++ means that the child is-a specific kind of the parent. A sphere is not a circle!

Besides, if you do want to get the surface area of the sphere, you should make your surface function virtual:

class Circle {
    public:
    virtual double surface();
};

That way, when you override it in Sphere, the Sphere version will be called.

answered Apr 10, 2015 at 14:07

user2030052

2 Comments

Not a real meerkat Over a year ago

Wrong. He only needs to make surface() virtual if he needs the Sphere version to be called when using a pointer to Circle. If he uses a pointer to Sphere, or a regular non-pointer object, he's fine.

user2030052 Over a year ago

I agree; I assumed that since he was using inheritance that he would want this kind of behavior, and not that he wants to hide the base class implementation.

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