Subclass getting superclass private variables c++

Question

I've got a superclass which has certain private variables, and I need to build a constructor for a subclass. Here's the superclass:

class Shape {
public:
    Shape(double w, double h);
    string toString();

private:
    double width;
    double height;

};

Superclass constructor:

Shape::Shape(double w, double h) {
    width = w;
    height = h;
}

Subclass:

class Rectangle : public Shape {

public:
    Rectangle(double w, double h, int s);
    string toString();

private:
    int sides;

};

My subclass constructor is as follows:

Rectangle::Rectangle(double w, double h, int s) : Shape(w, h) {

    width = w;
    height = h;
    sides = s;

}

I cannot modify the superclass at all. All I can do is modify the constructor class to get the values which are private in the superclass.

Possible duplicate of [Private and Protected Members : C++](http://stackoverflow.com/questions/224966/private-and-protected-members-c) — Matías Fidemraizer, Mar 24 '16 at 19:46
First of all, don't try this, after a while it will be impossible to debug why variable have a certain value if you can't correctly who changes it. Secondly, that is why they invented 'protected' — JVApen, Mar 24 '16 at 19:46
Sorry to say I literally CANNOT change from private to protected. This is part of a homework assignment, and I am not allowed to use protected for whatever reason. — Nathan Lundgren, Mar 24 '16 at 19:48
Why do you want this? You can set them by calling superclass constructor. — Revolver_Ocelot, Mar 24 '16 at 19:50
@Barry: The main point of my comment was to say "what's the problem?" but yeah okay — Lightness Races in Orbit, Mar 24 '16 at 19:55

score 2 · Answer 1 · answered Mar 24 '16 at 19:53

The derived constructor is not responsible for setting width and height. Indeed, it can't even see them. You could make them protected or even public, but why? It's a good thing that only the base can manage its own members.

You're already specifying the arguments for the base constructor, so you're good to go. There's no problem here once you remove the redundant and broken lines:

struct Shape
{
    Shape(double w, double h)
    {
       width  = w;
       height = h;
    }

private:
    double width;
    double height;
};

struct Rectangle : Shape
{
    Rectangle(double w, double h, int s)
      : Shape(w, h)
    {
       sides = s;
    }

private:
    int sides;
};

As an aside, you should standardise on using the ctor-initialiser where possible, instead of using it only sometimes and then assignment from the body at other times:

struct Shape
{
    Shape(double w, double h)
      : width(w)
      , height(h)
    {}

private:
    double width;
    double height;
};

struct Rectangle : Shape
{
    Rectangle(double w, double h, int s)
      : Shape(w, h)
      , sides(s)
    {}

private:
    int sides;
};

Sithideth Bouasavanh · Answer 2 · 2016-03-24T20:18:10.233

If I understand it right, you want to modify superclass' private variables after they have been constructed via subclass.

Firstly, implement set() methods in superclass so that we can access its private variables:

Shape.h

class Shape {
public:
    Shape(double w, double h);
    string toString();
    //here
    setWidth(double);
    setHeight(double);

private:
    double width;
    double height;

};

Shape.cpp

Shape::setWidth(double w) { width = w; }
Shape::setHeight(double h) { height = h; }

then, you can call superclass set() methods via subclass:

Rectangle constructor:

Rectangle(double w, double h, int s)
      : width(w)       // subclass width
      , height(h)
{
    // modify subclass width & height as desired here

    //then reflect superclass width & height according to subclass
    Shape::setWidth(width);    
    Shape::setHeight(height);
}

Shape::setWidth() and Shape::setHeight() can be called wherever we want in subclass and superclass private variables will be reflected accordingly.

Subclass getting superclass private variables c++

2 Answers2