Accessing private members of a derived class C++

Question

I am trying to access derived class' private members via an object of the base class. Here is what I'm trying to do :

 class A {

      private:

           int a, b;

      public:
           A(_a, _b) : a(_a), b(_b) {}
           int getA() { return a; }
           int getB() { return b; }
 };

 class B : public A {

      private:
           int c;

      public:
           B(_a, _b, _c) : A(_a, _b), c(_c) {}
           int getC() { return c; }

 };

 vector<A*> objects;
 A* retVal = new B(1,2,3);
 objects.push_back(retVal);

Now how is it possible to access this?

 objects[0] -> getC();

I am a little confused.

Thanks in advance.

If you are sure that the object in the vector is a 'B' instance you can cast — Matt Coubrough, May 25 '14 at 08:21
The line `int getA() { return a; }` should be `int getA() const { return a; }` — Ed Heal, May 25 '14 at 08:23
I tried casting it but I still get the same error : class a does not have member c — user3279394, May 25 '14 at 08:32

score 2 · Accepted Answer · answered May 25 '14 at 08:41

2

If you know the object is actually of derived type B, you can do this:

static_cast<B*>(objects[0])->getC();

If you are wrong and the object is not actually of type B (or a subclass of B), your program will invoke undefined behavior, so don't do that.

answered May 25 '14 at 08:41

John Zwinck

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2

Also one could use `dynamic_cast` and a `nullptr` check to determine what the actual derived class is. – πάντα ῥεῖ May 25 '14 at 08:51

score 1 · Answer 2 · answered May 25 '14 at 08:22

1

You can provide a pure virtual function in your base class

 class A {

      private:

           int a, b;

      public:
           A(_a, _b) : a(_a), b(_b) {}
           int getA() { return a; }
           int getB() { return b; }
           virtual int getC() = 0;
 };

 class B : public A {

      private:
           int c;

      public:
           B(_a, _b, _c) : A(_a, _b), c(_c) {}
           virtual int getC() { return c; }

 };

answered May 25 '14 at 08:22

πάντα ῥεῖ

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I have other derived classes too. I think this does not work. Is there another way? – user3279394 May 25 '14 at 08:31
@user3279394 _'Is there another way?'_ Not in a simple straightforward manner. May be inheritance isn't the right instrument to solve your real use case. Hard to give you advice without knowing it. – πάντα ῥεῖ May 25 '14 at 08:34

score 0 · Answer 3 · answered May 25 '14 at 09:26

I don't see any clean way to do what you want. I think it is at odds with the way inheritance is meant to be used in c++: if your derived class adds new methods then it should really be regarded as a new type, i.e. something more than just a morphing of the base type. (What you are trying to do is more objective-C-like philosophy, where methods calls are resolved runtime.) My suggestion is to reconsider your design and keep B-object conceptually separated from A-objects.

Accessing private members of a derived class C++

3 Answers3

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