Polymorphism

Hello.
I have 3 classes. B and C inherit from A. I want that class B will be able to call functions of C, so I tried to do this:

Code:

---

class A
{
    public:
      A(int b=0) {a=b;}
    protected:
      void Print() {cout<<"Hello world"<<endl;}
    private:
      int a;
};
class B: public A
{
    public:
      B(int b=0): A(b){}
      void Do(A& a) {a.Print();} //error
};
class C: public A
{
    C(int b=0): A(b){}

    int c;
};

---

but i get an error:
error: `virtual void A::Print()' is protected.

How can I do it without using "friend" (and without making Print public) ?

Why does A not have any virtual functions, not even a virtual destructor? It looks like you cannot use inheritance based polymorphism unless you change A.

Other than that... what does B have to do with C?

Quote:

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Originally Posted by omerd

Hello.
I have 3 classes. B and C inherit from A. I want that class B will be able to call functions of C, so I tried to do this:

---

In that case B and C must be related by inheritance like for example,

class A {};
class C : public A {};
class B : public C {};

Both B and C inherits A, C directly and B indirectly via C.

This is also possible,

class A {};
class C : public A {};
class B : public A, public C {};

It's essentially as before apart from that if A carries implementation it's now an example of the (in)famous so called diamond inheritance. You can resolve the ambiguity by the use of virtual inheritance like this,

class A {};
class C : virtual public A {};
class B : virtual public A, public C {};

If the purpose if inheriting C in B was only to get at the implementation of C then you can inherit C privately, like

class A {};
class C : virtual public A {};
class B : virtual public A, private C {};

Now B isn't a C class as it was before, Just an A class.

As you can see there are many options and ample opportunity to make a mess out of a design in C++. :)

Quote:

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error: `virtual void A::Print()' is protected.

---

This error shows the code you shown is incorrect. Please post the actual snippet, otherwise we can't help you too much.

Your thread title, code and description don't have much in common.

There's no polymorphism in the code you provided.

You're not trying to call in functions in C and the code you provided doesn't produce the error you report.

What are you really trying to do?

maybe
I changed the code a little bit. Now it is more specific, but I also tried to keep it simple.
Actually, B and C manage a list of objects of type D, and in some cases B need to get C's D objects. B and C create objects of D in different ways.
If it's help you, C holds a list of object of type B.

Code:

---

class D
{
    public:
      D(string s): name(s)
        {another_data =-1;}
      D(string s,int data): name(s),another_data(data)
        {}
      D(const D& d)
      {/*code*/}
      string GetName() {return name;}
      int GetData() {return another_data;}

    private:
      string name;
      int another_data;
};

class A
{
    public:
      A(int b=0) {a=b;}
      virtual ~A()
      {
          //code;
        }
    protected:
      virtual D* FindD(string)=0;
    private:
      int a;
};
class B: public A
{
    friend class A;
    public:
      B(int b=0): A(b){}

    D* FindD(string name)
    {
        //code that find a specific D
        //and return it
    }
      void Do(A& a, string name)
      {
          a.FindD(name); //error: `virtual D A::FindD(std::string)' is protected|
      }

    private:
    list<D> names;

};
class C: public A
{
    C(int b=0): A(b){}

    D* FindD(string name)
    {
        //code that find a specific D
        //and return it
    }

    list<D> names;
};

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Why not put the list in A?

you right, but it doesn't help. class B need to call "FindD" on objects of type C (which is the argument in function "Do")

Quote:

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Originally Posted by omerd

you right, but it doesn't help. class B need to call "FindD" on objects of type C (which is the argument in function "Do")

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Okay, so class B would need an instance of class C to do that. What's the problem? Just call FindD from your B object using your instance of C.

From an inheritance POV, if B and C both have a list of D, that list should be in a common base class.

Quote:

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Originally Posted by GCDEF

Okay, so class B would need an instance of class C to do that. What's Just call FindD from your B object using your instance of C.

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that what's I did. B call "FindD" on C's object, and I get an error.

Quote:

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Originally Posted by omerd

that what's I did. B call "FindD" on C's object, and I get an error.

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Show the smallest and simplest compilable program that demonstrates that error.

Quote:

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Originally Posted by omerd

that what's I did. B call "FindD" on C's object, and I get an error.

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Don't you think explaining the error a bit may help us help you?

Quote:

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Originally Posted by GCDEF

Don't you think explaining the error a bit may help us help you?

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I wrote the error above:
error: `virtual void A::Print()' is protected.

Quote:

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Originally Posted by laserlight

Show the smallest and simplest compilable program that demonstrates that error.

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I will edit this message in a few hours with the code.

So either make FindD public in C, or else declare that B is a friend of C. I'm not sure which is more appropriate in this specific situation.

But B and C are both derived from A. Both classes contain a list of D. Why not move both the list and the FindD function into A?