Polymorphism

[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:

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) ?

[laserlight]

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?

[nuzzle]

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++.

[cilu]

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.

[GCDEF]

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?

[omerd]

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.

 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;
};

[GCDEF]

Why not put the list in A?

[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")

[GCDEF]

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")

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.

[omerd]

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.

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

[laserlight]

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

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

[GCDEF]

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

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

[omerd]

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

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

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

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

[Lindley]

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.

[GCDEF]

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?