C++ Polymorphism

I'm trying to get along with C++ and polymorpism. Therefore I created the following code example. When try to compile i'm getting the following error message. Actually the code sample was Java exam and and now i try to translate it to C++.

Code:

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$ g++ main.cpp -o main
main.cpp: In member function `void A::func5(C*)':
main.cpp:18: Fehler: invalid use of undefined type `struct C'
main.cpp:6: Fehler: forward declaration of `struct C'

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Can anyone help me what's wrong with the example below. I guess it's related to forward declaration but don't know what's wrong. Any code improvements are appreciated as well.

Code:

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#include <iostream>


using namespace std;


class C; // forward declaration


class A
{
    public:
        A()                  { cout << "A::A()"  << endl; }
        virtual ~A()        { cout << "A::~A()" << endl; }


        virtual void func1() { cout << "A::func1()"  << endl; }
        virtual void func2() { cout << "A::func2()"  << endl; }
        virtual void func3() { cout << "A::func3()"  << endl; }
        void func4(A* pA)    { cout << "A::func4()"  << endl; pA->func1(); }
        void func5(C* pC)    { cout << "A::func5()"  << endl; pC->func1(); }
};


class B : public A
{
    public:
        B()                { cout << "B::B()"    << endl; }
        ~B()                { cout << "B::~C()"    << endl; }
        void func1()        { cout << "B::func1()" << endl; }
        void func2(int* pI) { cout << "B::func2"  << endl; }
};


class C : public B
{
    public:
        C()                  { cout << "C::C()"  << endl; }
        ~C()                { cout << "C::~B()" << endl; }


        virtual void func1() { cout << "C::func1()"  << endl; }
        virtual void func2() { cout << "C::func2()"  << endl; }
        virtual void func3() { cout << "C::func3()"  << endl; }
        void func4()        { cout << "C::func4()"  << endl; }
};


int main()
{
    A* ap = new C();


    cout << endl;


    ap->func1();
    ap->func2();
    ap->func3();
    ap->func4(ap);
    ((C*)ap)->func4();
    // static_cast<B*>(ap)->p()


    cout << endl;


    delete ap;


    return 0;
}

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Commenting out the line "void func5(C* pC) ..." and the program works...

Code:

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A::A()
B::B()
C::C()


C::func1()
C::func2()
C::func3()
A::func4()
C::func1()
C::func4()


C::~B()
B::~C()
A::~A()

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Here is a simple example in classes forward declaration :)
Forward declarations in C++

The compiler does know that a class C is going to exist,but it does not know it's members,because it is an incomplete declaration of C,that later becomes complete.

Ordinarily you wouldn't encounter this problem because implementation of the member functions is located after the declaration of the class or in a separate source file.

Additionally, the B and C destructors display lies about which one is being called. If anything I would fix that.

It isn't generally a good idea to have a base class which knows about its children. (Here, your A::func5)
That makes the design somewhat brittle.

I agree. As mentioned it's orignially form a Java exam, to confusue students if either A::func4 or A::func5 is called

Thanks for your help! It works now, code is attached.