Nested class inside an abstract class

Question

I have an abstract class:

class A
{
public:
    void func() = 0;
};

Can I make my implementations also have a nested iterator class?

#include <iterator>

template<typename T>
class A
{
public:
    class Iterator : public std::iterator<std::forward_iterator_tag, T>
    {
    };

    virtual Iterator begin() const = 0;
    virtual void func() = 0;
};

template<typename T>
class B : public A<T>
{
public:
    B() {}
    class Iterator : public std::iterator<std::forward_iterator_tag, T>
    {
    };

    Iterator begin() const
    {
        return Iterator();
    }

    virtual void func()
    {
    }
};

int main()
{
    B<int> b;
}

I just want to know if this is possible, and if so, what am I missing? Since the iterator class will depend on how class A is implemented, I don't know if formal implementation is possible.

Answer 1

Try the following:

template<typename T>
class A
{
public:
    class Iterator : public std::iterator<std::forward_iterator_tag, T>
    {
    public:
        virtual void DoSomething() = 0;
    };

    virtual Iterator * begin() const = 0;
    virtual void func() = 0;
};

template<typename T>
class B : public A<T>
{
public:
    B() {}
    class BIterator : public A<T>::Iterator
    {
    public:
        void DoSomething()
        {
            std::cout << "Hello world from B::BIterator!";
        }
    };

    A<T>::Iterator * begin() const
    {
        return new BIterator();
    }

    virtual void func()
    {
    }
};

int main(int argc, char * argv[])
{
    B<int> b;

    A<int>::Iterator * iter = b.begin();
    iter->DoSomething();
    delete iter;

    getchar();
}

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Answer 2

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