Strange VC++ compile error, C2244

Question

Take a look at this peice of code:

template 
Pointer::Iterator> BinaryTree::GetBegi         


        

Answer 1

For those interested, I tried writing a minimal sample reproducing the problem:

template <typename T>
struct Pointer {};

template <typename T>
struct Collection {
    struct Iterator {};
};

template <typename K,typename T>
struct BinaryTree : Collection<T>{
    Pointer<typename Collection<T>::Iterator> GetBeginning() const;

    struct BinaryTreeIterator : Collection<T>::Iterator {
        template <typename X>
        BinaryTreeIterator(BinaryTreeIterator*, X) {}
        struct Position {
            static int atBeginning() { return 0; }
        };
    };
};

template <typename K,typename T>
Pointer<typename Collection<T>::Iterator> BinaryTree<K,T>::GetBeginning() const
{
    return Pointer<typename Collection<T>::Iterator>();
}

int main(){
    BinaryTree<int, float> bt;
    bt.GetBeginning();
}

And yes, I get the error too. I can't see any obvious errors in what we've seen of your code, but then again, just this example has used more nested classes and inheritance than most sane C++ programmers do in a year, so I can't say for sure that your code is or isn't correct.

Moreover, I've had to guess quite a bit to piece this together. (What's atBeginning supposed to be? What are the actual class interfaces?)

But I suspect it'd work better (and be more readable and easier to debug) if you didn't inherit everything from everything else.

Update I tried compiling the above with GCC and Comeau's online compiler, and both accepted it. So it seems like it could be a compiler bug.

Answer 2

The obvious solution which you probably considered is to just define the function inside the class definition instead of redefining it later.

Also, Putting the iterator type in a typedef like so:

template <typename T>
struct Pointer {};

template <typename T>
struct Collection {
    struct Iterator {};
};

template <typename K,typename T>
struct BinaryTree : Collection<T>{
    typedef typename Collection<T>::Iterator Iter;
    Pointer<Iter> GetBeginning() const;

    struct BinaryTreeIterator : Collection<T>::Iterator {
    };
};

template <typename K,typename T>
Pointer<typename BinaryTree<K,T>::Iter> BinaryTree<K,T>::GetBeginning() const
{
    return new BinaryTreeIterator(this,BinaryTreeIterator::Position::atBeginning);
}

int main(){
    BinaryTree<int, float> bt;
    bt.GetBeginning();
}

seems to fix it. Not sure why, possibly a bug...

Answer 3

It will compile if you change it to this:

template <typename K,typename T>
struct BinaryTree : Collection<T> {
    Pointer<typename BinaryTree<K,T>::Iterator> GetBeginning() const;

};

template <typename K,typename T>
Pointer<typename BinaryTree<K,T>::Iterator> BinaryTree<K,T>::GetBeginning() const
{
    return Pointer<BinaryTree<K,T>::Iterator>();
}

In general, the original code isn't quite right because it implies that GetBeginning() can return any collection, while (I'm assuming) it can only return binary tree collections.

EDIT:

After some digging, it seems that VC++ doesn't handle well injected class names. That is, the original code will compile if you remove from Collection::Iterator in the method declaration:

template <typename K, typename T>
struct BinaryTree : Collection<T> {
    Pointer<typename Collection::Iterator> GetBeginning() const;

};

template <typename K, typename T>
Pointer<typename Collection<T>::Iterator> BinaryTree<K,T>::GetBeginning() const
{
    return Pointer<Collection<T>::Iterator>();
}