未定义引用与嵌套类模板静态const成员

Question

I have something like this, which is inherited by other 3 classes (list, queue, stack...):

template <typename generic>
class singlyLinkedList
{
    protected:

    class innerNode
    {
        public:

        innerNode *NextNode;
        generic Value;

        innerNode( generic value, innerNode *next = NULL ) :
            NextNode(next), Value(value) {}
    };

    innerNode *FirstNode;
    innerNode *LastNode;

    size_t CurrentSize;

    public:

    class iterator
    {
        private:

        innerNode *InnerNode;
        const bool IsDeepCopy;

        static const innerNode DefaultNode;

        public:

        iterator( innerNode *node = NULL ) : InnerNode(node), IsDeepCopy(false)
        {
            if (node == NULL) 
                this->InnerNode = (innerNode*)(&(iterator::DefaultNode));
        }

        iterator( innerNode *node, bool deepCopy ) : IsDeepCopy(deepCopy)
        {
            if (deepCopy) this->InnerNode = new innerNode(node->Value, node->NextNode);
            else this->InnerNode = node;
        }

        ~iterator()
        {
            if (this->IsDeepCopy) delete this->InnerNode;
        }

        inline bool isNull() const
        {
            return (this->InnerNode != &(iterator::DefaultNode));
        }

        // ...other iterator members which doesn't use the static member
    };

    // ...other singlyLinkedList members, some returning iterators
};

I used to have a separate singlyLinkedNode class to serve both as node and iterator, using friendships to give special access to the list classes... Then I've decided I'd change everything to how it is now (as in the above section).

Since iterator is nested in a class template and, as far as I know, it's possible to define static const members of class templates in headers, I saw no reasons to get Undefined Reference error from the linker for this code. However, I'm getting this:

/tmp/cc6vtGDj.o: In function `basicSinglyLinkedList<double>::iterator::iterator(basicSinglyLinkedList<double>::innerNode*)':
oo_test.cpp:(.text._ZN21basicSinglyLinkedListIdE8iteratorC2EPNS0_9innerNodeE[_ZN21basicSinglyLinkedListIdE8iteratorC5EPNS0_9innerNodeE]+0x2d): undefined reference to `basicSinglyLinkedList<double>::iterator::DefaultNode'
/tmp/cc6vtGDj.o: In function `basicBinarySearchTree<int>::iterator::iterator(basicBinarySearchTree<int>::innerNode*)':
oo_test.cpp:(.text._ZN21basicBinarySearchTreeIiE8iteratorC2EPNS0_9innerNodeE[_ZN21basicBinarySearchTreeIiE8iteratorC5EPNS0_9innerNodeE]+0x2a): undefined reference to `basicBinarySearchTree<int>::DefaultNode'
collect2: error: ld returned 1 exit status
Compilation failed.

Can somebody explain me why am I getting a linker error for singlyLinkedList<>::iterator::DefaultNode OR... what alternatives do I have to set a "default node" to control invalid iterators and where they point to?

Answer 1

It's perfectly fine to define a static const member of a template class in a header. But the point is you have to define it -- you have only declared it here.

Outside the class definition, you should add

template <typename T>
singlyLinkedList::innerNode singlyLinkedList::iterator::defaultNode = T();

(or something similar) to initialise the storage, like you would in an implementation file for a static member of a regular class. The linker will take care of merging all the templated static variables together in the final executable, as it does for template functions.

EDIT:

The above was off the top of my head: having given it a bit more thought the definition will need to be more like

template <typename T>
const typename singlyLinkedList<T>::innerNode singlyLinkedList<T>::iterator::defaultNode = T{};

but I can't try it right now. The compiler should guide towards what it actually wants you to say, though.