Why am I getting unresolved externals?

Question

I am writing an immutable binary search tree in c++. My terminating nodes are represented by a singleton empty node. My compiler (visual c++) seems to be having trouble resolving the protected static member that holds my singleton. I get the following error:

error LNK2001: unresolved external symbol "protected: static class boost::shared_ptr > node::m_empty" (?m_empty@?$node@HH@@1V?$shared_ptr@V?$node@HH@@@boost@@A)

I am assuming this means it cant resolve the static m_empty member for the type node. Is this correct? If so how do I fix it?

Code follows:

using namespace boost;
template<typename K, typename V>
class node {
protected:
    class empty_node : public node<K,V> {
    public:
        bool is_empty(){ return true; }
        const shared_ptr<K> key() { throw cant_access_key; }
        const shared_ptr<V> value()  { throw cant_access_value; }
        const shared_ptr<node<K,V>> left()  { throw cant_access_child; }
        const shared_ptr<node<K,V>> right()  { throw cant_access_child; }
        const shared_ptr<node<K,V>> add(const shared_ptr<K> &key, const shared_ptr<V> &value){
            return shared_ptr<node<K,V>>();
        }
        const shared_ptr<node<K,V>> remove(const shared_ptr<K> &key) { throw cant_remove; }
        const shared_ptr<node<K,V>> search(const shared_ptr<K> &key) { return shared_ptr<node<K,V>>(this); }
    };

    static shared_ptr<node<K,V>> m_empty;
public:
    virtual bool is_empty() = 0;
    virtual const shared_ptr<K> key() = 0;
    virtual const shared_ptr<V> value() = 0;
    virtual const shared_ptr<node<K,V>> left() = 0;
    virtual const shared_ptr<node<K,V>> right() = 0;
    virtual const shared_ptr<node<K,V>> add(const shared_ptr<K> &key, const shared_ptr<V> &value) = 0;
    virtual const shared_ptr<node<K,V>> remove(const shared_ptr<K> &key) = 0;
    virtual const shared_ptr<node<K,V>> search(const shared_ptr<K> &key) = 0;


    static shared_ptr<node<K,V>> empty() {
        if(m_empty.get() == NULL){
            m_empty.reset(new empty_node());
        }
        return m_empty;
    }
};

the root of my tree is initialized as:

shared_ptr<node<int,int>> root = node<int,int>::empty();

Answer 1

m_empty is static and so you'll need to have a source (.cpp) file with something like the following:

template <typename K, typename V> shared_ptr<node<K,V> > node<K,V>::m_empty;

Note: My original answer was incorrect and did not take into account that this was a template. This is the answer that AndreyT gave in his answer; I've updated this answer with the correct answer because this is the accepted answer and appears at the top of the page. Please upvote AndreyT's answer, not this one.