MFC中定义全局静态变量出错，搞不懂？

class Share
{public:
Share();
virtual ~Share();
static HTREEITEMSelectNode;//子树节点
static CStringPublicDir;//存放各种目录
static BOOL isConnect;
static SOCKET server;
static HTREEITEM m_hRoot;
};
error LNK2001: unresolved external symbol "public: static struct _TREEITEM * Share::SelectNode" (?SelectNode@Share@@2PAU_TREEITEM@@A)
这是为什么？