// The pointer to my tree control.
extern CTreeCtrl* pmyTreeCtrl;

UINT i, uCount = pmyTreeCtrl->GetVisibleCount();
HTREEITEM hItem = pmyTreeCtrl->GetFirstVisibleItem();

// Toggle the check state of all the visible items.
for (i=0;i < uCount;i++)
{
   ASSERT(hItem != NULL);
   pmyTreeCtrl->SetCheck(hItem, !pmyTreeCtrl->GetCheck(hItem));
   hItem = pmyTreeCtrl->GetNextVisibleItem(hItem);
}


这段代码为什么提示unresolved external symbol "class CTreeCtrl * pmyTreeCtrl" ？