Implementing Undo the Simple Way

Over the years I have implemented all kinds of undos for the editors and tools I have made. I have tried all kinds of ways, ranging from storing delta information to abstract command class that can undo redo itself. There is one kind, though, that is by far my favorite:

class Document ()
{
   static int MAX_UNDO = 10;
   struct UndoState
   {
       String cmd;
       String path;
   };
   UndoState m_undoStack[MAX_UNDO];

   int m_undoHead = 0;    // Current state.
   int m_undoTip = 0;    // The highest state that is set.
}

// Adds undo state to the undo stack.
Document:: addUndoState(String cmd, String path)
{
   // This kills all the states after the current one
   m_undoTip = m_undoHead+1;
   // Buffer getting full, shift down.
   if (m_undoTip >= MAX_UNDO) {
       for (int i = 0; i < MAX_UNDO-1; ++i)
           m_undoStack[i] = m_undoStack[i+1];
       m_undoTip--;
   }
   // Store state
   m_undoHead = m_undoTip;
   m_undoStack[m_undoTip].cmd = cmd;
   m_undoStack[m_undoTip].path = path;
}

// Takes snapshot of the document.
Document::snapshot(String cmd)
{
   String path = getTempFileName();
   ZipWriter zip;
   saveState(&zip);
   zip.save(path);
   addUndoState(cmd, path);
}

// Returns true if can undo.
Document::canUndo()
{
   return m_undoHead > 0;
}

// Returns true if can redo.
Document::canRedo()
{
   return m_undoHead < m_undoTip;
}

// Reverts to previous state of the document.
Document::undo()
{
   if (!canUndo()) return;
   m_undoHead--;

   ZipReader zip;
   zip.load(m_undoStack[m_undoHead].path);
   loadState(&zip);
}

// Advances to next state of the document.
Document::redo()
{
   if (!canRedo()) return;
   m_undoHead++;

   ZipReader zip;
   zip.load(m_undoStack[m_undoHead].path);
   loadState(&zip);
}

// Saves document state.
Document::saveState(Writer* out)
{
   // Save document state
   ...
}

// Loads document state.
Document::loadState(Reader* in)
{
   // Load document state
   ...
}