To serialize the tree view control override the Serialize() function. The Serialize() function is a virtual function defined in Cobject. 
In the code below we save the outline to a text file and can read it back from a text file. When saving the outline to the archive, tabs are used to indent the item text. Again, when reading back, tabs are used to determine the level that the newly read item should be placed at. 
      void CTreeCtrlX::Serialize(CArchive& ar) 
{
        if (ar.IsStoring())
        {        
                // storing code
                HTREEITEM hti = GetRootItem();
                while( hti )
                {
                        int indent = GetIndentLevel( hti );
                        while( indent-- )
                                ar.WriteString( "\t" );
                        ar.WriteString( GetItemText( hti )   "\r\n");
                        hti = GetNextItem( hti );
                }
                
        }
        else
        {        
                // loading code
                CString sLine;
                if( !ar.ReadString( sLine ) )
                        return;                    HTREEITEM hti = NULL;
                int indent, baseindent = 0;
                while( sLine[baseindent] == '\t' )
                        baseindent  ;
                do
                {
                        if( sLine.GetLength() == 0 )
                                continue;
                        for( indent = 0; sLine[indent] == '\t'; indent   )
                                ;                // We don't need a body
                        sLine = sLine.Right( sLine.GetLength() - indent );
                        indent -= baseindent;                            HTREEITEM parent;
                        int previndent = GetIndentLevel( hti );
                        if( indent ==  previndent)
                                parent = GetParentItem( hti );
                        else if( indent > previndent )
                                parent = hti;
                        else
                        {
                                int nLevelsUp = previndent - indent;
                                parent = GetParentItem( hti );
                                while( nLevelsUp-- )
                                        parent = GetParentItem( parent );
                        }
                        hti = InsertItem( sLine, parent ? parent : TVI_ROOT, TVI_LAST );
                }while( ar.ReadString( sLine ) );            }
}        int CTreeCtrlX::GetIndentLevel( HTREEITEM hItem )
{
        int iIndent = 0;            while( (hItem = GetParentItem( hItem )) != NULL )
                iIndent  ;
        return iIndent;
}    // GetNextItem        - Get next item as if outline was completely expanded
// Returns                - The item immediately below the reference item
// hItem                - The reference item
HTREEITEM CTreeCtrlX::GetNextItem( HTREEITEM hItem )
{
        HTREEITEM        hti;            if( ItemHasChildren( hItem ) )
                return GetChildItem( hItem );                // return first child
        else{
                // return next sibling item
                // Go up the tree to find a parent's sibling if needed.
                while( (hti = GetNextSiblingItem( hItem )) == NULL ){
                        if( (hItem = GetParentItem( hItem ) ) == NULL )
                                return NULL;
                }
        }
        return hti;
}