Info Catalog elisp: Markers elisp: Predicates on Markers

elisp: Overview of Markers

 
 30.1 Overview of Markers
 ========================
 
 A marker specifies a buffer and a position in that buffer.  A marker can
 be used to represent a position in functions that require one, just as
 an integer could be used.  In that case, the marker’s buffer is normally
 ignored.  Of course, a marker used in this way usually points to a
 position in the buffer that the function operates on, but that is
 entirely the programmer’s responsibility.  Positions, for a
 complete description of positions.
 
    A marker has three attributes: the marker position, the marker
 buffer, and the insertion type.  The marker position is an integer that
 is equivalent (at a given time) to the marker as a position in that
 buffer.  But the marker’s position value can change during the life of
 the marker, and often does.  Insertion and deletion of text in the
 buffer relocate the marker.  The idea is that a marker positioned
 between two characters remains between those two characters despite
 insertion and deletion elsewhere in the buffer.  Relocation changes the
 integer equivalent of the marker.
 
    Deleting text around a marker’s position leaves the marker between
 the characters immediately before and after the deleted text.  Inserting
 text at the position of a marker normally leaves the marker either in
 front of or after the new text, depending on the marker’s “insertion
 type” (Marker Insertion Types)—unless the insertion is done with
 ‘insert-before-markers’ (Insertion).
 
    Insertion and deletion in a buffer must check all the markers and
 relocate them if necessary.  This slows processing in a buffer with a
 large number of markers.  For this reason, it is a good idea to make a
 marker point nowhere if you are sure you don’t need it any more.
 Markers that can no longer be accessed are eventually removed (
 Garbage Collection).
 
    Because it is common to perform arithmetic operations on a marker
 position, most of these operations (including ‘+’ and ‘-’) accept
 markers as arguments.  In such cases, the marker stands for its current
 position.
 
    Here are examples of creating markers, setting markers, and moving
 point to markers:
 
      ;; Make a new marker that initially does not point anywhere:
      (setq m1 (make-marker))
           ⇒ #<marker in no buffer>
 
      ;; Set ‘m1’ to point between the 99th and 100th characters
      ;;   in the current buffer:
      (set-marker m1 100)
           ⇒ #<marker at 100 in markers.texi>
 
      ;; Now insert one character at the beginning of the buffer:
      (goto-char (point-min))
           ⇒ 1
      (insert "Q")
           ⇒ nil
 
      ;; ‘m1’ is updated appropriately.
      m1
           ⇒ #<marker at 101 in markers.texi>
 
      ;; Two markers that point to the same position
      ;;   are not ‘eq’, but they are ‘equal’.
      (setq m2 (copy-marker m1))
           ⇒ #<marker at 101 in markers.texi>
      (eq m1 m2)
           ⇒ nil
      (equal m1 m2)
           ⇒ t
 
      ;; When you are finished using a marker, make it point nowhere.
      (set-marker m1 nil)
           ⇒ #<marker in no buffer>
 

Info Catalog elisp: Markers elisp: Predicates on Markers