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(emacs-lisp-intro.info)kill-ring-yank-pointer

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The `kill-ring-yank-pointer' Variable
=====================================

   `kill-ring-yank-pointer' is a variable, just as `kill-ring' is a
variable.  It points to something by being bound to the value of what
it points to, like any other Lisp variable.

   Thus, if the value of the kill ring is:

     ("some text" "a different piece of text" "yet more text")

and the `kill-ring-yank-pointer' points to the second clause, the value
of `kill-ring-yank-pointer' is:

     ("a different piece of text" "yet more text")

   As explained in the previous chapter (Note: List Implementation),
the computer does not keep two different copies of the text being
pointed to by both the `kill-ring' and the `kill-ring-yank-pointer'.
The words "a different piece of text" and "yet more text" are not
duplicated.  Instead, the two Lisp variables point to the same pieces of
text.  Here is a diagram:

     kill-ring     kill-ring-yank-pointer
         |               |
         |      ___ ___  |     ___ ___      ___ ___
          ---> |   |   |  --> |   |   |    |   |   |
               |___|___|----> |___|___|--> |___|___|--> nil
                 |              |            |
                 |              |            |
                 |              |             --> "yet more text"
                 |              |
                 |               --> "a different piece of text
                 |
                  --> "some text"





   Both the variable `kill-ring' and the variable
`kill-ring-yank-pointer' are pointers.  But the kill ring itself is
usually described as if it were actually what it is composed of.  The
`kill-ring' is spoken of as if it were the list rather than that it
points to the list.  Conversely, the `kill-ring-yank-pointer' is spoken
of as pointing to a list.

   These two ways of talking about the same thing sound confusing at
first but make sense on reflection.  The kill ring is generally thought
of as the complete structure of data that holds the information of what
has recently been cut out of the Emacs buffers.  The
`kill-ring-yank-pointer' on the other hand, serves to indicate--that
is, to `point to'--that part of the kill ring of which the first
element (the CAR) will be inserted.

   The `rotate-yank-pointer' function changes the element in the kill
ring to which the `kill-ring-yank-pointer' points; when the pointer is
set to point to the next element beyond the end of the kill ring, it
automatically sets it to point to the first element of the kill ring.
This is how the list is transformed into a ring.  The
`rotate-yank-pointer' function itself is not difficult, but contains
many details.  It and the much simpler `yank' and `yank-pop' functions
are described in an appendix.  Note: Handling the Kill Ring.