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(elisp)Default Value

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The Default Value of a Buffer-Local Variable
--------------------------------------------

   The global value of a variable with buffer-local bindings is also
called the "default" value, because it is the value that is in effect
whenever neither the current buffer nor the selected frame has its own
binding for the variable.

   The functions `default-value' and `setq-default' access and change a
variable's default value regardless of whether the current buffer has a
buffer-local binding.  For example, you could use `setq-default' to
change the default setting of `paragraph-start' for most buffers; and
this would work even when you are in a C or Lisp mode buffer that has a
buffer-local value for this variable.

   The special forms `defvar' and `defconst' also set the default value
(if they set the variable at all), rather than any buffer-local or
frame-local value.

 - Function: default-value symbol
     This function returns SYMBOL's default value.  This is the value
     that is seen in buffers and frames that do not have their own
     values for this variable.  If SYMBOL is not buffer-local, this is
     equivalent to `symbol-value' (Note: Accessing Variables).

 - Function: default-boundp symbol
     The function `default-boundp' tells you whether SYMBOL's default
     value is nonvoid.  If `(default-boundp 'foo)' returns `nil', then
     `(default-value 'foo)' would get an error.

     `default-boundp' is to `default-value' as `boundp' is to
     `symbol-value'.

 - Special Form: setq-default [symbol form]...
     This special form gives each SYMBOL a new default value, which is
     the result of evaluating the corresponding FORM.  It does not
     evaluate SYMBOL, but does evaluate FORM.  The value of the
     `setq-default' form is the value of the last FORM.

     If a SYMBOL is not buffer-local for the current buffer, and is not
     marked automatically buffer-local, `setq-default' has the same
     effect as `setq'.  If SYMBOL is buffer-local for the current
     buffer, then this changes the value that other buffers will see
     (as long as they don't have a buffer-local value), but not the
     value that the current buffer sees.

          ;; In buffer `foo':
          (make-local-variable 'buffer-local)
               => buffer-local
          (setq buffer-local 'value-in-foo)
               => value-in-foo
          (setq-default buffer-local 'new-default)
               => new-default
          buffer-local
               => value-in-foo
          (default-value 'buffer-local)
               => new-default
          
          ;; In (the new) buffer `bar':
          buffer-local
               => new-default
          (default-value 'buffer-local)
               => new-default
          (setq buffer-local 'another-default)
               => another-default
          (default-value 'buffer-local)
               => another-default
          
          ;; Back in buffer `foo':
          buffer-local
               => value-in-foo
          (default-value 'buffer-local)
               => another-default

 - Function: set-default symbol value
     This function is like `setq-default', except that SYMBOL is an
     ordinary evaluated argument.

          (set-default (car '(a b c)) 23)
               => 23
          (default-value 'a)
               => 23