Info Catalog elisp: Creating Buffer-Local elisp: Buffer-Local Variables

elisp: Default Value

 
 11.10.3 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 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’ (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
 
    A variable can be let-bound (Local Variables) to a value.
 This makes its global value shadowed by the binding; ‘default-value’
 will then return the value from that binding, not the global value, and
 ‘set-default’ will be prevented from setting the global value (it will
 change the let-bound value instead).  The following two functions allow
 to reference the global value even if it’s shadowed by a let-binding.
 
  -- Function: default-toplevel-value symbol
      This function returns the “top-level” default value of SYMBOL,
      which is its value outside of any let-binding.
 
      (defvar variable 'global-value)
          ⇒ variable
      (let ((variable 'let-binding))
        (default-value 'variable))
          ⇒ let-binding
      (let ((variable 'let-binding))
        (default-toplevel-value 'variable))
          ⇒ global-value
 
  -- Function: set-default-toplevel-value symbol value
      This function sets the top-level default value of SYMBOL to the
      specified VALUE.  This comes in handy when you want to set the
      global value of SYMBOL regardless of whether your code runs in the
      context of SYMBOL’s let-binding.
 

Info Catalog elisp: Creating Buffer-Local elisp: Buffer-Local Variables