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#### 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` (see 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

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