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12.11.3 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

A variable can be let-bound (see 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.