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GNU Emacs Lisp Reference Manual

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11.10.2 Creating and Deleting Buffer-Local Bindings

Command: make-local-variable variable
This function creates a buffer-local binding in the current buffer for variable (a symbol). Other buffers are not affected. The value returned is variable.

The buffer-local value of variable starts out as the same value variable previously had. If variable was void, it remains void.

 
;; In buffer `b1':
(setq foo 5)                ; Affects all buffers.
     => 5
(make-local-variable 'foo)  ; Now it is local in `b1'.
     => foo
foo                         ; That did not change
     => 5                   ;   the value.
(setq foo 6)                ; Change the value
     => 6                   ;   in `b1'.
foo
     => 6

;; In buffer `b2', the value hasn't changed.
(save-excursion
  (set-buffer "b2")
  foo)
     => 5

Making a variable buffer-local within a let-binding for that variable does not work reliably, unless the buffer in which you do this is not current either on entry to or exit from the let. This is because let does not distinguish between different kinds of bindings; it knows only which variable the binding was made for.

If the variable is terminal-local, this function signals an error. Such variables cannot have buffer-local bindings as well. See section 29.2 Multiple Displays.

Note: Do not use make-local-variable for a hook variable. Instead, use make-local-hook. See section 23.6 Hooks.

Command: make-variable-buffer-local variable
This function marks variable (a symbol) automatically buffer-local, so that any subsequent attempt to set it will make it local to the current buffer at the time.

A peculiar wrinkle of this feature is that binding the variable (with let or other binding constructs) does not create a buffer-local binding for it. Only setting the variable (with set or setq) does so.

The value returned is variable.

Warning: Don't assume that you should use make-variable-buffer-local for user-option variables, simply because users might want to customize them differently in different buffers. Users can make any variable local, when they wish to. It is better to leave the choice to them.

The time to use make-variable-buffer-local is when it is crucial that no two buffers ever share the same binding. For example, when a variable is used for internal purposes in a Lisp program which depends on having separate values in separate buffers, then using make-variable-buffer-local can be the best solution.

Function: local-variable-p variable &optional buffer
This returns t if variable is buffer-local in buffer buffer (which defaults to the current buffer); otherwise, nil.

Function: buffer-local-variables &optional buffer
This function returns a list describing the buffer-local variables in buffer buffer. (If buffer is omitted, the current buffer is used.) It returns an association list (see section 5.8 Association Lists) in which each element contains one buffer-local variable and its value. However, when a variable's buffer-local binding in buffer is void, then the variable appears directly in the resulting list.

 
(make-local-variable 'foobar)
(makunbound 'foobar)
(make-local-variable 'bind-me)
(setq bind-me 69)
(setq lcl (buffer-local-variables))
    ;; First, built-in variables local in all buffers:
=> ((mark-active . nil)
    (buffer-undo-list . nil)
    (mode-name . "Fundamental")
    ...
    ;; Next, non-built-in buffer-local variables. 
    ;; This one is buffer-local and void:
    foobar
    ;; This one is buffer-local and nonvoid:
    (bind-me . 69))

Note that storing new values into the CDRs of cons cells in this list does not change the buffer-local values of the variables.

Command: kill-local-variable variable
This function deletes the buffer-local binding (if any) for variable (a symbol) in the current buffer. As a result, the default binding of variable becomes visible in this buffer. This typically results in a change in the value of variable, since the default value is usually different from the buffer-local value just eliminated.

If you kill the buffer-local binding of a variable that automatically becomes buffer-local when set, this makes the default value visible in the current buffer. However, if you set the variable again, that will once again create a buffer-local binding for it.

kill-local-variable returns variable.

This function is a command because it is sometimes useful to kill one buffer-local variable interactively, just as it is useful to create buffer-local variables interactively.

Function: kill-all-local-variables
This function eliminates all the buffer-local variable bindings of the current buffer except for variables marked as "permanent". As a result, the buffer will see the default values of most variables.

This function also resets certain other information pertaining to the buffer: it sets the local keymap to nil, the syntax table to the value of (standard-syntax-table), the case table to (standard-case-table), and the abbrev table to the value of fundamental-mode-abbrev-table.

The very first thing this function does is run the normal hook change-major-mode-hook (see below).

Every major mode command begins by calling this function, which has the effect of switching to Fundamental mode and erasing most of the effects of the previous major mode. To ensure that this does its job, the variables that major modes set should not be marked permanent.

kill-all-local-variables returns nil.

Variable: change-major-mode-hook
The function kill-all-local-variables runs this normal hook before it does anything else. This gives major modes a way to arrange for something special to be done if the user switches to a different major mode. For best results, make this variable buffer-local, so that it will disappear after doing its job and will not interfere with the subsequent major mode. See section 23.6 Hooks.

A buffer-local variable is permanent if the variable name (a symbol) has a permanent-local property that is non-nil. Permanent locals are appropriate for data pertaining to where the file came from or how to save it, rather than with how to edit the contents.


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