| [ < ] | [ > ] | [ << ] | [Plus haut] | [ >> ] | [Top] | [Table des matières] | [Index] | [ ? ] |
Each symbol has four components (or “cells”), each of which references another object:
The print name cell holds a string that names the symbol for reading
and printing. See symbol-name in @ref{Creating Symbols}.
The value cell holds the current value of the symbol as a variable.
When a symbol is used as a form, the value of the form is the contents of
the symbol's value cell. See symbol-value in Accessing Variable Values.
The function cell holds the function definition of the symbol. When a
symbol is used as a function, its function definition is used in its place.
This cell is also used to make a symbol stand for a keymap or a keyboard
macro, for editor command execution. Because each symbol has separate value
and function cells, variables names and function names do not conflict. See
symbol-function in Accessing Function Cell Contents.
The property list cell holds the property list of the symbol. See
symbol-plist in @ref{Property Lists}.
The print name cell always holds a string, and cannot be changed. The other three cells can be set individually to any specified Lisp object.
The print name cell holds the string that is the name of the symbol. Since symbols are represented textually by their names, it is important not to have two symbols with the same name. The Lisp reader ensures this: every time it reads a symbol, it looks for an existing symbol with the specified name before it creates a new one. (In GNU Emacs Lisp, this lookup uses a hashing algorithm and an obarray; see @ref{Creating Symbols}.)
The value cell holds the symbol's value as a variable (voir la section Variables).
That is what you get if you evaluate the symbol as a Lisp expression
(voir la section Evaluation). Any Lisp object is a legitimate value. Certain
symbols have values that cannot be changed; these include nil and
t, and any symbol whose name starts with ‘:’ (those are called
keywords). Voir la section Variables that Never Change.
We often refer to “the function foo” when we really mean the
function stored in the function cell of the symbol foo. We make the
distinction explicit only when necessary. In normal usage, the function
cell usually contains a function (@pxref{Functions}) or a macro
(voir la section Macros), as that is what the Lisp interpreter expects to see there
(voir la section Evaluation). Keyboard macros (voir la section Keyboard Macros), keymaps
(voir la section Keymaps) and autoload objects (voir la section Autoloading) are also
sometimes stored in the function cells of symbols.
The property list cell normally should hold a correctly formatted property list (@pxref{Property Lists}), as a number of functions expect to see a property list there.
The function cell or the value cell may be void, which means that the
cell does not reference any object. (This is not the same thing as holding
the symbol void, nor the same as holding the symbol nil.)
Examining a function or value cell that is void results in an error, such as
‘Symbol's value as variable is void’.
The four functions symbol-name, symbol-value,
symbol-plist, and symbol-function return the contents of the
four cells of a symbol. Here as an example we show the contents of the four
cells of the symbol buffer-file-name:
(symbol-name 'buffer-file-name)
⇒ "buffer-file-name"
(symbol-value 'buffer-file-name)
⇒ "/gnu/elisp/symbols.texi"
(symbol-function 'buffer-file-name)
⇒ #<subr buffer-file-name>
(symbol-plist 'buffer-file-name)
⇒ (variable-documentation 29529)
|
Because this symbol is the variable which holds the name of the file being
visited in the current buffer, the value cell contents we see are the name
of the source file of this chapter of the Emacs Lisp Manual. The property
list cell contains the list (variable-documentation 29529) which
tells the documentation functions where to find the documentation string for
the variable buffer-file-name in the ‘DOC-version’ file.
(29529 is the offset from the beginning of the ‘DOC-version’ file
to where that documentation string begins—see Documentation Basics.)
The function cell contains the function for returning the name of the file.
buffer-file-name names a primitive function, which has no read syntax
and prints in hash notation (voir la section Primitive Function Type). A symbol
naming a function written in Lisp would have a lambda expression (or a
byte-code object) in this cell.
| [ < ] | [ > ] | [ << ] | [Plus haut] | [ >> ] | [Top] | [Table des matières] | [Index] | [ ? ] |
Ce document a été généré par Eric Reinbold le 13 Octobre 2007 en utilisant texi2html 1.78.