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Each symbol has four components (or "cells"), each of which references another object:
symbol-name in 8.3 Creating and Interning Symbols.
symbol-value in
11.7 Accessing Variable Values.
symbol-function in 12.8 Accessing Function Cell Contents.
symbol-plist in 8.4 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 8.3 Creating and Interning Symbols.)
The value cell holds the symbol's value as a variable
(see section 11. Variables). That is what you get if you evaluate the symbol as
a Lisp expression (see section 9. 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). See section 11.2 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
(see section 12. Functions) or a macro (see section 13. Macros), as that is what the
Lisp interpreter expects to see there (see section 9. Evaluation). Keyboard
macros (see section 21.15 Keyboard Macros), keymaps (see section 22. Keymaps) and
autoload objects (see section 9.2.8 Autoloading) are also sometimes stored in
the function cells of symbols.
The property list cell normally should hold a correctly formatted property list (see section 8.4 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-plist 'buffer-file-name)
=> (variable-documentation 29529)
(symbol-function 'buffer-file-name)
=> #<subr buffer-file-name>
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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 24.1 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 (see section 2.3.15 Primitive Function Type). A
symbol naming a function written in Lisp would have a lambda expression
(or a byte-code object) in this cell.
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