We say that a variable is void if its symbol has an unassigned value cell (see Symbol Components).
Under Emacs Lisp's default dynamic scoping rule (see Variable Scoping), the value cell stores the variable's current (local or
global) value. Note that an unassigned value cell is not the
same as having
nil in the value cell. The symbol
a Lisp object and can be the value of a variable, just as any other
object can be; but it is still a value. If a variable is void, trying
to evaluate the variable signals a
void-variable error, instead
of returning a value.
Under the optional lexical scoping rule, the value cell only holds the variable's global value—the value outside of any lexical binding construct. When a variable is lexically bound, the local value is determined by the lexical environment; hence, variables can have local values even if their symbols' value cells are unassigned.
This function empties out the value cell of symbol, making the variable void. It returns symbol.
If symbol has a dynamic local binding,
makunboundvoids the current binding, and this voidness lasts only as long as the local binding is in effect. Afterwards, the previously shadowed local or global binding is reexposed; then the variable will no longer be void, unless the reexposed binding is void too.
Here are some examples (assuming dynamic binding is in effect):(setq x 1) ; Put a value in the global binding. ⇒ 1 (let ((x 2)) ; Locally bind it. (makunbound 'x) ; Void the local binding. x) error--> Symbol's value as variable is void: x x ; The global binding is unchanged. ⇒ 1 (let ((x 2)) ; Locally bind it. (let ((x 3)) ; And again. (makunbound 'x) ; Void the innermost-local binding. x)) ; And refer: it's void. error--> Symbol's value as variable is void: x (let ((x 2)) (let ((x 3)) (makunbound 'x)) ; Void inner binding, then remove it. x) ; Now outer
letbinding is visible. ⇒ 2
This function returns
tif variable (a symbol) is not void, and
nilif it is void.
Here are some examples (assuming dynamic binding is in effect):(boundp 'abracadabra) ; Starts out void. ⇒ nil (let ((abracadabra 5)) ; Locally bind it. (boundp 'abracadabra)) ⇒ t (boundp 'abracadabra) ; Still globally void. ⇒ nil (setq abracadabra 5) ; Make it globally nonvoid. ⇒ 5 (boundp 'abracadabra) ⇒ t