defined
- defined
Returns a Boolean value telling whether EXPR has a value other than the undefined value
undef
. If EXPR is not present,$_
is checked.Many operations return
undef
to indicate failure, end of file, system error, uninitialized variable, and other exceptional conditions. This function allows you to distinguishundef
from other values. (A simple Boolean test will not distinguish amongundef
, zero, the empty string, and"0"
, which are all equally false.) Note that sinceundef
is a valid scalar, its presence doesn't necessarily indicate an exceptional condition:pop
returnsundef
when its argument is an empty array, or when the element to return happens to beundef
.You may also use
defined(&func)
to check whether subroutine&func
has ever been defined. The return value is unaffected by any forward declarations of&func
. A subroutine that is not defined may still be callable: its package may have anAUTOLOAD
method that makes it spring into existence the first time that it is called; see perlsub.Use of
defined
on aggregates (hashes and arrays) is deprecated. It used to report whether memory for that aggregate had ever been allocated. This behavior may disappear in future versions of Perl. You should instead use a simple test for size:When used on a hash element, it tells you whether the value is defined, not whether the key exists in the hash. Use exists for the latter purpose.
Examples:
Note: Many folks tend to overuse
defined
and are then surprised to discover that the numberand
""
(the zero-length string) are, in fact, defined values. For example, if you say- "ab" =~ /a(.*)b/;
The pattern match succeeds and
$1
is defined, although it matched "nothing". It didn't really fail to match anything. Rather, it matched something that happened to be zero characters long. This is all very above-board and honest. When a function returns an undefined value, it's an admission that it couldn't give you an honest answer. So you should usedefined
only when questioning the integrity of what you're trying to do. At other times, a simple comparison toor
""
is what you want.