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 distinguish undef
from other values. (A simple Boolean test will not distinguish among undef
, zero, the empty string, and "0"
, which are all equally false.) Note that since undef
is a valid scalar, its presence doesn't necessarily indicate an exceptional condition: pop
returns undef
when its argument is an empty array, or when the element to return happens to be undef
.
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 an AUTOLOAD
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:
if (@an_array) { print "has array elements\n" }
if (%a_hash) { print "has hash members\n" }
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:
print if defined $switch{D};
print "$val\n" while defined($val = pop(@ary));
die "Can't readlink $sym: $!"
unless defined($value = readlink $sym);
sub foo { defined &$bar ? $bar->(@_) : die "No bar"; }
$debugging = 0 unless defined $debugging;
Note: Many folks tend to overuse defined
and are then surprised to discover that the number 0
and ""
(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 use defined
only when questioning the integrity of what you're trying to do. At other times, a simple comparison to 0
or ""
is what you want.