Returns a Boolean value telling whether EXPR has a value other than
the undefined value undef
. If EXPR is not present, $_
will be
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
. Note that a subroutine which 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 has 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 then are 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, despite the fact that 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 you're questioning the integrity of what
you're trying to do. At other times, a simple comparison to 0
or ""
is
what you want.