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Perl Language Reference Manual
by Larry Wall and others
Paperback (6"x9"), 724 pages
ISBN 9781906966027
RRP £29.95 ($39.95)

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defined EXPR


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 8.

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:

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 for the latter purpose.


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 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, 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.

See also , , .

ISBN 9781906966027Perl Language Reference ManualSee the print edition