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 &foo.
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:
if (@an_array) { print "has array elements\n" }
if (%a_hash) { print "has hash members\n" }
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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;
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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
The pattern match succeeds, and $1 is defined, despite the fact that it
matched "nothing". But it didn't really match nothing--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.
See also /undef, /exists, /ref.
