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# defined

Perl 5 version 18.2 documentation
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# defined

Perl functions A-Z | Perl functions by category | The 'perlfunc' manpage

- defined EXPR

- 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 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.

  See also undef, exists, ref.