Perl defined() function

In the Perl programming language has a built-in function defined().

Most often this feature is used to distinguish the value undef from any other value. But even with the help of defined() , you can learn whether you have defined a function.

Check undef

Main apply function defined() is a check on the value undef.

If the transfer function defined() value undef, the function will return false. The function will return 'true' if the value which it transferred is anything but undef. Here is an example code:

▶ Run
#!/usr/bin/perl

use Data::Dumper;

print Dumper defined(8);
print Dumper defined(0);
print Dumper defined('');
print Dumper defined(undef);

The result of this code:

$VAR1 = 1;
$VAR1 = 1;
$VAR1 = 1;
$VAR1 = '';

Check that function is defined

Most often, the function of defined() is used in order to distinguish undef from another value. But besides using defined() you can learn whether you have defined a function. Here is an example code:

▶ Run
#!/usr/bin/perl

use strict;
use warnings;

use Data::Dumper;

print Dumper defined(&check);
print Dumper defined(&Data::Dumper::Dumper);

Function check() is not defined, so defined() returns false, But the function Dumper a bundle Data::Dumper and defined() return 'true'. That's what this program displays on the screen:

$VAR1 = '';
$VAR1 = 1;

Arguments

If the function defined() not given no arguments, the function works with a variable $_:

If the transfer function defined() more than one argument, it will be an error and program execution will be stopped:

Too many arguments for defined operator at script.pl line 3, near "3)"
Execution of script.pl aborted due to compilation errors.

Return value

The result of the function defined() is always a Boolean value. True or false.

Usage with an array and Hesham

Very long function defined() worked in a special way if you tell her an array or hash as the argument. But in Perl 5.8 this use issued a warning, and since 5.22 this use began to fail.

Here is a sample code which in defined() is passed an array:

▶ Run
#!/usr/bin/perl

use strict;
use warnings;

use Data::Dumper;

my @arr;

print Dumper defined(@arr);

The result of this code in perl version 5.8:

defined(@array) is deprecated at script.pl line 10.
    (Maybe you should just omit the defined()?)
    $VAR1 = '';

The result of this when 5.30 perl:

Can't use 'defined(@array)' (Maybe you should just omit the defined()?) at script.pl line 10.

And the same code, but about the hash:

▶ Run
#!/usr/bin/perl

use strict;
use warnings;

use Data::Dumper;

my %h;

print Dumper defined(%h);

The results of the work on perl 5.8 and 5.30:

defined(%hash) is deprecated at script.pl line 10.
    (Maybe you should just omit the defined()?)
    $VAR1 = '';
Can't use 'defined(%hash)' (Maybe you should just omit the defined()?) at script.pl line 10.

Official documentation

Here is the output of the command perldoc -f defined:

       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".

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