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() to check whether a subroutine exists. On the other hand, use of defined() upon aggregates (hashes and arrays) is not guaranteed to produce intuitive results, and should probably be avoided.

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 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''. But it didn't really match nothing--rather, it matched something that happened to be 0 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.

Currently, using defined() on an entire array or hash reports whether memory for that aggregate has ever been allocated. So an array you set to the empty list appears undefined initially, and one that once was full and that you then set to the empty list still appears defined. 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"   }

Using undef() on these, however, does clear their memory and then report them as not defined anymore, but you shoudln't do that unless you don't plan to use them again, because it saves time when you load them up again to have memory already ready to be filled.

This counterintuitive behaviour of defined() on aggregates may be changed, fixed, or broken in a future release of Perl.

See also undef, exists, ref.