=over

=item defined EXPR

=item defined

Returns a Boolean value telling whether EXPR has a value other than
the undefined value C<undef>.  If EXPR is not present, C<$_> will be
checked.

Many operations return C<undef> to indicate failure, end of file,
system error, uninitialized variable, and other exceptional
conditions.  This function allows you to distinguish C<undef> from
other values.  (A simple Boolean test will not distinguish among
C<undef>, zero, the empty string, and C<"0">, which are all equally
false.)  Note that since C<undef> is a valid scalar, its presence
doesn't I<necessarily> indicate an exceptional condition: C<pop()>
returns C<undef> when its argument is an empty array, I<or> when the
element to return happens to be C<undef>.

You may also use C<defined()> to check whether a subroutine exists, by
saying C<defined &func> without parentheses.  On the other hand, use
of C<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 L</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 C<defined()>, and then are surprised to
discover that the number C<0> and C<""> (the zero-length string) are, in fact,
defined values.  For example, if you say

    "ab" =~ /a(.*)b/;

The pattern match succeeds, and C<$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 C<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 C<defined()> only when you're questioning the integrity of what
you're trying to do.  At other times, a simple comparison to C<0> or C<""> is
what you want.

Currently, using C<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 C<undef()> on these, however, does clear their memory and then report
them as not defined anymore, but you shouldn'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.  The normal way to 
free up space used by an aggregate is to assign the empty list.

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

See also L</undef>, L</exists>, L</ref>.

=back