Полезная информация


NAME

defined - test whether a value, variable, or function is defined


SYNOPSIS

defined EXPR

defined


DESCRIPTION

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() to check whether a subroutine exists, by saying defined &func without parentheses. 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 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 defined() on aggregates may be changed, fixed, or broken in a future release of Perl.

See also undef, exists, ref.


DISCLAIMER

We are painfully aware that these documents may contain incorrect links and misformatted HTML. Such bugs lie in the automatic translation process that automatically created the hundreds and hundreds of separate documents that you find here. Please do not report link or formatting bugs, because we cannot fix per-document problems. The only bug reports that will help us are those that supply working patches to the installhtml or pod2html programs, or to the Pod::HTML module itself, for which I and the entire Perl community will shower you with thanks and praises.

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--Tom Christiansen, Perl Documentation Compiler and Editor


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