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

- NAME
- SYNOPSIS
- DESCRIPTION

# 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(&func)` to check whether subroutine `&func` has ever been defined. The return value is unaffected by any forward declarations of `&func`. Note that a subroutine which 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 the perlsub manpage.

Use of `defined` on aggregates (hashes and arrays) is deprecated. It used to report whether memory for that aggregate has 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 in the perlfunc manpage 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''. 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 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.

See also undef in the perlfunc manpage, exists in the perlfunc manpage, ref in the perlfunc manpage.

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