| CARVIEW |
- #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
undefto indicate failure, end of file, system error, uninitialized variable, and other exceptional conditions. This function allows you to distinguishundeffrom other values. (A simple Boolean test will not distinguish amongundef, zero, the empty string, and"0", which are all equally false.) Note that sinceundefis a valid scalar, its presence doesn't necessarily indicate an exceptional condition:popreturnsundefwhen its argument is an empty array, or when the element to return happens to beundef.You may also use
defined(&func)to check whether subroutinefunchas ever been defined. The return value is unaffected by any forward declarations offunc. A subroutine that is not defined may still be callable: its package may have anAUTOLOADmethod that makes it spring into existence the first time that it is called; see perlsub.Use of
definedon aggregates (hashes and arrays) is no longer supported. It used to report whether memory for that aggregate had ever been allocated. 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
existsfor 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
definedand are then surprised to discover that the number0and""(the zero-length string) are, in fact, defined values. For example, if you say"ab" =~ /a(.*)b/;The pattern match succeeds and
$1is 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 usedefinedonly when questioning the integrity of what you're trying to do. At other times, a simple comparison to0or""is what you want.
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