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
funchas ever been defined. The return value is unaffected by any forward declarations of
func. A subroutine that is not defined may still be callable: its package may have an
AUTOLOADmethod that makes it spring into existence the first time that it is called; see perlsub.
Use of defined on 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:
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.
Note: Many folks tend to overuse defined and are then surprised to discover that the number
""(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 use defined only when questioning the integrity of what you're trying to do. At other times, a simple comparison to
""is what you want.