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perlfunc:defined

by gods
on Aug 24, 1999 at 22:42 UTC ( #270=perlfunc: print w/replies, xml ) Need Help??

defined

See the current Perl documentation for defined.

Here is our local, out-dated (pre-5.6) version:


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



defined EXPR

defined



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.


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[ambrus]: Hopefull the object isn't kept alive, the events are processed immediately, but you'd have to read a lot of source code to be sure about that.
[Corion]: ambrus: I think both of AnyEvent and Prima are pretty tight in their memory management because they both are cooperative multitasking and (I think) both use the Perl memory management for managing things
[Corion]: ambrus: And for Windows, I don't think that Prima knows if there still are messages queued for an object (in the Windows message loop). Finding that out would take lots of effort for little gain
[ambrus]: And even if this works, I'm still not sure you can't get double timeouts from a Timer.
[ambrus]: Corion: well Prima::Object says something like that the cleanup method will send an onDestory message and that you can't get more messages after cleanup, or something.
[Corion]: ambrus: Yeah - I don't think the deep source dive will be necessary if things are implemented as simple as they could be :)) And hopefully I won't need (more) timely object destruction. I can update the screen at 60Hz and hopefully even do HTTP ...
[Corion]: ... transfers in the background. Now that I think about it, this maybe even means that I can run the OpenGL filters on Youtube input :)
[ambrus]: Corion: I mentioned that the unix event loop of Prima always wakes up at least once every 0.2 seconds. Have you found out whether the win32 event loop of Prima does that too?
[Corion]: ambrus: Hmm - I would assume that the onDestroy message is sent from the destructor and doesn't go through the messageloop, but maybe it is sent when a window gets destroyed but all components are still alive...
[ambrus]: Corion: partly deep source dive, partly just conservative coding even if it adds an overhead.

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