FAQ 4.54 Why does defined() return true on empty arrays and hashes?

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4.54: Why does defined() return true on empty arrays and hashes?

The short story is that you should probably only use defined on scalars
or functions, not on aggregates (arrays and hashes). See "defined" in
perlfunc in the 5.004 release or later of Perl for more detail.



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Re: FAQ 4.54 Why does defined() return true on empty arrays and

I see the rules in perlfun:

perldoc -tf defined:

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:

But lost in the difference below:


case 1:

#! /bin/perl -l
print defined @a ? "defined--acutally means memory allocated" :
"undefined";

__END__
undefined


case 2:

#! /bin/perl -l
@a=undef;
print defined @a ? "defined--acutally means memory allocated" :
"undefined";

__END__
defined--acutally means memory allocated


case 3:
#! /bin/perl -l
@a=();
print defined @a ? "defined--acutally means memory allocated" :
"undefined";

__END__
undefined


Why case 2 is different from case 3? Does it means defined on
aggregator is totally meaningless in modern Perl?


-Todd

Re: FAQ 4.54 Why does defined() return true on empty arrays and

Quoth Todd :
>
> I see the rules in perlfun:
>
> perldoc -tf defined:
>
> 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:
>
> But lost in the difference below:
>
> case 1:
>
> #! /bin/perl -l
> print defined @a ? "defined--acutally means memory allocated" :
> "undefined";
>
> __END__
> undefined
>
> case 2:
>
> #! /bin/perl -l
> @a=undef;

This puts one element (undef) into @a. scalar(@a) at this point is 1.

> print defined @a ? "defined--acutally means memory allocated" :
> "undefined";
>
> __END__
> defined--acutally means memory allocated
>
>
> case 3:
> #! /bin/perl -l
> @a=();

This has no effect: @a was empty before, and is still empty afterwards.
defined on an array currently returns true if the array has at least one
element, or the array has 'get' magic, or the array is tied. It tells
you nothing about whether the underlying AV has actually been allocated
or not, because just referring to @a is enough to make sure that it has
been.

> print defined @a ? "defined--acutally means memory allocated" :
> "undefined";
>
> __END__
> undefined
>
> Why case 2 is different from case 3? Does it means defined on
> aggregator is totally meaningless in modern Perl?

Please don't use defined on aggregates. It doesn't tell you anything
useful, and may at some point change its behaviour. That's why it says
not to use it in perlfunc. Use scalar(@a) or scalar(keys %a) instead.

Ben

Re: FAQ 4.54 Why does defined() return true on empty arrays and

Thanks, Ben. You make it clear.

So it does have real & useful meaning when operate on aggregators. :-(

-Todd

Re: FAQ 4.54 Why does defined() return true on empty arrays and

Ben Morrow wrote:
> > @a=undef;
>
> This puts one element (undef) into @a. scalar(@a) at this point is 1.

I'm affected by lisp language: where `()' and `nil' are same:-(

-Todd

Re: FAQ 4.54 Why does defined() return true on empty arrays and hashes?

>>>>> "Todd" == Todd writes:

Todd> Ben Morrow wrote:
>> > @a=undef;
>>
>> This puts one element (undef) into @a. scalar(@a) at this point is 1.

Todd> I'm affected by lisp language: where `()' and `nil' are same:-(

So the first thing to learn is that Lisp's "nil" isn't Perl's "undef".
Get over that, and you wouldn't have made that mistake.

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