PERL Miscellaneous - FAQ 4.67 Why does passing a subroutine an undefined element in a hash create it?

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FAQ 4.67 Why does passing a subroutine an undefined element in a hash create it?

PerlFAQ Server

2005-07-27, 5:05 pm

This message is one of several periodic postings to comp.lang.perl.misc
intended to make it easier for perl programmers to find answers to
common questions. The core of this message represents an excerpt
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4.67: Why does passing a subroutine an undefined element in a hash create it?

If you say something like:

somefunc($hash{"nonesuch key here"});

Then that element "autovivifies"; that is, it springs into existence
whether you store something there or not. That's because functions get
scalars passed in by reference. If somefunc() modifies $_[0], it has to
be ready to write it back into the caller's version.

This has been fixed as of Perl5.004.

Normally, merely accessing a key's value for a nonexistent key does
*not* cause that key to be forever there. This is different than awk's
behavior.

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Paul Lalli

2005-07-27, 5:05 pm

PerlFAQ Server wrote:
> 4.67: Why does passing a subroutine an undefined element in a hash create it?
>
> If you say something like:
>
> somefunc($hash{"nonesuch key here"});
>
> Then that element "autovivifies"; that is, it springs into existence
> whether you store something there or not. That's because functions get
> scalars passed in by reference. If somefunc() modifies $_[0], it has to
> be ready to write it back into the caller's version.
>
> This has been fixed as of Perl5.004.
>
> Normally, merely accessing a key's value for a nonexistent key does
> *not* cause that key to be forever there. This is different than awk's
> behavior.

Er, if this was fixed a good 5 versions ago, is this FAQ really still
relevant? Are there really that many people still using anything less
than 5.004? (And if there are, and they have a problem about things
not working, shouldn't the answer be "Upgrade your Perl"?)

Paul Lalli

James Taylor

2005-07-28, 9:09 am

In article <1122489468.920709.91510@g43g2000cwa.googlegroups.com>,
Paul Lalli <mritty@gmail.com> wrote:
>
> PerlFAQ Server wrote:
>
> Er, if this was fixed a good 5 versions ago, is this FAQ
> really still relevant? Are there really that many people
> still using anything less than 5.004? (And if there are,
> and they have a problem about things not working,
> shouldn't the answer be "Upgrade your Perl"?)

Upgrading their Perl version may not be an option. It
certainly wouldn't be a practical option for me. Thankfully,
the RISC OS port by Nicholas Clark, is up to 5.005_03 but
there are plenty of newer Perl features that I'm stuck not
being able to use. Being unsympathetically told to upgrade
wouldn't be helpful.

I think this FAQ entry should stand as it helps anyone stuck
with an earlier version to discover the potential problem,
helps us all be more mindful of autovivification traps, and
gives people who *can* upgrade a good reason to do so.

--
James Taylor, London, UK PGP key: 3FBE1BF9
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