Dereferencing woes

shemp has asked for the wisdom of the Perl Monks concerning the following question:

Hi all, I have been a bit stumped about something for a while. Often in scripts, i'll have large hashrefs or arrayrefs, and pass them around as refs, to avoid memory consumption. But there are some things i end up doing with them, where i can never tell if im making deep copies or not. (i want to avoid this usually, for memory issues).
Take the following code for instance:

sub get_big_arrayref {
   # blah blah blah

   return \@big_array;
}

{
   ...
   my $big_arrayref = get_big_arrayref();

   foreach my $value (@$big_arrayref) {
       # whatever
   }
}
[download]

Now my concern is that when writing @$big_arrayref, that dereferences the array, and makes a copy. In and of itself, foreach() hasnt made a copy since old versions of perl, but that deref will.
I can get around the problem here with:

   for my $i (0 .. $#$big_arrayref) {
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Or at least i think this doesnt do any sublte dereferencing.

But, what happens when i have a big hashref. To use any of the hash traversal built-in functions, such as each, key, values, etc, the arguement must be a hash, not a hashref. So again, the big ref is dereferenced with something like:

   while ( my($key, $value) = each %$big_hashrerf ) {
[download]

And i dont see any way around this problem.

So, what i'm looking for:

Is my understanding of the dereferencing correct
Based on that answer, am i missing the boat on how to do some of this stuff

Thanks much for any insight, i hope im not too far off base.

Comment on Dereferencing woes Select or Download Code

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Re: Dereferencing woes by davido (Cardinal) on May 05, 2004 at 19:20 UTC
You don't get a copy when you dereference an array ref. But perhaps you're thinking of the anonymous array constructor, which will create a copy. For example: `my @array = ( 1, 2, 3 ); my $aref = \@array;` [download] The above example doesn't create a copy of @array. But the following will: `my @array = ( 1, 2, 3 ); my $aref = [ @array ];` [download] This may be desirable behavior. There are times, for example, where you really do need a copy to be made so that you're pointing to a new array rather than to the old one. Another construct to consider when using hashes is using keys in list context. For example: `foreach my $key ( keys %hash ) { ........ }` [download] IIRC, keys in list context will create a list of the keys, which means that in addition to your hash, you've also got in memory a temporary list of its keys; that could be a memory hog. If that's a concern, use the `while ( my( $key, $val ) = each %hash ) { ........ }` construct, since this shouldn't cause a list to be generated. So as the previous followup suggest, don't worry about it, it's not creating a copy. You do have to be concerned with things like sort though. Even if you're saying `@array = sort @array` sort is probably creating a temporary list somewhere to assign to @array. The assignment is done in one fell swoop, so the entire list must be created first. Dave	[reply] [d/l] [select]
Re: Re: Dereferencing woes by dave_the_m (Monsignor) on May 05, 2004 at 21:03 UTC
Even if you're saying @array = sort @array sort is probably creating a temporary list somewhere to assign to @array. From Perl 5.8.4 onwards, that particular construct is optimised to sort the array in-place, so no copying is done.	[reply]
Re: Dereferencing woes by japhy (Canon) on May 05, 2004 at 18:55 UTC
Rest easy: you're wrong. There is no copy being made. Update: what the dereferencing is doing is just telling Perl to use the data structure stored in the reference. It's not copying it anywhere. _____________________________________________________ Jeff`[japhy]`Pinyan: Perl, regex, and perl hacker, who'd like a job (NYC-area) `s++=END;++y(;-P)}y js++=;shajsj<++y(p-q)}?print:??;`	[reply]