Using SV as key in hash

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

For my new Data::Reuse module (now on CPAN), I'm looking for a way to create a hash entry in which both the value as well as the key point to the same SV, or more general: set the hash key to a specific SV.

From what I've been able to learn from looking at a bunch of XS code, and [hs]v.[ch], the following code should do the trick:

#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"

#include "ppport.h"

MODULE = Data::Reuse            PACKAGE = Data::Reuse

void
key_as_sv( hv, key, val )
    HV *hv
    SV *key
    SV *val
    PROTOTYPE: \%$$
    CODE:
        SvREFCNT_inc(key);
        SvREFCNT_inc(val);
        STRLEN length = SvCUR(key);
        hv_store( hv, SvPV( key, length ), length , val, 0 );
        HE *he = hv_fetch_ent( hv, key, 0, 0 );
        HeSVKEY_set( he, key );
[download]

which would then be called thus:

my %hash;
my $foo = 'foo';
Data::Reuse::key_as_sv( %hash, $foo, $foo );
print STDERR "found ".(keys %hash)." keys:\n";
print STDERR " $_: $hash{$_}\n" foreach keys %hash;
[download]

However, this produces the following output:

found 1 keys:
 foo:
[download]

or, in other words, the value of the key is gone.

I guess my question to any XS savvy Perl Monk is:

does it make sense to try to save memory this way?
if so, why doesn't the above work?

Thanks in advance for any insights.

Liz

Comment on Using SV as key in hash Select or Download Code

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Re: Using SV as key in hash by Fletch (Bishop) on Jul 17, 2006 at 01:41 UTC
Gratuitous handwaving here since it's been ages since I mucked with XS, but . . . I want to say that the keys to an `HV` are always stringified and kept as `char`, not as `SV`s. A good starting place might be to look at what Tie::RefHash does and then implement something similar in your XS (possibly something like keeping your own internal mapping of `char` to original `SV`).	[reply]
Re^2: Using SV as key in hash by liz (Monsignor) on Jul 17, 2006 at 06:16 UTC
I want to say that the keys to an HV are always stringified and kept as char, not as SVs. Well, that's just it. There is a provision in hv.c to actually have an SV as the key (rather than a char ). The `HeSVKEY_set` macro is supposed to be doing that. A good starting place might be to look at what Tie::RefHash does* I'm afraid that Tie::RefHash doesn't have any XS component to obtain any inspiriation from ;-( Liz	[reply] [d/l]
Re: Using SV as key in hash by grinder (Bishop) on Jul 17, 2006 at 08:11 UTC
I don't think you need to play with `HeSVKEY_set` directly. Assuming `key` is an SV, why not use `hv_store_ent` instead? `CODE: SvREFCNT_inc(key); SvREFCNT_inc(val); hv_store_ent( hv, key, val, 0 );` [download] • another intruder with the mooring in the heart of the Perl	[reply] [d/l]
Re^2: Using SV as key in hash by liz (Monsignor) on Jul 17, 2006 at 08:52 UTC
Hmmm... Are you implying that by default key values are already shared? As this code in sv.c seems to imply: `HV * Perl_newHV(pTHX) { register HV hv; register XPVHV xhv; hv = (HV)NEWSV(502,0); sv_upgrade((SV )hv, SVt_PVHV); xhv = (XPVHV)SvANY(hv); SvPOK_off(hv); SvNOK_off(hv); #ifndef NODEFAULT_SHAREKEYS HvSHAREKEYS_on(hv); / key-sharing on by default / #endif` [download] But I assume then that what is being shared, are char rather than SV's? And sharing between different hashes doesn't really solve my optimization "problem", as I only want to use one hash. It's just that in many cases the value is the same as the key, and that it would therefore make sense to have them share the same SV. Liz	[reply] [d/l]
Re: Using SV as key in hash by DrWhy (Chaplain) on Jul 17, 2006 at 19:17 UTC
Not really a solution, but a suggestion for finding out what is really going on here. Try using Data::Dumper to dump your test hash. It could be that the key does not end up being what you expect it to be. --DrWhy "If God had meant for us to think for ourselves he would have given us brains. Oh, wait..."	[reply]