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:

Thanks in advance for any insights.

Liz

In reply to Using SV as key in hash by liz

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