in reply to Re^2: HASH keys preserve class ?
in thread HASH keys preserve class ?

First you say keys are scalars not strings, then you say there might be a way of allowing things other than strings.

Except possibly through an XS trick, keys can only be strings. perldata confirms this: "Hashes are unordered collections of scalar values indexed by their associated string key." If you try to use a reference as a key, the reference will not be a key of the hash. It's string representation will.

I guess your point was that you could attempt to use a non-string as a key. That is indeed valid. The scalar will be stringified before being used as a key.

The following is an attempt to use a reference as a key in a hash: 

use strict;
use warnings;

my @var = qw( a b c );
my $ref = \@var;

my %hash;
$hash{$ref} = 1;

foreach (keys %hash) {
   print @$_, "\n";
}

[download]

In the latest Perl (5.8.8), the above code outputs 

Can't use string ("ARRAY(0x225fb8)") as an ARRAY ref while "strict ref
+s" in use

[download]

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Re^4: HASH keys preserve class ?
by Limbic~Region (Chancellor) on Oct 04, 2006 at 15:38 UTC
    ikegami,
    First you say keys are scalars not strings

    Hrmm. I guess I should have proofed my response a 4th time before submitting. The first paragraph was intended to address "values" and the second "keys". Where I said "...hash key values can only be scalars.", I meant hash values. In fact, I said that later on in the same paragraph "...so a hash value can...".

    Regarding non-string keys - I assume some trickery that can only be done in XS land would get around perldata. Of course, I don't remember who I heard discussing it or if I understood what they were saying but I suspect it would have been diotalevi or perhaps demerphq.

    In any case, my correction to your post was concerning values only. Sorry my incorrect wording implied otherwise.

    Cheers - L~R

[reply]

      It's certainly possible to reverse the stringyfication of references. 

      #! perl -slw
use strict;
use Data::Dumper;
use Devel::Pointer;


my @a = 1.. 10;
my %b = 'a'..'z';
my $c = \ 'A readonly scalar';
my $d = 'a scalar';
my %hash;
$hash{ $_ } = 1 for \( @a, %b, $c, $d );

for ( keys %hash ) {
    m[([^(]+)\(([^)]+)\)]
    and   $1 eq 'HASH'  ? print( Dumper unsmash_hv( oct( $2 ) ) )
        : $1 eq 'ARRAY' ? print( Dumper unsmash_av( oct( $2 ) ) )
        : $1 eq 'REF'   ? print( ${ unsmash_sv( oct( $2 )) } )
        : $1 eq 'SCALAR'? print( unsmash_sv( oct( $2 ) ) )
        : print "Unknown type '$1'";
}
__END__
C:\test>junk9
A readonly scalar
$VAR1 = [
          1,
          2,
          3,
          4,
          5,
          6,
          7,
          8,
          9,
          10
        ];

a scalar
$VAR1 = {
          'w' => 'x',
          'e' => 'f',
          'a' => 'b',
          'm' => 'n',
          's' => 't',
          'y' => 'z',
          'u' => 'v',
          'c' => 'd',
          'k' => 'l',
          'q' => 'r',
          'g' => 'h',
          'i' => 'j',
          'o' => 'p'
        };

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      Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
      Lingua non convalesco, consenesco et abolesco. -- Rule 1 has a caveat! -- Who broke the cabal?
      "Science is about questioning the status quo. Questioning authority".
      In the absence of evidence, opinion is indistinguishable from prejudice.
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