Overload + reblessing oddity

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

(This is with 5.8.8 on OSX.)

I'm working on getting DBM::Deep to properly handle a edgecase and I ran into an edgecase with overload and re-blessing objects. The code below demonstrates the problem.

{
    package Null;
    use overload
        bool => sub { undef },
        '""' => sub { undef },
        '0+' => sub { undef },
        fallback => 1,
        nomethod => 'AUTOLOAD';

    sub AUTOLOAD { return; }
}

{
    package Foo;
    sub new { return bless { a => 'b' }, shift; }
    sub apple { 'pear' }
}

my %x = (
    foo => Foo->new,
);
my $y = $x{foo};
print "Y1: $y\n";

bless $x{foo}, 'Null';
%{$x{foo}} = ();

print "Y2: $y\n";
my $z = $x{foo};
print "Z: $z\n";
[download]

What I'm seeing:

Y1: Foo=HASH(0x1801234)
Y2: Null=HASH(0x1801234)
Z:
[download]

I would expect Y2 and Z to be the same. It's as if the overloading doesn't happen unless the SV is created after the blessing. Reblessing doesn't seem to trigger the overloading. Am I missing something?

For the record, what I'm trying to do is fix the following edgecase:

$db->{foo} = { a => 'b' };
my $x = $db->{foo};
my $y = $db->{foo};
ok( $x == $y ); # This is broken in 1.0004 and before, but fixed in SV
+N.

delete $db->{foo};
is( $x, undef ); # This works
is( $x + 0, 0 ); # This does not
[download]

If someone has an idea of how to fix the underlying issue, I'm all ears.

My criteria for good software:

Does it work?
Can someone else come in, make a change, and be reasonably certain no bugs were introduced?

Comment on Overload + reblessing oddity Select or Download Code

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Re: Overload + reblessing oddity by kyle (Abbot) on Sep 29, 2007 at 02:41 UTC
Devel::Peek confirms the difference: `use Devel::Peek; #...etc... bless $x{foo}, 'Null'; print Dump( $y ); print Dump( $x{foo} );` [download] `$y` has `FLAGS = (PADBUSY,PADMY,ROK)` and `$x{foo}` has `FLAGS = (ROK,OVERLOAD)`. Fuller output: SV = RV(0x1822a6c) at 0x181c38c # $y REFCNT = 1 FLAGS = (PADBUSY,PADMY,ROK) RV = 0x18014a0 SV = PVHV(0x1821f90) at 0x18014a0 REFCNT = 2 FLAGS = (OBJECT,SHAREKEYS) IV = 1 NV = 0 STASH = 0x180b584 "Null" ARRAY = 0x301120 (0:7, 1:1) hash quality = 100.0% KEYS = 1 FILL = 1 MAX = 7 RITER = -1 EITER = 0x0 Elt "a" HASH = 0xca2e9442 SV = PV(0x1801678) at 0x181c4dc REFCNT = 1 FLAGS = (POK,pPOK) PV = 0x301110 "b"\0 CUR = 1 LEN = 2 SV = RV(0x1822a74) at 0x181c4f4 # $x{foo} REFCNT = 1 FLAGS = (ROK,OVERLOAD) RV = 0x18014a0 SV = PVHV(0x1821f90) at 0x18014a0 REFCNT = 2 FLAGS = (OBJECT,SHAREKEYS) IV = 1 NV = 0 STASH = 0x180b584 "Null" ARRAY = 0x301120 (0:7, 1:1) hash quality = 100.0% KEYS = 1 FILL = 1 MAX = 7 RITER = -1 EITER = 0x0 Elt "a" HASH = 0xca2e9442 SV = PV(0x1801678) at 0x181c4dc REFCNT = 1 FLAGS = (POK,pPOK) PV = 0x301110 "b"\0 CUR = 1 LEN = 2 [download] I notice that the `OVERLOAD` flag is on the scalar itself rather than the reference it's holding. The reference knows that its an object but not that it's overloaded. I guess to get `$y` to be overloaded, you'd have to rebless it directly. That said, given a reference, I don't know how you'd find other scalars that have the reference. Update: See also Is it possible to obtain all hardreferences to some memory address by knowing only one of them?	[reply] [d/l] [select]

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Re: Overload + reblessing oddity
by kyle (Abbot) on Sep 29, 2007 at 02:41 UTC

Devel::Peek confirms the difference:

use Devel::Peek;
#...etc...
bless $x{foo}, 'Null';

print Dump( $y );
print Dump( $x{foo} );
[download]

$y has FLAGS = (PADBUSY,PADMY,ROK) and $x{foo} has FLAGS = (ROK,OVERLOAD).

Fuller output:

SV = RV(0x1822a6c) at 0x181c38c        # $y
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,ROK)
  RV = 0x18014a0
  SV = PVHV(0x1821f90) at 0x18014a0
    REFCNT = 2
    FLAGS = (OBJECT,SHAREKEYS)
    IV = 1
    NV = 0
    STASH = 0x180b584   "Null"
    ARRAY = 0x301120  (0:7, 1:1)
    hash quality = 100.0%
    KEYS = 1
    FILL = 1
    MAX = 7
    RITER = -1
    EITER = 0x0
    Elt "a" HASH = 0xca2e9442
    SV = PV(0x1801678) at 0x181c4dc
      REFCNT = 1
      FLAGS = (POK,pPOK)
      PV = 0x301110 "b"\0
      CUR = 1
      LEN = 2
SV = RV(0x1822a74) at 0x181c4f4        # $x{foo}
  REFCNT = 1
  FLAGS = (ROK,OVERLOAD)
  RV = 0x18014a0
  SV = PVHV(0x1821f90) at 0x18014a0
    REFCNT = 2
    FLAGS = (OBJECT,SHAREKEYS)
    IV = 1
    NV = 0
    STASH = 0x180b584   "Null"
    ARRAY = 0x301120  (0:7, 1:1)
    hash quality = 100.0%
    KEYS = 1
    FILL = 1
    MAX = 7
    RITER = -1
    EITER = 0x0
    Elt "a" HASH = 0xca2e9442
    SV = PV(0x1801678) at 0x181c4dc
      REFCNT = 1
      FLAGS = (POK,pPOK)
      PV = 0x301110 "b"\0
      CUR = 1
      LEN = 2
[download]

I notice that the OVERLOAD flag is on the scalar itself rather than the reference it's holding. The reference knows that its an object but not that it's overloaded. I guess to get $y to be overloaded, you'd have to rebless it directly. That said, given a reference, I don't know how you'd find other scalars that have the reference.

[reply]
[d/l]
[select]