Re^2: how ! operator works

This "False" value is basically the same as

use Scalar::Util qw/dualvar/;
dualvar(0, '');
[download]

Which stems from the curious fact that a Perl scalar internally has a slot for both a numeric and a string value. By explicitly providing a numeric value, you don't get a warning when used as a number:

$ perl -wE 'say 1 + ""'
Argument "" isn't numeric in addition (+) at -e line 1.
1
$ perl -MScalar::Util=dualvar -wE 'say dualvar(0, "") + 1'
1
[download]

Note that when abused, these dualvars can be very confusing:

$ perl -MScalar::Util=dualvar -E 'my $x = dualvar(42, "23"); say $x; s
+ay $x + 1'
23
43
[download]

Devel::Peek shows that the False value is a wee bit more special than that:

perl -MDevel::Peek -MScalar::Util=dualvar -e 'Dump !1; Dump dualvar(0,
+ "")'
SV = PVNV(0x254ff70) at 0x7409c0
  REFCNT = 2147483647
  FLAGS = (IOK,NOK,POK,READONLY,pIOK,pNOK,pPOK)
  IV = 0
  NV = 0
  PV = 0x254ff50 ""\0
  CUR = 0
  LEN = 8
SV = PVNV(0x2550d90) at 0x2551220
  REFCNT = 1
  FLAGS = (TEMP,IOK,POK,pIOK,pPOK)
  IV = 0
  NV = 0
  PV = 0x255f610 ""\0
  CUR = 0
  LEN = 8
[download]

So it's not only a dualvar, but it's also readonly, and has a very high refcount - it's basically a singleton.

Perl 6 - second systems done right

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Re^3: how ! operator works by arkturuz (Curate) on Feb 04, 2011 at 09:12 UTC
That refcount (2147483647) looks strangely familiar. Is that a coincidence or just a hack in the interpreter so it (the object in question) never gets destroyed by the garbage collector (theoretically)?	[reply]
Re^4: how ! operator works by bart (Canon) on Feb 04, 2011 at 14:20 UTC
That refcount (2147483647) looks strangely familiar. Just print it out in hex and you'll see: 7FFFFFFF which is a clear indication that this is the largest positive integer you can stuff into a signed 32 bit integer (2's complement). So yeah, your guess is very likely correct.	[reply]
Re^4: how ! operator works by ikegami (Patriarch) on Feb 04, 2011 at 20:48 UTC
So I looked at the code to answer your question. It looks like it's a hack, not a magical value. The refcount can go down, but it gets reset back to the max for SvIMMORTAL variables (the true, the false, the undef and a special internal thing called "placeholder").	[reply]
Re^5: how ! operator works by arkturuz (Curate) on Feb 04, 2011 at 22:23 UTC
So I looked at the code to answer your question. Thanks! I searched the code myself today with no luck. I postoponed it for the weekend but now you have resolved this, I'll just have to stick to something else to do :)	[reply]
Re^6: how ! operator works by ikegami (Patriarch) on Feb 04, 2011 at 23:32 UTC
Re^4: how ! operator works by moritz (Cardinal) on Feb 04, 2011 at 09:44 UTC
I guess it's a hack, but I'm too lazy to check the source :-) Perl 6 - second systems done right	[reply]
Re^3: how ! operator works by !1 (Hermit) on Feb 04, 2011 at 18:53 UTC
...what'd you just call me?	[reply]


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