Search code examples
perlperlguts

Why are Perl's canonical "true" and "false" values PVNV internally?

Perl has a canonical "true" and "false" value which it uses for cases of Boolean negation with ! or not.

#!/usr/bin/env perl

use strict;
use warnings;

use Data::Dumper;

print Dumper !1;    # outputs $VAR1 = '';
print Dumper !!1;   # outputs $VAR1 = 1;

All good so far.

But, when I look at the values with Devel::Peek it is clear that they are not equivalent to literal '' and 1, they are both SV PVNV values.

$ perl -MDevel::Peek -E 'Dump ""; Dump !1;' 2>&1 | grep '^SV'
SV = PV(0x15e5140) at 0x1603298
SV = PVNV(0x15e3010) at 0x7814b0

$ perl -MDevel::Peek -E 'Dump 1; Dump !!1;' 2>&1 | grep '^SV'
SV = IV(0xfce228) at 0xfce238
SV = PVNV(0xfae030) at 0x7819f0

I have tested this with Perl 5.16.3 and Perl 5.20.0.

Isn't a PVNV much larger than a simple IV? Is this a case of Perl trading memory for speed?

Solution

  • Not just boolean negation; these values are returned by many builtins.

    There is really no issue of memory, since there is only one instance of each of them.

    The false value in particular must be a PVNV since its string value '' and its numeric value 0 are not derivable from each other (at least without triggering a warning).