Question

I was reading How can I access the ref count of a Perl hash?, and there both Devel::Refcount::refcount and Devel::Peek::SvREFCNT are suggested.

But they don’t return the same reference counts. Why is that?

Here a modified example from perldoc Devel::Refcount:

use Devel::Peek;
use Devel::Refcount;

my $anon = [];

printf "Anon ARRAY $anon has %d/%d reference\n",
    Devel::Refcount::refcount($anon),
    Devel::Peek::SvREFCNT($anon);

my $otherref = $anon;

printf "Anon ARRAY $anon now has %d/%d references\n",
    Devel::Refcount::refcount($anon),
    Devel::Peek::SvREFCNT($anon);

which prints out:

Anon ARRAY ARRAY(0x8b10818) has 1/1 reference
Anon ARRAY ARRAY(0x8b10818) now has 2/1 references

Notice the last 2/1 discrepancy…

(If it turns out I’m not doing something stupid, I’ll add a link from How can I access the ref count of a Perl hash? to here)

Answer 1

I can’t say that I grok all of it yet, but your question is answered prominently in the Devel::Refcount perldoc

COMPARISON WITH SvREFCNT

This function differs from Devel::Peek::SvREFCNT in that SvREFCNT() gives the reference count of the SV object itself that it is passed, whereas refcount() gives the count of the object being pointed to. This allows it to give the count of any referent (i.e. ARRAY, HASH, CODE, GLOB and Regexp types) as well.

Consider the following example program:

 use Devel::Peek qw( SvREFCNT );
 use Devel::Refcount qw( refcount );

 sub printcount
 {
    my $name = shift;

    printf "%30s has SvREFCNT=%d, refcount=%d\n",
       $name, SvREFCNT($_[0]), refcount($_[0]);
 }

 my $var = [];

 printcount 'Initially, $var', $var;

 my $othervar = $var;

 printcount 'Before CODE ref, $var', $var;
 printcount '$othervar', $othervar;

 my $code = sub { undef $var };

 printcount 'After CODE ref, $var', $var;
 printcount '$othervar', $othervar;

This produces the output

            Initially, $var has SvREFCNT=1, refcount=1
      Before CODE ref, $var has SvREFCNT=1, refcount=2
                  $othervar has SvREFCNT=1, refcount=2
       After CODE ref, $var has SvREFCNT=2, refcount=2
                  $othervar has SvREFCNT=1, refcount=2

Here, we see that SvREFCNT() counts the number of references to the SV object passed in as the scalar value – the $var or $othervar respectively, whereas refcount() counts the number of reference values that point to the referent object – the anonymous ARRAY in this case.

Before the CODE reference is constructed, both $var and $othervar have SvREFCNT() of 1, as they exist only in the current lexical pad. The anonymous ARRAY has a refcount() of 2, because both $var and $othervar store a reference to it.

After the CODE reference is constructed, the $var variable now has an SvREFCNT() of 2, because it also appears in the lexical pad for the new anonymous CODE block.