The number of references to a variable

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

Replies are listed 'Best First'.
Re: The number of references to a variable by Corion (Patriarch) on Nov 26, 2007 at 11:50 UTC
I tought there was a function in Scalar::Util that returns the refcount of an object, but I couldn't find it. But also in Scalar::Util, you find the `weaken` function, which lets you pass around weak references. From your description, I think you have a "master" object that holds all "relevant" children, but over time, some of the children become irrelevant yet are kept alive by some references. If you make your master hold strong (normal) references but pass out only weak references, your children will be removed if the master removes them from the list. Such an approach will have different problems, because weak references can turn into strong references again if you copy them, unfortunately. Maybe another approach would be to (always) ask the master whether a child is still relevant or not ...	[reply] [d/l]
Re^2: The number of references to a variable by jeanluca (Deacon) on Nov 26, 2007 at 14:25 UTC
I'm just curious, is the only way to make a weak reference strong, by copying it ? `$y = \($$y) ;` [download] LuCa	[reply] [d/l]
Re^3: The number of references to a variable by ikegami (Patriarch) on Nov 26, 2007 at 17:07 UTC
I wouldn't call that copying. Your snippet creates a new reference. And yes, that works.	[reply]
Re: The number of references to a variable by lima1 (Curate) on Nov 26, 2007 at 12:29 UTC
The SvREFCNT macro (perlguts) returns this number. I am sure this is already somewhere on cpan, but a simple XS routine could look like this: `/* generated by h2xs -c Scalar::RefCount / #include "EXTERN.h" #include "perl.h" #include "XSUB.h" #include "ppport.h" MODULE = Scalar::RefCount PACKAGE = Scalar::RefCount int refcount(s) SV s PROTOTYPE: $ CODE: RETVAL = SvREFCNT(s); OUTPUT: RETVAL` [download] `use Test::More tests => 6; BEGIN { use_ok('Scalar::RefCount') }; ######################### my $a; is(Scalar::RefCount::refcount($a),1, 'return 1'); my $b = \$a; is(Scalar::RefCount::refcount($a),2, 'return 2'); my $c = \$a; is(Scalar::RefCount::refcount($a),3, 'return 3'); { my $d = \$a; is(Scalar::RefCount::refcount($a),4, 'return 4'); } is(Scalar::RefCount::refcount($a),3, 'return 3');` [download] Update: Ah, thanks cdarke.	[reply] [d/l] [select]
Re^2: The number of references to a variable by cdarke (Prior) on Nov 26, 2007 at 13:12 UTC
SvREFCNT is exposed in Devel::Peek (no need to go to CPAN), which is a bit easier to use than embedding XS. For example: `use Devel::Peek qw(SvREFCNT); $x = 42; $y = \$x; $refs_for_x = SvREFCNT($x); print "$refs_for_x\n";` [download] Prints 2.	[reply] [d/l]
Re: The number of references to a variable by jbert (Priest) on Nov 26, 2007 at 11:54 UTC
If you want objects which aren't referenced to go away, you could use store a 'weak' reference to each of your objects in your array, and the weak reference will become undef if there are no 'real' references left to the object. That might do what you need. (See Scalar::Util for weaken). Otherwise, it seems an unusual route to go down and that makes me think it might not be the right path. Could you let us know more about your motivation for this, so we can offer an opinion on this?	[reply]
Re: The number of references to a variable by BrowserUk (Patriarch) on Nov 26, 2007 at 17:02 UTC
See Internals`::GetRefCount($object)` for direct access to refcounts without writing your own XS. Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". In the absence of evidence, opinion is indistinguishable from prejudice. "Too many [] have been sedated by an oppressive environment of political correctness and risk aversion."	[reply] [d/l]
Re: The number of references to a variable by Somni (Friar) on Nov 27, 2007 at 13:34 UTC
As has been mentioned, Devel::Peek provides an SvREFCNT. There's also a builtin function (probably undocumented) called Internals::SvREFCNT. Your statement: I only want to use those objects which are referenced didn't make a lot of sense, however. The only objects you have access to are those with references to them; if there are no references to an object then it is deallocated. Therefore, any refcount check would be guaranteed to be over 0. I simply wanted to point that out; perhaps it was just a miscommunication. If weaken was what you were looking for, great.	[reply]