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How to dereference a FORMAT reference?

by Jeffrey Kegler (Friar)
on Jun 23, 2009 at 18:01 UTC ( #774128=perlquestion: print w/ replies, xml ) Need Help??
Jeffrey Kegler has asked for the wisdom of the Perl Monks concerning the following question:

For most practical purposes, this question is totally useless. But I have a CPAN module Test-Weaken, which detects leaks in arbitrary Perl data structures. It needs to traffic in references to as many Perl data objects as possible. For all the Perl data types except one, I can duplicate a reference, as follows:

\@{$array_ref} # ARRAY \%{$hash_ref} # HASH \${$scalar_ref} # SCALAR, REF, LVALUE, VSTRING, Regexp \*{$glob_ref) # GLOB, IO, FILEHANDLE \&{$code_ref} # CODE

My question: how do I dup a FORMAT reference?

You can get a format reference with this syntax: $formatref = *foo{FORMAT}. Formats themselves are deprecated and why you'd want a reference to one I don't know, but my purpose is to allow Test::Weaken to be able to deal with whatever data is thrown at it. That data may come from perverse and/or legacy code.

UPDATED: corrected typos.

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Re: How to dereference a FORMAT reference?
by morgon (Chaplain) on Jun 23, 2009 at 23:17 UTC
    I am not entirely sure but I think in all cases you can just get a new reference via your typeglob-syntax.

    Consider this:

    use strict; use Scalar::Util qw(weaken isweak); my $a = 1; my $r1 = *main::a{SCALAR}; my $r2 = *main::a{SCALAR}; weaken $r1; print "r1 is weak\n" if isweak $r1; print "r2 is weak\n" if isweak $r2;

    Here I get two copies of the typeglob. Both are references to $a but they are different one as proved by weakening one and observing that the other one has not been weakened.

    So I guess you don't need the special snytax for array-refs, hash-refs etc, you just generically get a new copy of whatever is in the corresponding slot of the typeglob.

      That's a very good idea, and one I hadn't thought of.

      But it's just half the problem. The solution requires that I know the typeglob slot. All I have is a reference and I don't know a general way of going from the reference to the typeglob slot.

      In fact, I can't see from the documentation any situation in which a format reference can be used for any purpose whatsover. (Unless you count taking its address and using that as a very low quality pseudo-random number generator as a "use".)

        All I have is a reference and I don't know a general way of going from the reference to the typeglob slot.

        What? I thought that the typeglob slots were chosen to match what ref returns (except you'd want Scalar::Util::reftype because of the unfortunate decision to overload ref to returning class names and you have to deal with just a few special cases like the unfortunate and silly decision to have ref(\\$x) return "REF" instead of "SCALAR" and the more reasonable features of returning "LVALUE" or "VSTRING" instead of "SCALAR").

        I don't see how having to deal with 3 special cases prevents this from being a "general" solution.

        Updated with minor wording changes.

        - tye        

        I think it is even simpler:

        Consider:

        use strict; use Scalar::Util qw(weaken isweak); my $a = []; my $r1 = $a; my $r2 = $a; weaken $r1; print "a is weak\n" if isweak $a; print "r1 is weak\n" if isweak $r1; print "r2 is weak\n" if isweak $r2;

        So here we have a array_ref $a of which we make two further copies for which we prove again that they are different.

        So to answer your initial question:

        Given a reference you simple assign it to a scalar to get another reference to the same referent - no need for special syntax or type-glob fiddling...

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