overload and XS

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

I'm trying to use stringify overload on a tied XS variable (Perl 5.6.0) and I can't figure out how to turn on 'fallback'.

I was able to find SvAMAGIC_on() to turn on overload, and I provide a stringify method ('""') that works when you do "$ref", but I need

  $ref eq "some string"

to work also, and that means setting fallback => 1, which I can't figure out how to do to my XS-tied variable. My tie'ing sequence looks roughly like this:

      HV *hash = newHV();
      SV *hashref = newRV_noinc((SV*)hash);
      SV *tie = newSViv((IV)value);
      SV *tieref = newRV_noinc(tie);

      sv_bless(tieref, hash_tie_stash);
      sv_bless(hashref, xml_attribute_stash);
      SvAMAGIC_on(hashref);
      hv_magic(hash, (GV *)tieref, 'P');
      SvREFCNT_dec(tieref);
      return hashref;

Devel::Peek isn't giving me a clue (afaics) as to what magic is different between my XS-tied var and a pure-Perl tied var with overload magic.

Any clues appreciated!

Comment on overload and XS

Replies are listed 'Best First'.
(tye)Re: overload and XS by tye (Sage) on May 18, 2001 at 09:10 UTC
Why rewrite your Perl code in C?? Just because you have some XS code doesn't mean you have to do every single thing for that module in XS code! Do in C, only the stuff that is easy in C. Use a wrapper subroutine written in Perl to do stuff like what you are trying above. The code you have above is the kind of code that breaks when a new version of Perl comes out (or doesn't work when someone tries to use it on an older version of Perl than the one you developed it on). I've done this quite a bit and have been extremely happy with the results. I find I can make modules with much nicer, Perlish interfaces than I see in other XS modules. Plus the code ends up being much easier to understand, maintain, enhance, and debug. - tye (but my friends call me "Tye")	[reply]
Re: (tye)Re: overload and XS by kmacleod (Initiate) on May 18, 2001 at 16:56 UTC
This module is a bridge between C code and Perl code (similar to Inline), the '`value`' above is a C pointer to the structure that is tied, that I want to be able to stringify. '`xml_attribute_stash`' is the Perl wrapper package for that particular class, which has '`use overload`' with '`""`' and '`fallback`' specified. I found that I needed to use `SvAMAGIC_on` to enable overload on that tied C-structure so that the wrapper would see it as overloaded, but '`fallback`' still isn't enabled. I'm guessing there's deeper magic to turn on fallback, since the perl wrapper isn't.	[reply] [d/l] [select]
(tye)Re2: overload and XS by tye (Sage) on May 18, 2001 at 23:43 UTC
What I am saying is: Move all of that code out of the C part of the module/XS. Have the C part just return the 'value' as an IV or UV in an orinary Perl scalar and do all of the hash creation, taking of references, tieing, overloading, and magic in Perl code. - tye (but my friends call me "Tye")	[reply]

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