sv_derived_from gives a confsing result

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

O Monks, be pleased to offer wisdom to this lowly hacker:

I'm testing Audio::TagLib, and this statement:

print Audio::TagLib::ID3v2::Frame::size(1), "\n";
[download]

results in the runtime error:

THIS is not of type Audio::TagLib::ID3v2::Frame
[download]

I realize that this is not exactly a full example. I'm trying to keep things simple:-)

The short of it is this. We're accessing an XS interface to the C++ library taglib.. The XS code is:

unsigned int 
TagLib::ID3v2::Frame::size()
CODE:
    RETVAL = THIS->size();
OUTPUT:
    RETVAL
[download]

and the C code derived from this is:

...
if (sv_isobject(ST(0)) && sv_derived_from(ST(0), "Audio::TagLib::ID3v2
+::Fram
        THIS = INT2PTR(TagLib::ID3v2::Frame *, SvIV((SV *)SvRV(ST(0)))
+);
    else
        Perl_croak(aTHX_ "THIS is not of type Audio::TagLib::ID3v2::Fr
+ame");
[download]

So I see where the error is coming from, the question is: how is Perl determining what the type of THIS is? As far as I can tell, the various objects all have the correct specifications back to the C++ code. Additionally, is there a perlapi that will say what it thinks the type of THIS is?

Thanks.

Comment on sv_derived_from gives a confsing result Select or Download Code

Replies are listed 'Best First'.
Re: sv_derived_from gives a confsing result by ikegami (Patriarch) on Mar 27, 2011 at 05:49 UTC
You're passing «1» where an object of type «Audio::TagLib::ID3v2::Frame» is expected. The proper usage is `$frame->size()` [download]	[reply] [d/l]
Re: sv_derived_from gives a confsing result by syphilis (Archbishop) on Mar 27, 2011 at 06:13 UTC
Additionally, is there a perlapi that will say what it thinks the type of THIS is? You can find out what ST(0) is with: `warn("ST(0) is of type %s\n", HvNAME(SvSTASH(SvRV(ST(0)))));` [download] A demo: `use Inline C => Config => BUILD_NOISY => 1; use Inline C => <<'EOC'; void foo(SV * x) { dXSARGS; if (sv_isobject(ST(0))) warn("ST(0) is of type %s\n", HvNAME(SvSTASH(SvRV(ST(0))))); else warn("Not an object\n"); XSRETURN(0); } EOC use Math::BigInt; my $obj = Math::BigInt->new(17); foo($obj);` [download] Cheers, Rob	[reply] [d/l] [select]
Re^2: sv_derived_from gives a confsing result by geoffleach (Scribe) on Mar 29, 2011 at 05:08 UTC
Thanks, Rob. Your code, plus this: `warn("not an object %s\n",(char )SvPV_nolen(ST(0)));` [download] Revealed that ST(0) was '1', leading to the discovery that despite coding that looked* like C++, Perl was not treating it as such. Seems to be related to the fact that there was no new() method. Sigh!	[reply] [d/l]
Re^3: sv_derived_from gives a confsing result by ikegami (Patriarch) on Mar 29, 2011 at 08:52 UTC
`Audio::TagLib::ID3v2::Frame::size(1)` would be just as wrong in C++. The C++ equivalent of the correct code would be «`frame->size()`».	[reply] [d/l] [select]
Re^3: sv_derived_from gives a confsing result by syphilis (Archbishop) on Mar 29, 2011 at 10:43 UTC
Seems to be related to the fact that there was no new() method Sounds feasible. The method doesn't need to be called "new" (with C, at least), but if `sv_isobject(sv)` is to return true, then the reference (sv) does need to have been blessed into some package ... which is what "new" normally does, of course. Cheers, Rob	[reply] [d/l]