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";

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results in the runtime error: 
THIS is not of type Audio::TagLib::ID3v2::Frame

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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

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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");

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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.

In reply to sv_derived_from gives a confsing result by geoffleach

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