Re: [boost] Re: additional test for is_abstract type trait

From: Christoph Ludwig (cludwig_at_[hidden])
Date: 2004-11-11 10:37:53

On Thu, Nov 11, 2004 at 10:07:43AM -0500, David Abrahams wrote:
> Christoph Ludwig <cludwig_at_[hidden]> writes:
> 
> > As I mentioned in previous posts the following code program
> >
> >   #include <boost/type_traits/is_abstract.hpp>
> >   #include <iostream>
> >
> >   template<typename T>
> >   struct A { virtual void f() = 0; }
> >
> >   int main() {
> >     if(boost::is_abstract<A<int> >::value) {
> >       std::cout << "A<int> is abstract";
> >     else {
> >       std::cout << "A<int> is not abstract";
> >     }
> >   }
> >
> > erroneously outputs "A<int> is not abstract" if compiled with gcc 3.4.
> >
> > However, the regression tests report that gcc 3.4 passes the
> > is_abstract test. The reason is that above error is triggered only if
> > A<int> is not instantiated by the compiler; but is_abstract_test.cpp
> > does not test class template specializations at all.
> 
> Why don't we just get is_abstract to cause the instantiation?  It
> seems to me that you'd need to do that anyway in order to detect
> abstractness.  As a matter of fact, I have some doubts about your
> diagnosis, since I can't imagine any way that is_abstract could work
> without doing the instantiation.

Well, it's really Giovanni Bajo's diagnosis who implemented the
support for DR 337 in gcc 3.4, IIUC. So I am going to trust him on
this... :-) There was agreement on gcc bugzilla that section 14.7.1/p4
of the standard requires the instantiation of the class template and
that gcc is in error not to do so. But this bug won't be fixed in the
3.4 release series.

I will give a workaround within is_abstract that forces the
instantiation some thought later in the evening. But even if I come up
with a patch, I think it is worth explicitly testing whether
is_abstract interacts well with templates. This compiler bug has shown
again that errors creep in where you don't expect them.

Regards

Christoph

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