Re: [boost] [type_traits][result_of] Handling of cv-qualified functionpointers/references

From: John Maddock (john_at_[hidden])
Date: 2006-05-30 05:27:22

João Abecasis wrote:
> and a slightly more convoluted for type_traits::is_pointer:
>
>      #include <boost/type_traits/is_function.hpp>
>      #include <boost/static_assert.hpp>
>
>      void foo ();
>
>      template <class F>
>      void bar(F &)
>      {
>          BOOST_STATIC_ASSERT(boost::is_function<F const>::value);
>      }
>
>      template <class F>
>      void baz(F const & f)
>      {
>          bar(f);
>      }
>
>      int main()
>      {
>          baz(foo);
>      }

Right that should fail because:

*There is no such thing as a const-qualified function type in the C++ 
language*

So argument deduction for baz would fail because it would have an invalid 
signature.  Actually, having said that, I seem to remember that behaviour 
may have been changed by a DR that makes a cv-qualified function type (or 
reference) the same as the non-cv-qualified type.

Unfortunately this is a bug that crops up in compilers from time to time, 
often they allow you to generate cv-qualified function or reference types 
inadvertently, but don't let you test for them with partial specialistion 
because that part of the compiler "knows" the situation can never occur :-(

BTW your code compiles cleanly with gcc-3.4.4 and Intel 9 and VC++8 (after 
removing the const from the function baz).

I'm surprised that gcc-4 has this issue, I'm fairly sure we looked at this 
for gcc-2 and couldn't fix it (or maybe it was the cv-qualified reference 
issue we looked at, I don't remember now).  Whatever, if you have a patch 
that works then yes lets fix is_function to workaround the problem for those 
specific compilers.

John.

> Both failing with gcc-4.x. The first test case also fails with
> gcc-3.2.3. while the second case fails for a completely different
> reason
> in overload resolution.
>
> For unpack_args I worked around these issues by applying remove_cv to
> pointers in general and also before testing with is_function. In
> either
> case cv-qualification shouldn't affect the type of the arguments or
> the result, which is what I'm interested in.
>
> Anyway I'm thinking that the solution could be moved upstream so
> others
> won't stumble upon the same issues. Thoughts?
>
> Best regards,
>
>
> João
>
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