Re: [boost] type_traits problems on MSVC

From: John Maddock (John_Maddock_at_[hidden])
Date: 2001-08-26 06:21:52

>I experimented a bit with tuple.hpp to see what's wrong; here's a minimal
>example I've found that fails on both VC 6 and VC 7:
>
>#include <boost/type_traits.hpp>
>
>template<int N> boost::add_reference<int>::type get();

This is so weird, I've experimented a bit with this and the following
program fails with an internal compiler error on vc6:

template <int N> struct dummy_arg{};

template <class T>
struct dummy_traits
{
   typedef T type;
};

template<int N, class T>
typename dummy_traits<T>::type get_n(T* p, const dummy_arg<N>* up = 0)
{
   return p[N];
}

Note that the template is never used: it's the declaration itself that's
enough to trigger the error.

BTW the dummy_arg is used here because otherwise 
get_n<1,int> and
get_n<2,int> and
get_n<3,int> etc
all have the same mangled name with vc6 - and that really screws up things
at run time.  The macro to check for this is
BOOST_NO_EXPLICIT_FUNCTION_TEMPLATE_ARGUMENTS BTW.

However the above program does compile with vc7b2, in fact vc7 seems to be
doing quite well here - here's the full test program that I managed to get
working with vc7b2:

#include <boost/type_traits/transform_traits.hpp>
#include <cassert>

template <int N> struct dummy_arg{};

template <class T>
struct dummy_traits
{
   typedef T type;
};

template<int N, class T>
typename dummy_traits<T>::type get_n(T* p, const dummy_arg<N>* up = 0)
{
   return p[N];
}

template<int N, class T>
typename ::boost::add_reference<T>::type get_n2(T* p)
{
   return p[N];
}

int main()
{
   int i[3];
   int ii = get_n<2>(i);
   int& ir = get_n2<2>(i);
   assert(&ir == &(i[2]));
   double da[3];
   double& d = get_n2<1>(da);
   assert(&d == &(da[1]));
}

Unfortunately this doesn't help the tuple code any: that still fails with
vc7b2.

Confused again :-(

- John Maddock
http://ourworld.compuserve.com/homepages/john_maddock/