Re: [boost] [lexical_cast][STLport 4.5.3][trunk] static assertion failed in boost/detail/lcast_precision.hpp

From: Joaquín Mª López Muñoz (joaquin_at_[hidden])
Date: 2007-10-09 03:52:19

Paul A Bristow ha escrito:

>
>
> >-----Original Message-----
> >From: boost-bounces_at_[hidden]
> >[mailto:boost-bounces_at_[hidden]] On Behalf Of Joaquín Mª
> >López Muñoz
> >Sent: 08 October 2007 18:37
> >To: boost_at_[hidden]
> >Subject: [boost] [lexical_cast][STLport 4.5.3][trunk] static
> >assertion failed in boost/detail/lcast_precision.hpp
> >
> >Compiling the following snippet in MSVC++ 6.5 + STLport 4.5.3
> >with the trunk
> >
> >  #include <boost/lexical_cast.hpp>
> >
> >  int main()
> >  {
> >    return 0;
> >  }
> >
> >results in
> >
> >...\boost\detail\lcast_precision.hpp(79) : error C2027:
> >  use of undefined type 'STATIC_ASSERTION_FAILURE<0>'
> >  ...\boost\detail\lcast_precision.hpp(89) : see reference to
> >class template instantiation
> >  'boost::detail::lcast_precision<T>' being compiled

[...]

> I can't see an obvious cause (presume it works OK on other platforms), so I can only
> suggest breaking this down into separate assertions to see which one is causing the trouble,
> and perhaps getting the values of std::limits<T>::digits and boost::is_abstract<T>.
>
> It seems that the suitable precision (decimal digits to use to get the full accuracy for type T)
> can't be calculated.
>
> It would also be useful to see what T is?  Is this some funny type for which std::numeric_limits
> isn't properly specialised?

OK, I think I've got it: MSVC++ 6.5 behaves funny with static constants inside templates,
as it does not see their correct value during template parsing. For instance, the following:

  #include <boost/config.hpp>
  #include <boost/static_assert.hpp>

  template<typename T>
  struct foo
  {
    BOOST_STATIC_CONSTANT(int,x=2);
    BOOST_STATIC_ASSERT(x==2);
  };

  int main()
  {
    return 0;
  }

fails at the static assertion line (x is seen as 1, btw). After instantiation, however, the
static constant holds the correct value, this phenomenon only happens at the parsing
stage.
So,  this is the problem with boost/detail/lcast_precision.hpp in MSVC++  6.5
+ STLport 4.5.3; when using the native Dinkumware stdlib the static assertions
pass by pure chance, as they are affected by the same compiler bug. The attached
diff simply omits the offending assertions for MSVC++ 6.0/7.0. Is it OK to commit?

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo

Index: lcast_precision.hpp
===================================================================
--- lcast_precision.hpp	(revision 39833)
+++ lcast_precision.hpp	(working copy)
@@ -68,19 +68,23 @@
 
     BOOST_STATIC_CONSTANT(unsigned int, precision_dec = limits::digits10 + 1U);
 
+#if !defined(BOOST_MSVC)||!(BOOST_MSVC<1310)
     BOOST_STATIC_ASSERT(!is_specialized_dec ||
             precision_dec <= streamsize_max + 0UL
         );
+#endif
 
     BOOST_STATIC_CONSTANT(unsigned long, precision_bin =
             2UL + limits::digits * 30103UL / 100000UL
         );
 
+#if !defined(BOOST_MSVC)||!(BOOST_MSVC<1310)
     BOOST_STATIC_ASSERT(!is_specialized_bin ||
             limits::digits + 0UL < ULONG_MAX / 30103UL &&
             precision_bin > limits::digits10 + 0UL &&
             precision_bin <= streamsize_max + 0UL
         );
+#endif
 
     BOOST_STATIC_CONSTANT(std::streamsize, value =
             is_specialized_bin ? precision_bin