Re: [Boost-users] [mpl] what is not a placeholder expression

28 Jul 2007


      ""JOAQUIN LOPEZ MU?Z""  wrote in message 
news:f9a9e060691a.691af9a9e060@tid.es...
...
According to the MPL reference, a placeholder expression
X must satisfy (among others) the following
condition:
"All of X's template parameters, including the
  default ones, are types."
According to that the following should then compile
without errors:
#include 
  #include 
  #include 
using namespace boost;
  using namespace boost::mpl;
template
  struct foo
  {
    template<typename Q>
    struct apply{
      typedef Q type;
    };
  };
typedef apply1,int>::type t1;
BOOST_MPL_ASSERT((is_same));
int main(){}
However, I've tried with GCC 3.4.4 and the static
assertion fails! I'd like to know:
1. Whether I'm right in assuming that the code above
should compile without assertion failures.
Yes.
...
2. What the behavior is for other versions of GCC and
compilers: reports most welcome!!
Basically, this is a GCC-specific problem caused by a
non-conforming extension that was removed in 4.2.0
(http://gcc.gnu.org/gcc-4.2/changes.html, search for
"(undocumented) extension").
...
3. Whether this is a known problem specific to GCC
or, more generally, what the status of this issue is.
It is, and it's not a issue anymore starting with GCC 4.2.
You should be able to work around it for older versions
by providing a specialization of the
'boost::mpl::aux::template_arity' template along the
following lines (untested):

    template
    struct template_arity > : int_<-1> {};

HTH,
--
Aleksey Gurtovoy
MetaCommunications Engineering