""JOAQUIN LOPEZ MU?Z"" <joaquin@tid.es> wrote in message news:f9a9e060691a.691af9a9e060@tid.es...
According to the MPL reference, a placeholder expression X<a1,..,an> 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 <boost/mpl/apply.hpp> #include <boost/mpl/assert.hpp> #include <boost/type_traits/is_same.hpp>
using namespace boost; using namespace boost::mpl;
template<typename T,int=0> struct foo { template<typename Q> struct apply{ typedef Q type; }; };
typedef apply1<foo<_1>,int>::type t1;
BOOST_MPL_ASSERT((is_same<t1,int>));
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<typename T,int n> struct template_arity<foo<T,n> > : int_<-1> {}; HTH, -- Aleksey Gurtovoy MetaCommunications Engineering