Re: [Boost-users] [boost::mpl::apply] compiler dependent behavior

From: Samuel Charron (chammm_at_[hidden])
Date: 2007-03-28 18:25:18

> > Hi,
> >
> > I'm currently using the apply metafunction, and I have two different
> > behaviors depending on the used compiler.
> >
>
> <snip code>
>
> >
> > With g++ 4.1, there is no error.
> > With g++ 4.2, icc 9.1, an error occurs:
>
> I can't verify, but I believe it's might be:
>
> > - g++ :
> > no class template named 'apply' in 'class
> > std::tr1::unordered_set<mpl_::arg<1>, std::equal_to<int>,
> > std::tr1::hash<int>, std::allocator<int>, false>'
>                                              ^^^^^^
>
> Placeholder substitution won't work in argument lists of templates with
> non-type parameters (well, sometimes it does work, but only if there is
> a default and if the compiler deduces template template arguments in a
> certain, probably non-standard way).
>
> >
> > Can you give me some help to make this work ?
> >
>
> To work around this limitation, you can use a custom Metafunction
>
>      template<typename T>
>      struct specialize_unordered_set
>      {
>          typedef std::tr1:unordered_set<T> type;
>      };
>
> and then use it in a Placeholder Expression. Alternatively (this way
> might save you some compile time), you can write a Metafunction Class to
> replace the Placeholder Expression:
>
>      struct specialize_unordered_set
>      {
>          template<typename T>
>          struct apply
>          {
>              typedef std::tr1::unordered_set<T> type;
>          };
>      };
>
>
>

Working on this bug this afternoon, I found what I think is a cause to that bug.
It comes from template_arity. It returns -1 with buggy compilers and
the right numbers of parameters with working compilers.
The issue comes from non-type parameters in a template with default values.

An example of a code with different behaviours :

# include <boost/mpl/aux_/template_arity.hpp>
# include <iostream>

template < typename A, bool t = false>
struct foo
{
};

int main()
{
  std::cout << boost::mpl::aux::template_arity < foo < int > >
::type::value  << std::endl;
}

Here, with a template < template < class > >, the match works with
some compilers, while others consider that the bool must be part of
the template template expression.

To make this work, I finally wrapped my class with a templated
structure typedef'ing the unordered_set < template_parameters > as
::type. The apply metafunction did the necessary automagically,
without defining an apply structure.

-- 
Samuel