Re: [Boost-users] Auto dispatch metaprogramming tricks

Subject: Re: [Boost-users] Auto dispatch metaprogramming tricks
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2010-05-21 11:20:46

AMDG

Kim Kuen Tang wrote:
> as indicated by strasser i have also difficulties to compile your 
> code. But your code seems to be more generic, since you dont need to 
> provide the result_type of the member function.
>
> Can you provide a working example?

A couple bugs:
* CV qualifiers we're causing an ambiguity.
* I missed a pair of parentheses.
* At least one overload of on is required.

The following compiles for me:

#include <boost/mpl/bool.hpp>
#include <boost/mpl/assert.hpp>

typedef char no;
struct yes { char dummy[2]; };

struct has_on_result {
    has_on_result operator,(int);
};

no check_on_result(const has_on_result&);

yes check_on_result(...);

template<class T>
struct has_on_impl : T {
    using T::on;
    has_on_result on(...) const;
};

template<class T, class M>
struct has_on : boost::mpl::bool_<
    sizeof(check_on_result((((has_on_impl<T>*)0)->on(*((M*)0)), 0))) != 
sizeof(no)
 > {};

struct has_on_struct {
   int on(int) const
   {
       return 1;
   }
};

struct arbitrary_struct {
   int on(void*) const
   {
       return 1;
   }
};

int main() {
   BOOST_MPL_ASSERT((has_on<has_on_struct,int>));
   BOOST_MPL_ASSERT_NOT((has_on<arbitrary_struct,int>));
}

In Christ,
Steven Watanabe