Re: [Boost-users] Visual C++ does not recognize lambda metafunction as such

From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2008-03-13 16:04:05

AMDG

Matt Brown wrote:
> Thanks for the reply, Steven.  You were able to give me enough
> direction to solve the problem.  I simply had to write a wrapper class
> around apply that had the correct template signature.
>
>
> template <typename MetaFunc>
> struct apply1
> {
> 	template <typename T>
> 	struct apply : MetaFunc::apply<T>
> 	{};
> };
>
> int main(void)
> {
> 	test<
> 		apply1<	
> 			boost::mpl::lambda<
> 				boost::is_same<boost::mpl::_1, int>
> 			>::type
> 		>::apply
> 	>::print();
> };
>
>
> I've gotta say, though, that this behavior of MPL's lambda, bind,
> quoten, etc Meftafunctions is counter intuitive (to put it nicely).
> The runtime bind library, as a counter example, is intuitive.  When
> you bind one argument to a second order function, you get a first
> order function.  If the runtime bind library was designed similar to
> the bind facility in MPL, it would give you a function with a variable
> argument list, which I believe anyone would agree is counter
> intuitive.  I'm sure there is a good reason MPL was designed this way,
> though I doubt its a better reason than the reason it should have been
> designed intuitively.
>   

As a matter of fact the runtime bind library /does/ give you a function
object that can take a variable number of arguments.

boost::bind(std::plus<int>(), _1, _1)(0, 2, 1, 3, 4);  // OK.  extra 
arguments ignored.

What you're trying to do with MPL is the equivalent of saying:

int (*f)(int, int) = boost::bind(std::plus<int>(), _1, _2);

Which of course will not work.

In short you can use the nested apply template, but there
are no guarantees about exactly how it is defined.  The MPL
way of achieving your end is to pass metafunction classes around
rather than template template arguments.

#include <iostream>
#include <boost/mpl/lambda.hpp>
#include <boost/mpl/apply.hpp>
#include <boost/type_traits/is_same.hpp>
#include <boost/mpl/placeholders.hpp>

template <class boolfunc>
class test
{
public:
       static void print()
       {
               if(mpl::apply1<boolfunc, int>::type::value)
               {
                       std::cout << "int: true\n";
               }
       }
};

int main(void)
{
       test<
               boost::mpl::lambda<
                       boost::is_same<boost::mpl::_1, int>
               >::type
       >::print();
};

In Christ,
Steven Watanabe