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

From: Matt Brown (deadguysfrom_at_[hidden])
Date: 2008-03-13 15:27:38

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.

-Matt

On Thu, Mar 13, 2008 at 9:31 AM, Steven Watanabe <watanabesj_at_[hidden]> wrote:
> AMDG
>
>
>
>  Matt Brown wrote:
>  > I just tried to build this code in visual studio 2005, and got error:
>  >
>  > error C3201: the template parameter list for class template
>  > 'boost::mpl::bind2<F,T1,T2>::apply' does not match the template
>  > parameter list for template parameter 'boolfunc'
>  >
>  > This code works fine with gcc, and boost mpl portability page
>  > (http://www.boost.org/libs/mpl/doc/tutorial/portability.html) says my
>  > version of visual studio should be able to compile this.  However, it
>  > appears to think the lambda< ... >::type::apply is not
>  > a metafunction.  Any help will be greatly appreciated.
>  >
>  > -Matt Brown
>  >
>  >
>  >
>  > #include <iostream>
>  > #include <boost/mpl/lambda.hpp>
>  > #include <boost/type_traits/is_same.hpp>
>  > #include <boost/mpl/placeholders.hpp>
>  >
>  > template <template <typename> class boolfunc>
>  > class test
>  > {
>  > public:
>  >        static void print()
>  >        {
>  >                if(boolfunc<int>::type::value)
>  >                {
>  >                        std::cout << "int: true\n";
>  >                }
>  >        }
>  > };
>  >
>  > int main(void)
>  > {
>  >        test<
>  >                boost::mpl::lambda<
>  >                        boost::is_same<boost::mpl::_1, int>
>  >                >::type::apply
>  >        >::print();
>  > };
>
>  Don't do that.
>
>
>  boost::mpl::lambda<
>                        boost::is_same<boost::mpl::_1, int>
>                >::type::apply<int>::type
>
>
>  will work, but apply actually takes extra default arguments and thus cannot
>  be bound to a template template argument.
>
>  i.e.  (For exposition only)
>
>  template<class T>
>  struct lambda {
>     struct type {
>         template<class T0 = na, class T1 = na, ...>
>         struct apply {
>             typedef /unspecified/ type;
>         };
>     };
>  };
>
>  In Christ,
>  Steven Watanabe
>
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