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

From: Matt Brown (deadguysfrom_at_[hidden])
Date: 2008-03-13 16:15:35

ah, I understand.  thanks again.

On Thu, Mar 13, 2008 at 1:04 PM, Steven Watanabe <watanabesj_at_[hidden]> wrote:
> 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
>
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