[Boost-users] [mpl] Accessing the nested value of an invoked placeholder

Subject: [Boost-users] [mpl] Accessing the nested value of an invoked placeholder
From: Nathan Crookston (nathan.crookston_at_[hidden])
Date: 2010-10-27 14:29:14

All,

I'm writing some code to filter a vector_c of enum values.  I have
some existing structs which are templatized on those enum types and
I'm having some trouble accessing those structs using placeholders.
Following is code which hopefully clarifies:

#include <boost/cstdint.hpp>
#include <boost/mpl/assert.hpp>
#include <boost/mpl/copy_if.hpp>
#include <boost/mpl/vector_c.hpp>
#include <boost/type_traits.hpp>
#include <iostream>

using boost::mpl::placeholders::_;

enum MyEnum
{
        val1,
        val2,
};

template <MyEnum me>
struct trait;

template <>
struct trait<val1>
{
        typedef boost::uint16_t type;
};

template <>
struct trait<val2>
{
        typedef boost::uint8_t type;
};

//PROBLEM: The 'T' passed in is _, not what it substitutes for.
template <typename T>
struct traitMeta : trait<T::value> {};

typedef boost::mpl::vector_c<MyEnum, val1, val2> Vec;

//For each item in Vec, determine if trait<_>::type matches T.  If so, add it.
template <typename T>
struct TypeIs : boost::mpl::copy_if<Vec, boost::is_same<typename
traitMeta<_>::type, T> > {};

BOOST_MPL_ASSERT((TypeIs<boost::uint16_t>::type,
boost::mpl::vector_c<MyEnum, val1>));

int main()
{ return 0; }

//END CODE

I would assume from reading the book and documentation that mpl::apply
or mpl::lambda would be best to use here, but my attempts to do so
haven't been successful.  I'd appreciate any guidance.

Thanks,
Nate