At Wed, 27 Oct 2010 12:29:14 -0600, Nathan Crookston wrote:

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:

Hi Nathan,

#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> > {};

This is a problem; reaching inside a template for a member (like ::type) always causes it to be instantiated. That means traitMeta<_>::type gets evaluated early, which isn't what you want. I think if you just drop the ::type you'll get the semantics you want.

//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.

mpl::copy_if uses apply (which uses lambda) internally, so you don't have to. -- Dave Abrahams BoostPro Computing http://www.boostpro.com