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

Subject: Re: [Boost-users] [mpl] Accessing the nested value of an invoked placeholder
From: David Abrahams (dave_at_[hidden])
Date: 2010-10-27 16:59:57

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