Re: [Boost-users] [MPL] Applying a boost::mpl::list to the template parameter of a type.

Subject: Re: [Boost-users] [MPL] Applying a boost::mpl::list to the template parameter of a type.
From: Igor R (boost.lists_at_[hidden])
Date: 2010-12-18 13:22:33

> I have a class that requires a boost::variant containing shared pointers to
> various types as follows:
> Â Â  Â template <typename ToySharedPtrVariant, typename ColorSharedPtrVariant>
> Â Â  Â class ToyPicker {
> Â Â  Â  Â  typedef std::pair<
> Â Â  Â  Â  Â  ToySharedPtrVariant,
> Â Â  Â  Â  Â  ColorSharedPtrVariant
> Â Â  Â  Â  > toyAndColorPair;
> Â Â  Â  Â  typedef std::map<
> Â Â  Â  Â  Â  std::string,
> Â Â  Â  Â  Â  std::vector<
> Â Â  Â  Â  Â  Â  toyAndColoPair
> Â Â  Â  Â  Â  >
> Â Â  Â  Â  > stringToToyColorPairMap;
> Â Â  Â  Â  // ... methods that use the defined types...
> Â Â  Â }
> This class currently requires template parameters of the following form to
> compile:
> Â Â  Â ToyPicker<
> Â Â  Â  Â  Â  Â  Â  Â  boost::variant<
> Â Â  Â  Â  Â  Â  Â  Â  Â  boost::shared_ptr<ToyModel>
> Â Â  Â  Â  Â  Â  Â  Â  >,
> Â Â  Â  Â  Â  Â  Â  Â  boost::variant<
> Â Â  Â  Â  Â  Â  Â  Â  Â  boost::shared_ptr<BlueToy>,
> Â Â  Â  Â  Â  Â  Â  Â  Â  boost::shared_ptr<RedToy>,
> Â Â  Â  Â  Â  Â  Â  Â  Â  boost::shared_ptr<GreenToy>
> Â Â  Â  Â  Â  Â  Â  Â  >
> Â Â  Â  Â  Â  Â  Â  > toyPicker;
> How do I use an mpl list so that I can allow the following much simpler
> definition for users, then convert it into the one above inside my class?
> Â Â  Â ToyPicker<
> Â Â  Â  Â  Â  Â  boost::mpl::list<
> Â Â  Â  Â  Â  Â  Â  ToyModel
> Â Â  Â  Â  Â  Â  >,
> Â Â  Â  Â  Â  Â  boost::mpl::list<
> Â Â  Â  Â  Â  Â  Â  BlueToy,
> Â Â  Â  Â  Â  Â  Â  RedToy,
> Â Â  Â  Â  Â  Â  Â  GreenToy
> Â Â  Â  Â  Â  Â  >
> Â Â  Â  Â  Â  > toyPicker;

If you got you right, you map a Model to a list of Colors, right?
I'm not a TMP expert, but I'd propose something like this:

#include <boost/mpl/map.hpp>
#include <boost/mpl/list.hpp>
#include <boost/mpl/transform.hpp>
#include <boost/mpl/placeholders.hpp>
#include <boost/variant.hpp>
#include <boost/shared_ptr.hpp>

using namespace boost::mpl;
using boost::shared_ptr;
using boost::variant;
using boost::make_variant_over;

// a metafunction that will map for us maps list<a, b,...> -->
list<shared_ptr<a>, shared_ptr<b>, ...>
template<class Seq, template<class> class Wrapper>
struct add_wrapper : transform<Seq, Wrapper<_> >{};

// now your picker class get template args of the form pair<model,
list<color1, color2,...> >
template<typename ModelColors>
struct picker
{
  // wrapping the model with shared_ptr and variant manually
  typedef variant<shared_ptr<typename ModelColors::first> > wrapped_model;
  // applying our metafunction to the sequence of colors, then
applying make_variant_over
  typedef typename make_variant_over<typename add_wrapper<typename
ModelColors::second, shared_ptr>::type>::type wrapped_colors;

  wrapped_colors colors_;
  wrapped_model model_;
};

struct Model1;
struct Color1;
struct Color2;
struct Color3;

int main(int argc, char* argv[])
{
  picker<pair<Model1, list<Color1, Color2> > > p;
  return 0;
}