How to iterate through the indices of a Boost Multi_index container?

Question

I have a boost::multi_index::multi_index_container container having six different ordered_non_unique indices. The idea is to be able to sort the data along these six indices (as a means of ranking solutions using multiple criteria).

The issue I'm facing is while retrieving the indices in a loop. Boost requires me to use the following syntax to get (say) the 4th index:

const result_multi::nth_index<1>::type &legs_index = result.get<4>();

What I'm trying to do is to put the above statement in a loop that runs between 0 to 5, so that I can use the same code on all six indices. Of course, the following code fragment would not compile:

for (size_t i = 0; i < 5; ++i) {
  const result_multi::nth_index<1>::type &index = result.get<i>();
  ...
  ... Display result sorted along the i-th index
  ...
 }

As the get<i> is a template that needs to be defined during compilation.

How can I use achieve the above functionality so that I don't need to duplicate the code 6 times? It seems boost:preprocessor may help in doing so, but am unable to figure out exactly how to use it - any pointers would be really appreciated!

EDIT: I would really appreciate a non-C++11 solution as well, to complement the excellent answer using one. (For non-technical reasons, I'm forced to use a legacy version of gcc).

Answer 1

If you can't use C++14, the backporting to C++03 with Boost could look like this:

Live Coliru Demo

#include <boost/type_traits/integral_constant.hpp>

template<typename T,T N0,T N1,typename F>
void static_for(F f)
{
  static_for<T,N0,N1>(f,boost::integral_constant<bool,(N0<N1)>());
}

template<typename T,T N0,T N1,typename F>
void static_for(F f,boost::true_type)
{
  f(boost::integral_constant<T,N0>());
  static_for<T,N0+1,N1>(f);
}

template<typename T,T N0,T N1,typename F>
void static_for(F f,boost::false_type)
{
}

#include <boost/multi_index_container.hpp>
#include <boost/multi_index/ordered_index.hpp>
#include <boost/multi_index/identity.hpp>

using namespace boost::multi_index;
typedef multi_index_container<
  int,
  indexed_by<
    ordered_non_unique<identity<int> >,
    ordered_non_unique<identity<int>,std::greater<int> >,
    ordered_non_unique<identity<int> >,
    ordered_non_unique<identity<int>,std::greater<int> >,
    ordered_non_unique<identity<int> >,
    ordered_non_unique<identity<int>,std::greater<int> >
  >
> result_multi;

#include <iostream>

struct body
{
  body(result_multi& result):result(result){}

  template<typename I>
  void operator()(I){
    typename result_multi::nth_index<I::value>::type& index=
      result.get<I::value>();

    std::cout<<"index #"<<I::value<<": ";
    for(typename result_multi::nth_index<I::value>::type::iterator
          b=index.begin(),
          e=index.end();
        b!=e;++b){
      std::cout<<*b<<" ";
    }
    std::cout<<"\n";
  }

  result_multi& result;
};

int main()
{
  result_multi result;
  for(int i=0;i<3;++i)result.insert(i);

  static_for<int,0,6>(body(result));
}

which is considerably uglier. Another alternative is to use the preprocessor with BOOST_PP_REPEAT. I'm not sure myself which solution looks best, though I think I'd favor the first as it's better prepared for C++14 upgrading:

Live Coliru Demo

#include <boost/multi_index_container.hpp>
#include <boost/multi_index/ordered_index.hpp>
#include <boost/multi_index/identity.hpp>

using namespace boost::multi_index;
typedef multi_index_container<
  int,
  indexed_by<
    ordered_non_unique<identity<int> >,
    ordered_non_unique<identity<int>,std::greater<int> >,
    ordered_non_unique<identity<int> >,
    ordered_non_unique<identity<int>,std::greater<int> >,
    ordered_non_unique<identity<int> >,
    ordered_non_unique<identity<int>,std::greater<int> >
  >
> result_multi;

#include <boost/preprocessor/repetition/repeat.hpp>
#include <iostream>

int main()
{
  result_multi result;
  for(int i=0;i<3;++i)result.insert(i);

#define BODY(z,i,_)                                        \
{                                                          \
  result_multi::nth_index<i>::type& index=result.get<i>(); \
                                                           \
  std::cout<<"index #"<<i<<": ";                           \
  for(result_multi::nth_index<i>::type::iterator           \
        b=index.begin(),                                   \
        e=index.end();                                     \
      b!=e;++b){                                           \
    std::cout<<*b<<" ";                                    \
  }                                                        \
  std::cout<<"\n";                                         \
}

BOOST_PP_REPEAT(6,BODY,~)

#undef BODY
}

Answer 2

You need some metaprogramming to do this, namely replacing the run-time for with a compile-time construct that can iterate over types representing the constants 0,...,5. A very simple static_for relying on C++14 capabilities is shown below. Note that the generic lambda function substituting for the for body is passed a std::integral_constant i whose numerical value is obtained through operator(), hence the "i()" in "auto& index=result.get<i()>();".

Live Coliru Demo

#include <type_traits>

template<typename T,T N0,T N1,typename F>
void static_for(F f)
{
  static_for<T,N0,N1>(f,std::integral_constant<bool,(N0<N1)>{});
}

template<typename T,T N0,T N1,typename F>
void static_for(F f,std::true_type)
{
  f(std::integral_constant<T,N0>{});
  static_for<T,N0+1,N1>(f);
}

template<typename T,T N0,T N1,typename F>
void static_for(F f,std::false_type)
{
}

#include <boost/multi_index_container.hpp>
#include <boost/multi_index/ordered_index.hpp>
#include <boost/multi_index/identity.hpp>

using namespace boost::multi_index;
using result_multi=multi_index_container<
  int,
  indexed_by<
    ordered_non_unique<identity<int>>,
    ordered_non_unique<identity<int>,std::greater<int>>,
    ordered_non_unique<identity<int>>,
    ordered_non_unique<identity<int>,std::greater<int>>,
    ordered_non_unique<identity<int>>,
    ordered_non_unique<identity<int>,std::greater<int>>
  >
>;

#include <iostream>

int main()
{
  result_multi result={0,1,2};

  static_for<int,0,6>([&](auto i){
    auto& index=result.get<i()>();

    std::cout<<"index #"<<i()<<": ";
    for(int x:index)std::cout<<x<<" ";
    std::cout<<"\n";
  });
}

Output:

index #0: 0 1 2 
index #1: 2 1 0 
index #2: 0 1 2 
index #3: 2 1 0 
index #4: 0 1 2 
index #5: 2 1 0