get_elements.h

Go to the documentation of this file.

#ifndef LARCOREALG_COREUTILS_GET_ELEMENTS_H
#define LARCOREALG_COREUTILS_GET_ELEMENTS_H

// LArSoft libraries
#include "larcorealg/CoreUtils/StdUtils.h" // util::get()
#include "larcorealg/CoreUtils/span.h"     // util::make_transformed_span()

// C/C++ libraries
#include <cstddef> // std::size_t
#include <type_traits>
#include <utility> // std::get()

namespace util {

  // -- BEGIN -- Transformed iterations ----------------------------------------

  template <std::size_t... Indices, typename Coll>
  decltype(auto) get_elements(Coll&& coll);

  template <std::size_t... Indices, typename Coll>
  decltype(auto) get_const_elements(Coll&& coll);

  // -- END -- Transformed iterations ------------------------------------------

} // namespace util

//==============================================================================
//=== template implementation
//==============================================================================
//------------------------------------------------------------------------------
//--- util::values()
//------------------------------------------------------------------------------
namespace util::details {

  //----------------------------------------------------------------------------
  template <typename Coll, std::size_t First, std::size_t... Others>
  struct get_elements_impl {

    template <typename T>
    static constexpr decltype(auto) iterate(T&& coll) noexcept
    {
      auto extractor = [](auto&& item) -> decltype(auto) {
        if constexpr (sizeof...(Others) == 0U) { return util::get<First>(item); }
        else {
          return std::forward_as_tuple(util::get<First>(item), util::get<Others...>(item));
        }
      };
      return util::make_transformed_span(coll, extractor);
    }

  }; // struct get_elements_impl

  //----------------------------------------------------------------------------

} // namespace util::details

//------------------------------------------------------------------------------
template <std::size_t... Indices, typename Coll>
decltype(auto) util::get_elements(Coll&& coll)
{
  static_assert(sizeof...(Indices) >= 1U, "At least one index must be specified");
  return details::get_elements_impl<std::decay_t<Coll>, Indices...>::iterate(
    std::forward<Coll>(coll));
} // util::get_elements()

//------------------------------------------------------------------------------
template <std::size_t... Indices, typename Coll>
decltype(auto) util::get_const_elements(Coll&& coll)
{
  return get_elements<Indices...>(std::as_const(coll));
}

//------------------------------------------------------------------------------

#endif // LARCOREALG_COREUTILS_GET_ELEMENTS_H