Implementation C++14 make_integer_sequence

Question

I tried to implement the C++14 alias template make_integer_sequence, which simplifies the creation of the class template integer_sequence.

templ         


        

Answer 1

This is basically me hacking around Xeo's solution: Making community wiki - if appreciative, please upvote Xeo.

...just modified until I felt it couldn't get any simpler, renamed and added value_type and size() per the Standard (but only doing index_sequence not integer_sequence), and code working with GCC 5.2 -std=c++14 could run otherwise unaltered under older/other compilers I'm stuck with. Might save someone some time / confusion.

// based on http://stackoverflow.com/a/17426611/410767 by Xeo
namespace std  // WARNING: at own risk, otherwise use own namespace
{
    template 
    struct index_sequence
    {
        using type = index_sequence;
        using value_type = size_t;
        static constexpr std::size_t size() noexcept { return sizeof...(Ints); }
    };

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

    template 
    struct _merge_and_renumber;

    template 
    struct _merge_and_renumber, index_sequence>
      : index_sequence
    { };

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

    template 
    struct make_index_sequence
      : _merge_and_renumber::type,
                            typename make_index_sequence::type>
    { };

    template<> struct make_index_sequence<0> : index_sequence<> { };
    template<> struct make_index_sequence<1> : index_sequence<0> { };
}

Notes:

• the "magic" of Xeo's solution is in the declaration of _merge_and_renumber (concat in his code) with exactly two parameters, while the specilisation effectively exposes their individual parameter packs

• the typename...::type in...

  struct make_index_sequence
    : _merge_and_renumber::type,
                          typename make_index_sequence::type>
  

    avoids the error:

invalid use of incomplete type 'struct std::_merge_and_renumber, std::index_sequence<0ul> >'