c++ reversed integer sequence implementation

Question

Who knows how to implement C++ std::make_index_sequence reverse version. To get - make_index_sequence_reverse<int, 5> = <4,3,2,1,0>. Thank you!

Laruelarum

Welcome to SO! Please take a look at how to ask questions .- can you post any attempts you've made? – Greenbrier

@Andrey Avraliov Also accept an answer! – Frayne

Answer 1

IMHO, there is no reason for a index_sequence_reverse: std::index_sequence support sequences of indexes and are order neutral (or even without order).

If you can use std::make_index_sequence, for a makeIndexSequenceReverse you can make something as follows

#include <utility>
#include <type_traits>

template <std::size_t ... Is>
constexpr auto indexSequenceReverse (std::index_sequence<Is...> const &)
   -> decltype( std::index_sequence<sizeof...(Is)-1U-Is...>{} );

template <std::size_t N>
using makeIndexSequenceReverse
   = decltype(indexSequenceReverse(std::make_index_sequence<N>{}));

int main ()
 {
   static_assert( std::is_same<std::index_sequence<4U, 3U, 2U, 1U, 0U>,
      makeIndexSequenceReverse<5U>>::value, "!" );
 }

Strobel

This was not the OP's question, but I would like to note something here: Inverting a sequence with sizeof...(Is)-1u-Is only works if the index sequence covers the full range of the original type sequence (e.g., from a tuple). If the index sequence is only a subset, one must consider the size of the original type sequence and the base index of the subset. – Frayne

Answer 2

Here's a way to do it with inheritance:

template <std::size_t, typename>
struct make_reverse_index_sequence_helper;

template <std::size_t N, std::size_t...NN>
struct make_reverse_index_sequence_helper<N, std::index_sequence<NN...>> 
   : std::index_sequence<(N - NN)...> {};

template <size_t N>
struct make_reverse_index_sequence 
   : make_reverse_index_sequence_helper<N - 1, 
        decltype(std::make_index_sequence<N>{})> {};

The helper struct is used to deduce the parameters and apply the subtraction. It can be used just like std::make_index_sequence because it derives from std::index_sequence, as you can see here:

std::index_sequence<4, 3, 2, 1, 0> x = make_reverse_index_sequence<5>{};

Heterolecithal

This solution is general and consistent with the STL, much appreciated! – Perkin