Sort 2 dimensional c array with std::sort

Question

I cannot seem to sort a 2 dimensional c array with std::sort. I can however sort a one dimensional array. This is in a case where I am being handed a c array in a c++ progra

Answer 1

Maybe there is some way to turn it into a std::array without copying it?

Perhaps not turning into a std::array per se, but an alternative approach might be to cast the 2D C-style arrays into a std::array reference just for the sorting. Doing so in reliance on the standard saying an std::array representation in memory at least begins with its C-style array equivalent. See here under [array.overview§2]:

An array is an aggregate that can be list-initialized with up to N elements whose types are convertible to T.

In practice, the following usage of reinterpret_cast is most probably safe, but do note that unless there is a special exception for it somewhere in the standard, it would formally be undefined behaviour:

#include <algorithm>
#include <array>
#include <iostream>

int main() {
  auto two_dim_less = [](std::array<int, 2>& a, std::array<int, 2>& b) {
      return a[1] < b[1]; };

  int two_dim[][2] = {{1, 8}, {2, 4}, {3, 10}, {4, 40}, {5, 1}};

  std::array<std::array<int, 2>, 5>& arr =
    *reinterpret_cast<std::array<std::array<int, 2>, 5>*>(&two_dim);

  std::sort(arr.begin(), arr.end(), two_dim_less);

  for (int i = 0; i < 5; i++)
    std::cout << two_dim[i][0] << ", " << two_dim[i][1] << '\n';

  return 0;
}

Output:

5, 1
2, 4
1, 8
3, 10
4, 40

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Regarding the use of std::qsort(), note that it is potentially slower than std::sort() due to the latter allowing to inline the comparisons while the former doesn't.

Answer 2

std::sort() requires the objects it's used to sort to be MoveAssginable.

Arrays are not MoveAssginable (nor assignable at all).

Try using an array of structures or std::pairs instead.