std::indirect_array<T>::operator=

#include <iomanip>
#include <iostream>
#include <numeric>
#include <valarray>
 
void print(int n, std::valarray <int> const& v)
{
 std::cout << n << ':';
 for (int e : v)
 std::cout << std::setw (3) << e;
 std::cout << '\n';
}
 
int main()
{
 std::valarray <int> v(8);
 std::iota (std::begin (v), std::end (v), 0);
 print(1, v);
 
 std::valarray <std::size_t > idx{1, 3, 5, 7};
 const std::indirect_array <int> ia = v[idx];
 // 'ia' refers to v[1], v[3], v[5], v[7]
 ia = -1; // (1), effectively:
 // v[1] = v[3] = v[5] = v[7] = -1;
 print(2, v);
 
 ia = /*std::valarray<int>*/{-1, -2, -3, -4}; // (2),
 // effectively: v[1] = -1, v[3] = -2, v[5] = -3, v[7] = -4;
 print(3, v);
 
 std::valarray <std::size_t > idx2{0, 2, 4, 6};
 const std::indirect_array <int> ia2 = v[idx2];
 // 'ia2' refers to v[0], v[2], v[4], v[6]
 ia = ia2; // (3), effectively:
 // v[1] = v[0], v[3] = v[2], v[5] = v[4], v[7] = v[6];
 print(4, v);
}

1: 0 1 2 3 4 5 6 7
2: 0 -1 2 -1 4 -1 6 -1
3: 0 -1 2 -2 4 -3 6 -4
4: 0 0 2 2 4 4 6 6