#include <iostream>
#include <valarray>

int main()
{
    std::valarray<int> data{0, 1, 2, 3, 4, 5, 6, 7, 8, 9};

    std::valarray<std::size_t> idx{0, 2, 4, 6, 8};

    std::cout << "Original valarray: ";
    for (int n : data)
        std::cout << n << ' ';
    std::cout << '\n';

    data[idx] += data[idx]; // double the values at indices 'idx'

    // the type of data[idx] is std::indirect_array<int>

    std::cout << "After indirect modification: ";
    for (int n : data)
        std::cout << n << ' ';
    std::cout << '\n';
}

Original valarray: 0 1 2 3 4 5 6 7 8 9 
After indirect modification: 0 1 4 3 8 5 12 7 16 9