std::indirect_array

#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 indexes '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';
}