C++17 / C++1z parallel usage of std::for_each

Question

I want to use C++17 parallel capabilities to divide every element of a std::vector by some constant and store the result in another std::vector of same length and (!!) order.

E.g.

{6,9,12} / 3 = {2,3,4}

I have a not compiling example

#include <execution>
#include <algorithm>

template <typename T>
std::vector<T> & divide(std::vector<T> const & in)
{
  std::vector<T> out(in.size(), 0);

  float const divisor = 3;

  std::for_each
  ( std::execution::par_unseq
  , in.begin()
  , in.end()
  , /* divide each element by divisor and put result in out */ );

  return out;
}

How can I get this running, lockless and threadsafe?

Answer 1

something like this:

#include <vector>
#include <algorithm>
#include <execution>

template <typename T>
std::vector<T> divide(std::vector<T> result)
{
    // ^^ take a copy of the argument here - will often be elided anyway 

    float const divisor = 3;

    // the following loop mutates distinct objects within the vector and
    // invalidates no iterators. c++ guarantees that each object is distinct
    // and that neighbouring objects may be updated by different threads
    // at the same time without a mutex.
    std::for_each(
        std::execution::par, 
        std::begin(result), 
        std::end(result), 
        [divisor](T& val) {  // copies are safer, and the resulting code will be as quick.
            // modifies value in place
            val /= divisor;
        });

    // implicit fence here. Safe to manipulate the vector as a whole.
    // from here on

  // return by value. Allows RVO.
  return result;
}