lib/parforeach.cpp

The following code example is taken from the book
C++17 - The Complete Guide by Nicolai M. Josuttis, Leanpub, 2017
The code is licensed under a Creative Commons Attribution 4.0 International License.

---

// raw code

#include <vector>
#include <iostream>
#include <algorithm>
#include <numeric>
#include <execution>  // for the execution policy
#include <cmath>      // for sqrt()
#include "timer.hpp"

int main()
{
  int numElems = 1000;

  struct Data {
    double value;  // initial value
    double sqrt;   // parallel computed square root
  };

  // initialize numElems values without square root:
  std::vector<Data> coll;
  coll.reserve(numElems);
  for (int i=0; i<numElems; ++i) {
    coll.push_back(Data{i * 4.37, 0});
  }

  // parallel computation of square roots:
  for_each(std::execution::par,
           coll.begin(), coll.end(),
           [](auto& val) {
             val.sqrt = std::sqrt(val.value);
           });
}