async: Concurrent Calculation

// dotProductAsync.cpp
#include <iostream>
#include <future>
#include <random>
#include <vector>
#include <numeric>
using namespace std;
static const int NUM= 100000000;
long long getDotProduct(vector<int>& v, vector<int>& w){
  
  auto vSize = v.size();
  auto future1 = async([&]{ 
    return inner_product(&v[0], &v[vSize/4], &w[0], 0LL); 
  });
  
  auto future2 = async([&]{ 
    return inner_product(&v[vSize/4], &v[vSize/2], &w[vSize/4], 0LL); 
  });
  
  auto future3 = async([&]{ 
    return inner_product(&v[vSize/2], &v[vSize* 3/4], &w[vSize/2], 0LL);
  });
  
  auto future4 = async([&]{ 
    return inner_product(&v[vSize * 3/4], &v[vSize], &w[vSize * 3/4], 0LL); 
  });
  return future1.get() + future2.get() + future3.get() + future4.get();
}
int main(){
  cout << endl;
  random_device seed;
  // generator
  mt19937 engine(seed());
  // distribution
  uniform_int_distribution<int> dist(0, 100);
  // fill the vectors
  vector<int> v, w;
  v.reserve(NUM);
  w.reserve(NUM);
  for (int i=0; i< NUM; ++i){
    v.push_back(dist(engine));
    w.push_back(dist(engine));
  }
  cout << "getDotProduct(v, w): " << getDotProduct(v, w) << endl;
  cout << endl;
}