#include <iostream>
#include <vector>
using namespace std;
 
template <typename ELEM>
ostream& operator << (ostream &out, const vector<ELEM> &vec)
{
  const char *sep = "{ ";
  for (const ELEM &elem : vec) { out << sep << elem; sep = ", "; }
  return out << " }";
}
 
int main()
{
  vector<double> v1{ 1.0, 2.0, 3.0 }, v2{ 4.0, 5.0, 6.0 };
  // in one line:
  vector<decltype(v1.front() * v2.front())> v3 { v1[0] * v2[0], v1[0] * v2[1], v1[0] * v2[2], v1[1] * v2[0], v1[1] * v2[1], v1[1] * v2[2], v1[2] * v2[0], v1[2] * v2[1], v1[2] * v2[2] };
  // output:
  cout << v3 << endl;
  // done
  return 0;
}

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