#include <iostream>
#include <algorithm>
#include <vector>
#include <functional>

class IdGen{
  int id;
public:
  IdGen(): id{0}{std::cout<<"constructed an IdGen with initial id="<<id<<"\n";} 
  int operator()(void){return (id++);};
};

int main(){
  std::vector<int> v(5),w(5);
  std::cout<<"call std::generate with class name IdGen()\n";
  std::generate(v.begin(), v.end(), IdGen());  
  for (auto vv:v) {std::cout<<vv<<" ";}
  std::cout<<"\n";
  std::generate(w.begin(), w.end(), IdGen());
  for (auto ww:w) {std::cout<<ww<<" ";}
  std::cout<<"\n";
 
  std::cout<<"call std::generate with std::ref(idgen), class IdGen object idgen\n";
  IdGen idgen;
  std::generate(v.begin(), v.end(), std::ref(idgen));
  for (auto vv:v) {std::cout<<vv<<" ";}
  std::cout<<"\n";
  std::generate(w.begin(), w.end(), std::ref(idgen));
  for (auto ww:w) {std::cout<<ww<<" ";}
  std::cout<<"\n";
}