#include <iostream>
#include <functional>
 
template <typename OBJ>
int foo(int n, OBJ o)
{
  int x = 0;
  for (int i = 0; i < n; ++i) {
    x += o[i];
  }
  return x;
}
 
template <typename> struct Functor;
 
template <typename R> struct Functor<R(int)>
{
   using ftype = std::function<R(int)>;
   Functor(ftype f) : f_(f) {}
 
   R operator[](int i) const { return f_(i); }
 
   ftype f_;
};
 
int main()
{
   Functor<int(int)> f = {[](int i) -> int {return i*i;}};
   std::cout << foo(10, f) << std::endl;
}
 

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