operator() for structs in C++

structs_as_functions.cpp

#include <iostream>
#include <vector>
using namespace std;

// an illustration for my Julia-related question on julia-users

struct datfun {
   // some data
   vector<double> x;
   vector<double> y;

   // operator() lets the object act like a function;
   // note that here we operate on the object's fields
   // (one function, different "hidden" input data)
   vector<double> operator()(int i) {
      vector<double> ret(2);
      ret[0] = x[i];
      ret[1] = y[i];
      return ret;
   }


   datfun(const vector<double>& _x, const vector<double>& _y) { // auxiliary
      x = _x; y = _y;
   }
};

ostream& operator<<(ostream& out, const vector<double>& v) { // auxiliary
   for (int i=0; i<v.size(); ++i)
      out << v[i] << " ";
   out << endl;
}


int main() {
   // example vectors:
   vector<double> x1(10), y1(10), x2(10), y2(10);
   for (int i=0; i<10; ++i) {
      x1[i] = i+1;
      y1[i] = 10*(i+1);
      x2[i] = -x1[i];
      y2[i] = -y1[i];
   }

   // create two objects:
   datfun f1(x1, y1);
   datfun f2(x2, y2);

   // and now:
   cout << f1(1); // 2 20
   cout << f2(1); // -2 -20

   x1[1] = 1000;
   cout << f1(1); // 2 20 (no change -> deep copy)

   return 0;
}