Exercise solution:
A variant of the unique_ptr program

// Variation of unique-ptr-1.cc

#include <memory>
#include <iostream>
#include <string>
#include "point.h"

using namespace std;

unique_ptr<Point> f(unique_ptr<Point> pp1){   // Notice the new signature.
  unique_ptr<Point>ap1(move(pp1)),      // Cannot copy unique_ptr. Cast to rvalue by move.  
                   ap2(new Point(3,4));

  cout << "ap1: " << *ap1 << endl; 
  cout << "ap2: " << *ap2 << endl; 

  ap2->displace(1,1);
  cout << "ap2: " << *ap2 << endl; 

  cout << "ap1-ap2 dist: " << ap1->distance_to(*ap2) << endl; 

  // Destructive copying:  The point referred by ap2 is deleted.
  //                       The pointer encapsualated in ap1 is moved to ap2.
  //                       ap1 is unbound (becomes nullptr).
  cout << "Now assigning ap1 to ap2" << endl;
  ap2 = move(ap1);        

  if (ap1.get())                        // ap1 is nullptr
     cout << "ap1: " << *ap1 << endl;  
  else
     cout << "ap1 is nullptr" << endl;

  if (ap2.get())                        // (1,2)
     cout << "ap2: " << *ap2 << endl;
  else
     cout << "ap2 is nullptr" << endl;

  return ap2;        // ap2 is moved out of f.
}

int main(){
  unique_ptr<Point> p(new Point(1,2));
  p = f(move(p));   // notice move

  cout << "p: " << *p << endl; // Now OK. (1,2).
                               // p is moved back from f.
}

ap1: (1,2)
ap2: (3,4)
ap2: (4,5)
ap1-ap2 dist: 4.24264
Now assigning ap1 to ap2
Point deleted
ap1 is nullptr
ap2: (1,2)
p: (1,2)
Point deleted