start:                  a = 13, b = 3
  inside swapByValue:     x = 3, y = 13
after swapByValue:      a = 13, b = 3
after swapByPointer:    a = 3, b = 13
after swapByReference:  a = 13, b = 3
after std::swap:        a = 3, b = 13
