class X:
  field x1
  field x2
  field x3
  func get_x1:
    return x1

f(x1, x2):
  - we know that we only need x1, x2
  - we can use x1 and x2 directly, we don't care about how we get them
  - we don't rely on class X getters

f(X):
  - we don't know which attributes we need, we can use x3 by mistake
  - should get attributes (x1, x2), we will write more code
  - if f needs more x3 we can get it easily without modifying the signature