Handling friendship with the attorney-client idiom

Granting functions and classes access to the non-public parts of a class with a friend declaration has been usually seen as a sign of bad design, as friendship breaks encapsulation and ties classes and functions. Friends, whether they are classes or functions, get access to all the private parts of a class, although they may only need to access parts of it.

The attorney-client idiom provides a simple mechanism to restrict friends access to only designated private parts of a class.

Getting ready

To demonstrate how to implement this idiom, we will consider the following classes: Client, which has some private member data and functions (the public interface is not important here), and Friend, which is supposed to access only parts of the private details, for instance, data1 and action1(), but has access to everything:

class Client
{
  int data_1;
  int data_2;
  void action1() {}
  void action2() {}
  friend class...