FAQ 19.04 Do friends violate the encapsulation barrier?

Not necessarily.

If the encapsulation barrier is narrowly defined as the suite of member functions on a class, then friends violate the encapsulation barrier. However, this is a naive view of encapsulation, and applying it consistently actually degrades the overall encapsulation of a system. For example, if another entity needs to be part of the same abstraction, this naive approach suggests that the first class should expose its implementation technique via an unnecessarily large suite of get/set member functions.

The enlightened view is that the encapsulation barrier encapsulates an abstraction, not just a class. For example, the earlier example of a database with multiple cursors illustrates an abstraction that is too rich to be implemented by a single class. In cases like this, friend classes are a valuable way of hiding the (possibly complex) interrelationships between the various pieces of the abstraction.

Friends don't violate the encapsulation barrier; they are part of the encapsulation barrier.