FAQ 19.02 What s a good mental model for friend classes?

FAQ 19.02 What's a good mental model for friend classes?

A secret handshake or other technique to exchange information with a confidant in such a way that normal people are unable to access the same secrets. Friend classes prevent users (the normal people) from seeing the information being exchanged through the secret codes that connect the various objects (confidants).

The overall effect is to keep private things private.

The alternative to granting special access privileges between the classes would be for the classes to declare public: member functions that allow anyone to manipulate the class's private: members. For instance, in the DatabaseCursor example, class Database would need to provide public: member functions to manipulate its cache, B-tree, file system, and so on. Although the implementation bits would be encapsulated, the implementation technique would be exposed. Subsequent changes to the implementation technique would break users' code.

In the traditional software realm, friendship is called tight cohesion, and is, within limits, considered good.