FAQ 19.07 What are some guidelines to make sure friend functions are used properly?

Friend functions should be part of the class's public: interface, and their code should be owned by the class's owner.

Guideline 1: Friend functions should make the user's code easier to understand. Look at some sample syntax of how a user would use the class with the friend function, and compare it with the moral equivalent of the sample syntax if the friend function were changed into a member function. The friend function version should be used if and only if it results in more intuitive user code (see FAQ 19.06).

Guideline 2: Friend functions should be used only for operations that are part of the public: interface of a class. They should not be used every time someone wants to do something tricky with the class. If a user has a function that needs to access the innards of your class (for example, because the class's current public: interface isn't powerful enough), fix the problem (the interface) rather than patching the symptoms. Don't grant friendship to everyone.

Guideline 3: A friend function or class should be under the political and technical authority of the same team that owns the class itself. Granting friendship status to a function or class under the political authority of a team other than the one implementing the class results in a scheduling headache changes that involve coordinating multiple participants who may not always be in a position to handle the requested modifications in a timely manner is a nightmare.