Good OOP practice prefers to minimize interfaces that have access to internal state (including 'friend' functions or 'friend' classes), and prefers non-member, non-friend functions located in the same namespace--because of Koenig Lookup, these functions are still effectively a part of the class interface, but better promote encapsulation.

True member functions should be used to provide the minimum necessary interface needed to implement higher-level interface functions or when the object is itself modified by the operation. Using these member functions should be the preferred means of implementing higher-level functionality through non-member, non-friend functions, but non-member friend functions can be used in cases when a function requires non-modifying access to object internals (though this usually indicates that your "true member" interface is lacking) using a const reference to the object.

Good OOP practice prefers to minimize interfaces that have access to internal state (including 'friend' functions or 'friend' classes), and prefers non-member, non-friend functions located in the same namespace--because of Koenig Lookup, these functions are still effectively a part of the class interface, but better promote encapsulation.

True member functions should be used to provide the minimum necessary interface needed to implement higher-level interface functions or when the object is itself modified by the operation. Using these member functions should be the preferred means of implementing higher-level functionality through non-member, non-friend functions, but non-member friend functions can be used in cases when a function requires non-modifying access to object internals (though this usually indicates that your "true member" interface is lacking).