C++ Primer Plus

Shared Friends

Another use for friends is when a function needs to access private data in two separate classes. Logically, such a function should be a member function of each class, but that’s impossible. It could be a member of one class and a friend to the other, but sometimes it’s more reasonable to make the function a friend to both. Suppose, for example, that you have a Probe class that represents some sort of programmable measuring device and an Analyzer class that represents some sort of programmable analyzing device. Each has an internal clock, and you would like to be able to synchronize the two clocks. You could use something along the following lines:

class Analyzer;  // forward declarationclass Probe{    friend void sync(Analyzer ...