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The Power of Hidden Friends in C++

by Anthony Williams

From the article:

"Friendship" in C++ is commonly thought of as a means of allowing non-member functions and other classes to access the private data of a class. This might be done to allow symmetric conversions on non-member comparison operators, or allow a factory class exclusive access to the constructor of a class, or any number of things.

However, this is not the only use of friendship in C++, as there is an additional property to declaring a function or function template a friend: the friend function is now available to be found via Argument-Dependent Lookup (ADL). This is what makes operator overloading work with classes in different namespaces...