CppCon 2018

A friend of a class is a function or class that is not a member of that class, but nonetheless has access to all members of that class. The conventional wisdom is that granting friendship is something you do should sparingly. This lecture introduces a distinctive use for friend function declarations in class templates that's not about granting access to non-public members. Rather, it's about using a template as a
factory for "making" new friends. Along the way, this session explains various subtleties about operator overloading, overload resolution,
template instantiation, and name lookup for friend functions.