The template friend factory
What we need is to automatically generate a non-template function for every T type that’s used to instantiate the class template C. Of course, it is impossible to generate all of these functions in advance - there is a nearly unlimited number of T types that could, in theory, be used with the template class C. Fortunately, we do not need to generate operator+() for every one of such types - we only need them for the types that were actually used with this template in our program.
Generating friends on demand
The pattern that we are about to see is a very old one, and was introduced by John Barton and Lee Nackman in 1994 for a completely different purpose - they used it to work around certain limitations of the compilers that existed at the time. The inventors proposed the name Restricted Template Expansion, which was never widely used. Years later, Dan Sacks coined the name Friends Factory, but the pattern is also sometimes referred to simply...
