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 an operator+() for every one of such types—we only need them for the types that were actually used with this template in our program.
The template friend factory
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...
