This article can be found in the book "C++ Gems" (I think it originally appeared in the C++ Report). It's interesting from the historical perspective. Rich Hickey is well-known today as the creator of Clojure, but in the 1990ies he was apparently doing C++ development. Back in 1994 when this article was authored, C++ templates were not as well developed as they are today. Beyond generic collection, templates were not much in use. The state of the art for callbacks was a base class with a pure virtual function that you were supposed to implement in a derived class. Most (all?) well knows C++ libraries did it this way. Nobody thought of `std::function` yet. Besides, implementing something like `std::function` using templates of 1994 presented a significant challenge. But Rich Hickey not only recognized the need of such abstraction, but also provided a quite reasonable implementation.