Callback implementations in C++

This paper presents pattern histories for deriving two different callback libraries. The pattern histories document the important design decisions behind each library. The two resulting libraries are compared to determine which patterns lead to a better solution. The patterns community is identifying many different patterns that solve the same problems. This paper shows that it is possible to use objective comparisons of the resulting solutions, as opposed to relying on personal reactions, the expertise of the author, and finding three previous uses of the pattern, to determine which patterns lead to better solutions