From: Andrew Sutton
I guess it might be worthwhile to distinguish formally between optional parameters and those with default values. A parameter with default values is still a required input to a function even if the user doesn't explicitly pass a corresponding value. It's value is supplied by the specification. An optional parameter is one affects the function's behavior by its absence or presence in the argument list.
There are actually a number of examples in Boost.Graph. Consider dijkstra_shortest_paths(). There are two optional (output) parameters that can be passed - a predecessor map that records the parents in the shortest path tree and a distance map that records the distance to each vertex. If they aren't supplied the algorithm doesn't do any extra work. If one or both is supplied, the algorithm records parents and/or distances. This isn't an extreme example of affected behavior, but it is probably the most common.
Aahh. Light dawns.
The idea of using functors is interesting... but for some reason, I'm having trouble seeing how the solution would work this morning.
That's because I was thinking of something like "copy from here to there, optionally transforming the copied values", where a default of template <class T> T& identity(T& x) { return x; } would be fine (modulo dangling references). I'm sure someone will be along shortly to show how to do it :-)
I'll have to give it some thought. I think, however, that I'm doing something similar with my `not_given` type - it's being used as a sort of token that, when passed to template functions doesn't do anything.
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