on Tue Jul 10 2007, Andrew Sutton <asutton-AT-cs.kent.edu> wrote:
I'm not clear what your "truly optional" parameters are actually for, but if (for example) they supply a functor could the default value be some sort of identity functor?
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.
In that particular case I would make the defaults for those parameters be stubs that satisfy the syntactic concept requirements but on which the "recording" operations compile away to nothing. In general, I would try hard to end up with one very generic algorithm operating on parts that may or may not do anything. In cases where it's impossible to share the same code without loss of efficiency, I would use something like "tag dispatching" to choose among different implementations. It would be easy enough to dispatch on boost::parameter::void_, for example. HTH, -- Dave Abrahams Boost Consulting http://www.boost-consulting.com The Astoria Seminar ==> http://www.astoriaseminar.com