On Sun, May 11, 2003 at 01:47:00PM -0400, David Abrahams wrote: > > I have a suggestion for another "orthogonal" feature of iterators, which is > > available for plain pointers : NULL. More precisely, the fact that the > > default initialized iterator is a unique (singular) value which you can > > reliably test equality with. > > > > It is very useful to have this capability : in the CGAL library, we store > > the nodes of a graph in containers. Graphs are a bunch of nodes storing > > pointers/iterators to each other. Having a value like NULL is very useful > > there, because the alternative is either to allocate a specific object > > whose iterator you use for this purpose (but it requires access to the > > associated container to test with, which is painful), or storing a bool, > > which is a waste of memory. > > > > I don't know if these issues arise in the BGL as well. > > > > I think this feature doesn't make sense for all iterators, especially for > > Input and Output iterators. However, for iterators connected with > > containers, they are usually trivial to provide because the iterator > > usually stores a pointer, which can then be default initialized to NULL. > > > > In the case of CGAL, it would simply allow us to re-use the standard > > containers for storing our graphs, while right now we re-implement > > containers... > > What, specifically, would allow you to re-use the standard > containers? I'm looking for a statement along the lines of, "if ... > then we could re-use the standard containers". > > I'm guessing "..." is something like, "if default-constructed standard > container iterators were non-singular and guaranteed to be unequal to > any iterator obtained by other means".
That would be something more like "if default-constructed standard container iterators were non-singular and guaranteed to be unequal to any iterator obtained by other means, and compare equal to any default-constructed iterator." I.e. the default constructed value must be unique (as far as operator== is concerned). > > Another feature (less important but connected to the previous one) that > > pointers have and iterators don't : automatic conversion to bool. > > > > What do you think ? > > It sounds like a useful concept, if I've understood you correctly. > > At the same time, I doubt it belongs in our proposal: we don't say > anything about containers (except vector<bool>, and then only in the > rationale). You should write a proposal for extension and submit it > to the commitee if you care about this. It should be easy, compared > to the iterator adaptors/categories proposal. OK. I'll think about writing something. -- Sylvain _______________________________________________ Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost
