Jeremy Maitin-Shepard <jbms <at> attbi.com> writes:

I think there may be the problem that the various predicates used for the indices do not generally define comparison operators which compare predicates specialized on different types.

I think Gary and you are talking about different things. As you point out, there are difficulties in using the internal comparison objects of indices for defining operator <. Moreover, the standard clearly dictates that comparison between containers a and b be implemented as lexicographical_compare(a.begin(),a.end(),b.begin(),b.end()) leaving no option to use other comparison predicate than operator < between element objects (see std 23.1.5) A rationale for the decision of not using the internal comparison predicates is given in http://www.sgi.com/tech/stl/FAQ.html (section "Why doesn't map's operator< use the map's comparison function?") So, IMHO there's little doubt that operator < between indices of an indexed_set must use value_type::operator <, conterintuitive as it may seem. A similar reasoning applies to equality comparison between indices. Now, what Gary proposes (if I understand it right) is that we extend operator == and < so that indices on different element types can be compared. As I see it, this can be legally done, and it should do no harm as it merely extends the current functionality. It'd be something along these lines: template<...(1),...(2)> bool operator==( const *index class*<...(1)>& x, const *index class*<...(2)>& y) { return x.size()==y.size()&&std::equal(x.begin(),x.end(),y.begin()); } template<...> bool operator<( const *index class*<...(1)>& x, const *index class*<...(2)>& y) { return std::lexicographical_compare(x.begin(),x.end(),y.begin(),y.end()); } where ...(1) and ...(2) are (possibly different) sets of template intantiation arguments for *index class*. Is this what's being proposed? Joaquín M López Muñoz Telefónica, Investigación y Desarrollo