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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?

Right! Sorry I was too terse in my previous email. There is no reason why not to extend operator==() and operator<() to allow for two different container types to be compared. Either there is a comparison that will work for the two types or there is not, but it isn't the indexed set that enforces it, but rather std::equal and std::lexigraphical_compare. So the proprosal is to also extend all of the comparison operators to take the two complete sets of templated arguments. ie. Old: template<typename Value,typename IndexSpecifierList,typename Allocator> bool operator==( const indexed_set<Value,IndexSpecifierList,Allocator>& x, const indexed_set<Value,IndexSpecifierList,Allocator>& y); New: template<typename Value1,typename IndexSpecifierList1,typename Allocator1> template<typename Value2,typename IndexSpecifierList2,typename Allocator2> bool operator==( const indexed_set<Value1,IndexSpecifierList1,Allocator1>& x, const indexed_set<Value2,IndexSpecifierList2,Allocator2>& y); etc.. AFAIK there is no penalty for the library and much to be gained for the user. Yours, -Gary-