RE: [boost] Re: Formal Review: Indexed Set

From: Powell, Gary (powellg_at_[hidden])
Date: 2004-03-24 11:57:57

>> 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-