[boost] Re: [range] breaking changes

From: Thorsten Ottosen (nesotto_at_[hidden])
Date: 2005-04-06 11:01:24

"David Abrahams" <dave_at_[hidden]> wrote in message 
news:usm242h75.fsf_at_boost-consulting.com...
| "Thorsten Ottosen" <nesotto_at_[hidden]> writes:
|
| > | > yes, but in return the library only has two provide one overload
| > |                                          ^^^-"to"
| > | > of begin/end.
| > |
| > | Meaning that you don't need a separate overload taking Seq const& just
| > | to handle mutable rvalues.  That's good.  However, that benefit
| > | doesn't depend on uglification.  We could use the same old names; the
| > | interface would just not be useful for mutable rvalues unless the 2nd
| > | overload were provided.
| >
| > but now you introduce the ADL lookup problem again.
|
| What is "the ADL lookup problem?"

the problem that emerges when you want ADL to happen, but
cannot use an unqualified call because it would create conflicts
between

boost::begin;
sequence::begin
foo::begin

| > | Another point is that the whole iterator/const_iterator thing is
| > | artificial and broken anyway.  My work has evolved towards a "cursors
| > | and property maps" arrangement where cursors encode position/traversal
| > | and property maps encode access.  In that scheme there's no difference
| > | between the cursors used when traversing constant and mutable
| > | containers.  Of course, if you want to be able to get read access to a
| > | non-const property map rvalue you may need two overloads for that
| > | purpose.
| >
| > How would I traverse a range in that framework?
|
| It depends whether the cursors are homogeneous (like iterators, the
| usual case) or heterogeneous (a different type at each position).

so heterogeneous means "compile-time" iteration?

| The
| latter requires a recursive formulation; I won't go into it here
| because it's not really relevant to what we're discussing.  The former
| looks like:
|
|    end_cursor<S>::type finish;
|    for (begin_cursor<S>::type p = sequence::begin(s); p != finish; ++p)
|        ...
|
| In either case, elements are accessed via:
|
|    elements_type<S>::type e = sequence::elements(s); // the property map
|    value_type<S>::type v = e(p);                     // read
|    e(p,v);                                           // write

if e is a "map", then why dont you allow e[p] and e[p] = v ?

| > | We need to decide whether we're going to accept sub-optimal
| > | designs or not.  If we want to use standard iterators (for which
| > | there are obvious positive arguments) we are forced into some
| > | compromises that we wouldn't have to accept if using cursors,
| >
| > How does ADL lookup change in the new framework you're working on?
|             ^ ^^^^^^----- redundant ;-)

redundant? you will use the existing iterators as cursors then?

| Only that sequences will be providing begin(s) and not
| uglified_begin(s).  I was going with "uglified" names like
| sequence_begin() until I thought harder about Thomas' argument and
| considered the likelihood of bad collisions to be low.

it could fairly easily happen IMO. more and more code is put into
header files qualified syntax hence becomes more important

| > I don't see how it relates to the the proposed changes to
| > boost.range which has as its main purpose to enable ADL lookup while
| > using qualified notation.
|
| It's related to the argument that you only have to provide one
| overload of begin/end.

right, so it doesn't solve what I called "the ADL lookup problem".

| > | > |AFAICS there are two ways out of this
| > | > |
| > | > | a) X provides the unprefixed begin as well.
| > | >
| > | > how does that solve anything?
| > |
| > | It allows clients of X to use it without boost.range.  Go back and
| > | read Peter Dimov's posts in this thread: http://tinyurl.com/55j7b
| > |
| > 
(http://news.gmane.org/find-root.php?message_id=%3c004801c522aa%24ee97fd10%246501a8c0%40pdimov2%3e).
| > | Shouldn't a Boost.Range-conformant range type be usable without the
| > | dispatching functions in Boost.Range?
| >
| > well, for vector<int> v; you can use v.begin() etc. for pair<I,I> p, you
| > can use p.first etc.
|
| And what about a new range type that someone invents?  Should they
| really pick a different interface and not use something like the one
| Boost.Range already provides for its ranges?

So assume don't want to use boost.Range. They can use what ever syntax they
want and call their functions

namespace foo
{
    iterator begin( X& );
}

Assume they later want to use boost.range, just provide a simple overload

namespace foo
{
    iterator range_begin( X& r ) { return ::foo::begin(r); }
}

| > So I don't think that makes much sense; boost.range is a
| > framework---either you use it or you don't.
|
| I repeat, go back and read Peter's posts in http://tinyurl.com/55j7b.
| The framework ceases to own its concepts eventually.

Was it the concept it its customizarion points?. I recall it as it was the 
costumization points
that a framework ceases to own.

begin() is a bad customization point; hence we need two functions.

-Thorsten