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Subject: Re: [boost] Cxx dual library
From: Edward Diener (eldiener_at_[hidden])
Date: 2016-06-05 20:30:50

On 6/5/2016 5:55 PM, Rob Stewart wrote:
> On June 4, 2016 10:59:16 PM EDT, Edward Diener <eldiener_at_[hidden]> wrote:
>> On 6/4/2016 9:28 PM, Rob Stewart wrote:
>>> Edward, you've missed that Vicente doesn't see that importing either
>> a Boost or a Standard Library solution, into a common namespace
>> requires anything more of the user.
>>> The user would always use the new, common namespace name for
>> something, regardless of its original namespace. For example, foo::x is
>> the common name, but x may have been introduced into the foo namespace,
>> by a using directive, from the boost or the std namespace.
>>> In the end, the user always includes the foo header and uses the foo
>> name. There are no macros in the user's code. Both solutions select one
>> implementation or the other. Yours refers to the namespace of the
>> selected implementation with a macro, while his just uses namespace
>> foo.
>> It does involve more work and macros are still being used, although not
>> to name the namespace. I honestly think that CXXD's solution is cleaner
>> and more flexible. Lifting constructs which are normally accessed
>> through one namespace to another namespace in that way seems to me a
>> kludge. I am not sure of all the ramifications of doing this but I
>> would
>> be wary of doing such a thing myself.
>> You also don't have the technique of overriding the dual library choice
>> that you have with CXXD, unless you manually change some source, which
>> you may not have access to in the first place. But if you think the
>> macros are going to cause problems, just because they are macros and
>> therefore "evil', by all means roll your own solution as Vicente has
>> done.
> See whether you still think so after reading my reply to Niall.

My thought is generally that CXXD represents the easiest approach and
that importing either side of a dual library's namespace into another
common namespace, no matter how it is controlled, does not represent
anything better than what CXXD has to offer.

Furthermore, although I haven't had the time or knowledge to work
through all possible situations I believe that Vicente's approach could
have problems involving customization points for a library, ADL, and/or
template specializations.

Vicente's approach is essentially changing the namespace in which
constructs operate and any prior extensions to those constructs, in
whichever side of the dual library is chosen, seem like they could be
lost. The extensions will refer to a namespace in which the rest of the
constructs are no longer operating or the extensions will be written in
a namespace in which the rest of the constructs will no longer be
operating. Either way I anticipate some C++ name lookup problems along
the lines of using constructs in the common namespace no longer finding
template specializations or extension points in a different namespace or
referring, through ADL, to a different namespace. Of course I could be
wrong about this and everything just "works" despite the different
namespace in which all those constructs are now operating. But why deal
with those possible problems in the first place.

In CXXD the namespace stays the same and I think this is much safer. The
fact that a simple object-like macro refers to the correct namespace at
all times bothers me not at all simply because it is a macro.

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