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Subject: Re: [boost] Update on status of C++ 11 only Boost fork/modular distro
From: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2014-10-08 07:28:33

On 7 Oct 2014 at 16:55, Mikael Persson wrote:

> Personally, I find this proposal very scary. I'm just gonna give my opinion
> on this, make what you want of it.
> One little thing, I'm not sure that STL11 is a particularly good name for
> this, since most of the new C++11 libraries would not be classified as
> "STL" libraries (except for unordered containers and <array>). But that's
> just a trivial issue of terminology.

Interestingly, one of the most obvious things you see in the bindings
output is the word 'template' due to how template heavy the C++ 11
additions are. But I mainly chose it for being short and descriptive.

> You state in your readme for your generator:
> ".. or write some brittle bindings which manually replicate STL items .."
> In my experience, what can make those kinds of manual bindings brittle are
> all those subtle incompatibilities (often undocumented) between analogous
> components of the alternative implementations.

In the use case situation of providing some/any C++ 11 STL
implementation, I would see Boost as just one of Dinkumware,
libstdc++ and libc++. If your code already doesn't mind which STL it
runs on, it is highly unlikely to get suddenly brittle with Boost,
and moreover if it does, there is probably a bug in there.

Of course, existing Boost libraries (with a few exceptions like ASIO)
have only ever been tested against one C++ 11 STL implementation, the
one coming with Boost. That is probably a bad thing. I would expect
any surprises to emerge from binding to another C++ 11 STL to be a
good thing.

Even with AFIO, I found it broke in useful ways when I ported to
libc++. That found sloppiness in my lack of use of noexcept which
merely caused silent poor performance with libstdc++.

> From a distribution point of view, I see some big problems too. I guess the
> idea, if I understand correctly, is to distribute your binding generator
> along with the select-few boost header-only libraries, and then, the user
> would have to run the binding generator for his particular platform to
> generate the implementation-detail binding headers of those boost library
> ports. Or, if you want to spare the user from having to do this, then you
> must distribute a specific version of all the headers that will work for
> their particular platform (OS, compiler and std-lib implementation) and the
> particular versions of those platform components. Either way, this is a
> distribution nightmare.

The idea is that the bindings generator makes a starting set of
bindings. The library maintainer then adjusts those to fit their
library and bundles them with their library. The bindings are then
fixed, and are part of the library implementation. Remember that
using Boost also goes through bindings, we basically allow the end
user to choose which STL 11 implementation for a Boost library to
use. I am proposing that we "virtualise" the STL implementation used
by Boost libraries so it is end user selectable.

> As for "forking" boost based on this, I don't understand why this is
> necessary, or maybe I don't quite understand what it is meant to really
> involve. Most boost libraries already contain a significant amount of
> conditional code and workarounds to deal with platform-specific issues or
> to use beneficial features that only some platforms support (including
> C++11 features). Why would it be necessary to have a completely separate
> version or distribution of those libraries that are targeted at platforms
> with the (mythical) feature-complete C++11 library implementation? That is,
> if you really mean to "fork" the boost libraries in that way, there is
> going to be enormous costs associated to doing that, mostly in terms of
> maintenance and man-power.

There is plenty of discussion in the archives of this mailing list to
explain all that, and I would prefer to not repeat all that yet
again. I will say this: this is part of one low cost path to
achieving modular Boost. If you consider actual useful modularity
desirable, this is one step along that path as somehow one needs to
create separation between a Boost library and its dependencies, and
it's not like we've been making progress on this for some years now
other than endlessly debating header analysed dependencies, which
while helpful doesn't actually achieve real modularisation.

The local namespace binding technique lets the library maintainer
explicitly mark up which dependencies are substitutable (e.g. STL
ones), which are mandatory, and which are optional.

We have yet to think of a formal set of guidelines for marking up
mandatory dependencies and optional dependencies. Baby steps.

> I think that a much better alternative strategy is to do what boost has
> done with Boost.TR1. Why not create a library wrapper like Boost.TR1 but
> for C++11/14? Isn't that what users want, i.e., to be able to use standard
> C++11/14 library components and not have to worry about running their code
> on platforms that don't support it or only partially? I mean, people who
> use Boost.Thread today, instead of <thread>, use it because it works
> everywhere. But if they could include something like
> <boost/cpp11/thread.hpp>, and use "std::thread" and friends, then the only
> remaining platform-specific work would be to install and link against
> libboost_thread for platforms that do not provide the standard <thread>
> library.

Thing is, TR1 was a set of new libraries, so it was straightforward
to make a Boost.TR1 and it was contained. C++11/14 involves an
enormous number of changes to *existing* Boost libraries, so we have
been enhancing Boost libraries with C++ 11/14 features with varying
rates of progress.

Speaking only as someone on the Boost.Thread team as I can't speak
much for other Boost libraries ... Boost.Thread already provides a
significant superset of the C++ 11 STL, and the intent is that by
next year it shall provide a significant portion of a potential C++
17 threading STL too. However, we have a problem: what if a user
library wants to use Boost.Thread's potential C++ 17 facilities with
std:: not boost:: threading facilities - a perfectly reasonable
request, especially if a user library only and exclusively uses
std::thread et al and has zero wish to buy into the boost ecosystem
apart from the 17 functionality?

My opinion on this is that the potential C++ 17 features we add to
Thread also need to be capable of existing in std::experimental, so
as an example of a current work item of mine, the proposed
expected<T, E> based non-allocating future-promise could appear in
either or both of:

1. namespace boost { namespace expected_future { inline namespace v1
{ ...

2. namespace std { namespace experimental { inline namespace
expected_future { inline namespace v1 { ...

However, if you think it through, expected_future::future<T, E> needs
to understand both std::exception_ptr, std::chrono etc as well as
boost::exception_ptr, boost::chrono etc. If a user configures a
std::experimental target they may still wish to feed it boost STL
items in addition to std STL items. They also may wish both
expected_future implementations to coexist in the same code base, and
to interoperate seamlessly.

These are some of the issues we face moving forwards. C++ is complex,
not helped by our collective historical failure to deal properly with
ODR violation resolution or solving object componentisation as my C++
Now 2014 paper describes.

> An additional way of doing this is to simply require that the maintainers
> of the libraries that have been standardized to update their library such
> that it binds to the standard C++11 library implementation (as thinly as
> possible) whenever available and sufficiently compatible (I think that most
> of the boost libraries are today, they seem to have recovered from all the
> mayhem following the C++11 standard and all its slight differences w.r.t.
> Boost). In other words, if you include <boost/thread.hpp> on a
> feature-complete C++11 platform, it should do little more than include
> <thread> and bind its components to the boost namespace.

Requiring maintainers to do anything, including do any maintenance,
has proven to be hard. We already have many unmaintained libraries,
some with decade old unfixed issues, so enforcing raised maintenance
requirements just makes more unmaintained libraries. As a personal
opinion, I would personally welcome that as an opportunity to
deprecate and eliminate a large number of deteriorating Boost
libraries, but mine is not the consensus opinion on this, and is
definitely not the opinion of the steering committee.

> In both cases, the responsibility of maintaining the C++11/14 compatibility
> in those boost library rests on the shoulder of the individual maintainers.
> No forks required.

At no stage did I ever propose the forked code stops being compatible
with legacy monolithic Boost. Forked code simply becomes dual use.

Why use the word fork then? It's still a fork in approach,
methodology, intent, practice and most especially in Boost
philosophy. It makes possible my alternative method of ranking
library quality via automated scripts. That is definitely a fork in
the definition of what qualifies a library to gain the Boost stamp of

> And the local bindings (generated or not, from boost to
> std or vice versa) can be maintained more easily. Incompatibilities and
> other platform-specific can be dealt with more practically by manually
> maintaining the bindings. And users simply need to be advised about what
> features a platform should have to allow a specific boost library to be
> used standalone and / or without having to install and link to a
> pre-compiled library.
> Maybe I completely misunderstood your goals, and all I said are things you
> had already considered. I hope so.

I think what I am proposing actually is mostly what you are looking

> This is just the way I see things. Hearing the words "fork" in the same
> sentence as "boost", gives me chills... (unless it means "fork" only in the
> sense of a temporary developement branch that is meant to be merged back
> quickly).

The other aspect of this being a fork is that new Boost libraries
joining my modular distro simply wouldn't bother joining the
monolithic Boost distro. That is already happening anyway to an
extent - as my C++ Now presentation showed, exciting new C++
libraries are increasingly not trying to enter Boost any more and the
Boost participation rate has been declining steadily for two years

I am hoping that through modularation and my alternative automated
ranking of quality, the archaic buy ins demanded by Boost to enter
Boost go away and thus shall boost (!) the greenfield library
participation rate. I in particular want
the boost namespace to go away where possible, it screams
monolithicism and
cathedral instead of bazaar. And besides with inline namespaces and
full fat namespace binding in 11/14, separate namespaces for
everything is now very straightforward to do - let the user bind
dependencies into their local namespace instead of Boost
monolithically deciding for you.


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