Re: The new Boost 17.0.0

From: Antony Polukhin (antoshkka_at_[hidden])
Date: 2020-12-01 21:39:12

On Wed, Dec 2, 2020, 00:22 Edward Diener via Boost <boost_at_[hidden]>
wrote:

> On 12/1/2020 2:44 PM, Antony Polukhin via Boost wrote:
> > On Fri, Nov 27, 2020, 15:54 Edward Diener via Boost
> >>
> >> If a library provides support for C++98/2003 but also works perfectly
> >> fine using C++17, please explain to me what is wrong with using that
> >> library when compiling for C++17 ? Thank you !
> >>
> >
> > For example, that library may use boost/type_traits.hpp and boost/mpl/*.
> > Those headers include a bunch of text and variadic templates emulation,
> and
> > compile times getting drastically worse.
> >
> > Another example: library that uses boost/optional.hpp includes ~500kB of
> > code. To make things funnier, ln some standard libraries inclusion of
> > boost/optional.hpp
> > leads to inclusion of <optiknal>, so removing the dependency drops 500kb
> of
> > text from each translation unit.
> >
> > Final example: library work in c++98 and it almost does not depend on
> other
> > Boost libraries. Great! It's a perfect candidate for Boost17.
> >
> > Nothing wrong in c++98 support, the problem is in the dependencies and
> huge
> > translation units to compile.
>
> So if you use in a C++17 project a Boost library which supports
> C++98/C++03 you have huge translation units but if you use a Boost
> library which supports C++11 on up or if you use C++ standard libraries
> which often depend on each other you never have huge translation units.
> Is that what you are saying ?
>

More or less. Users will use the standard libraries anyway, adding Boost
libraries that duplicate the functionality increase the size of translation
unit. If you have no standard alternative - you use Boost, you have no
choice.

> Linker may merge not ABI
> > compatible Boost
> > symbols, leading to breakages. More specifically, the layout of
> > Boost.Variant changed multiple times since 1.45, but the signature of the
> > boost::apply_visitor function did not change. As a result you get
> > ODR-violation.
>
> Are you saying that Boost is not ABI compatible between releases but any
> compiler's C++ standard library is always ABI compatible between releases ?
>

Not any, but many.