Re: The new Boost 17.0.0

From: Edward Diener (eldiener_at_[hidden])
Date: 2020-12-01 22:21:57

On 12/1/2020 4:39 PM, Antony Polukhin via Boost wrote:
> 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.

I am not against C++98/C++03 libraries being upgraded to C++11 in order 
to use C++ standard library equivalents to Boost types. Nor am I against 
a separate C++11 library of a current C++98/C++03 library being added to 
Boost which uses C++ standard library equivalents to Boost types and in 
all other respects is the same as the original library. But the latter 
solution does require extra maintenance and who is going to do that ?

> 
> 
>> 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.

I believe the great majority of Boost libraries attempt to maintain ABI 
compatibility between releases. But just like with compilers it is not 
always possible between releases.