Re: [boost] C++03 / C++11 compatibility question for compiled libraries

Subject: Re: [boost] C++03 / C++11 compatibility question for compiled libraries
From: Edward Diener (eldiener_at_[hidden])
Date: 2018-02-19 18:30:52

On 2/19/2018 4:51 AM, Mike via Boost wrote:
>     Hello everyone,
>     I hope it is ok, for me as a non-boost-dev, to chime in here.
>     What I generally expect " dropping c++03 support" to mean are the
>     following things:
>     - Any library may start to use c++11 features at any point without
>     prior announcement and/or without having to wait a few releases (I.e.
>     it is no longer considered a breaking change).

I think that it is a breaking change in that end-users who previously 
compiled in C++03 mode when using such a library can no longer do so. Of 
course if Boost wants to tell end-users that compiling any particular 
library in C++03 mode is no longer guaranteed to work, even if it has 
worked previously, then this would not longer be a breaking change.

>     - c++03 builds are removed from the test matrices

The test matrices are up to those contributing them. If you would like 
to suggest to them that testing in C++03 mode is wasteful, please feel 
free to do so.

>     - Compilation paths that are c++03 only can be removed (thus
>     simplifying the code base and future refactoring/development)

Any library developer can always do this now as a potential breaking change.

>     - My hope (obviously no one can or should force a volunteer to do
>     anything) is that - where applicable - libraries start to replace boost
>     types & functions that have been merged into the c++11 standard with
>     their std equivalent or the equivalent c++11 language feature.

Using std versus boost libraries is almost always equivalent to working 
only in C++11 mode or above.

>     - In the long run, libraries that have been fully merged into c++11
>     could be deprecated and removed all together.

What does "fully merged into c++11" mean ? Requires C++11 mode to work ? 
If a library which previously worked in C++03 mode is changed so that it 
requires C++11 mode on up, why would you want to remove or deprecate it ?

>     Just to be clear: Dropping c++03 support doesn't mean a developer is
>     forced to use c++11 constructs (if we ignore the potential future
>     removal of boost libraries) - it primarily means that a user can no
>     longer rely on the lib being usable in c++03 mode.

OK, at least this is your definition of what "dropping support for 
C++03" means.

>     And yes, this could be done on a per library basis, but especially
>     older boost libraries are so tightly coupled that such decisions affect
>     a lot of other libraries anyway. So my guess is that very few
>     maintainers would dare to drop c++03 support as long as that hasn't
>     been deemed acceptable by larger parts of the boost community.j

The problem with your definition of "dropping support for C++03" is that 
end-users of Boost libraries need to be told that Boost no longer can 
guarantee that a library which currently works in C++03 mode will 
continue to do so. This announcement as I see it would have to be 
determined by the Boost Steering Committee and announced on the website, 
as a minimum. I am not personally against doing so, but I am also glad I 
am not involved in any way ofd making such a decision. I am personally 
against dropping libraries from Boost just because they happen to work 
in C++03 mode, when that library also works fine in C++11 mode on up.

>     Best regards
>     Mike
>     --
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>     gesendet.