Re: [boost] Seeking advice regarding inline namespaces

Subject: Re: [boost] Seeking advice regarding inline namespaces
From: Gavin Lambert (gavinl_at_[hidden])
Date: 2015-11-11 22:10:53

On 12/11/2015 06:26, Louis Dionne wrote:
> (1) How can you guarantee ABI compatibility from one version of a library
>      to the next? IIUC, ABI can be broken by merely changing the size or
>      the layout of a type, so guaranteeing backwards ABI compatibility
>      means freezing those things in time, right?

It's really hard to do this absolutely (since compiler settings can 
affect things as well), which is why it's rarely done.  (But nice where 
possible.)

> (2) Assuming that it is unreasonable for a library L to guarantee ABI
>      compatibility from one version to the next, would there still be a
>      reason to use inline namespaces to version that library?

Yes, this is the best reason to do so.  Imagine a static library A that 
has been built against version 1 of L, and then an application that is 
trying to consume version 2 of L and the prebuilt library A.

Without version-specific namespaces, there is a very good chance that 
this will successfully link, but then fail in some unspecified (and 
probably horrible, or worse: subtle) way at runtime.  With 
version-specific namespaces, this will fail to link until the user 
either switches the application to use v1 of L or rebuilds A to use v2 
as well.

The above assumes that v2 of L does not define the namespace that v1 
used.  If instead v2 defined "ABI compatible symbols" in a v1-named 
namespace as well (which is harder), then the above scenario *might* 
link and even work correctly, assuming that either the app and lib A did 
not pass lib L's objects to each other, or that lib L defined ways to 
make them interoperable (which can also be hard).