[boost] Seeking advice regarding inline namespaces

Subject: [boost] Seeking advice regarding inline namespaces
From: Louis Dionne (ldionne.2_at_[hidden])
Date: 2015-11-11 12:26:51

Dear community,

I am seeking advice regarding the usage of inline namespaces for header-only
metaprogramming libraries. I was at Eric Niebler's talk on C++11 library
design back in 2014, and I understand the problem that inline namespaces
solve. I also read all of this past thread [1] on the mailing list, but did
not find
what I was looking for.

I also understand how they can be useful for a normal header-only library 
__that would provide guarantees about ABI compatibility__. Indeed, if the
ABI of the library changes from v1 to v2, then any client C1 exposing types 
from v1 in its interface and trying to link with a client C2 built against
v2 
will get a clean link-time error instead of a runtime error.

Now, my question has several aspects:

(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?

(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?

(3) When using inline namespaces to version a library (at all, not
necessarily 
    for header only libraries), are there other guidelines to follow? 

    For example, I see that Eric Niebler's range-v3 library is all stuffed
in
    the range/v3 subdirectory. IIUC, this would allow an eventual range-v4 
    library to cohabit with the range-v3 library, because people explicitly 
    state which version of the library they're including. Is this the reason
    behind the v3 subdirectory?

Any answer or pointer to answers is appreciated.

Regards,
Louis

[1]:
http://boost.2283326.n4.nabble.com/type-traits-general-Best-practice-for-inline-namespaces-td4671315.html

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