question.md

Leaving aside what is correct and what is not, I have problems putting these two sentences from the chat together (from a purelly technical low-level POV).

1. We can do any change in the WhateverImpl classes without affecting ABI compatibility "because users of library does not use Impl".

2. A change in the public Bcache breaks ABI compatibility "for all programs linked to libstorage-ng, no matter if bcache stuff is used at all".

Why changing the Bcache class has an binary influence in the other public classes but changing WhateverImpl does not? How are they different in that regard? As I understand, the fact that WhateverImpl is non-public is a convention, not something the compiler knows.

(2) is oversimplifying things a lot.

It is safe to extend the Bcache class.

It is not safe to modify its behavior, the semantics of any parameters to its methods (including the constructors!), or to remove any method.

Sure. In both cases (1 and 2) I meant doing intrusive changes like changing parameters or removing methods.

My statement is a bit out of context: Changing the major library version (a consequence of the ABI break) breaks linking for all programs (unless both the old and the new library are provided (and thus also both maintained)).

Ok. Got it. Thanks.