Re: [boost] [afio] Formal review of Boost.AFIO

Subject: Re: [boost] [afio] Formal review of Boost.AFIO
From: Gavin Lambert (gavinl_at_[hidden])
Date: 2015-08-31 19:33:09

On 1/09/2015 02:57, Thomas Heller wrote:
>> You don't appear to understand well how ABI stable C++ is designed.
>> It requires imposing costs on everybody. Look at the PIMPL idiom.
>
> The PIMPL idiom has absolutely *nothing* to do with ABI stability.

This is incorrect.

PIMPL is one essential tool in maintaining public ABI stability, as it 
allows the size of the private members of a type to become irrelevant to 
the ABI size of the public type, and therefore permitted to vary without 
breaking ABI compatibility.

Granted, it has other purposes as well (such as breaking header 
dependency chains for build time improvement, or enforcing certain kinds 
of ownership semantics), but this is a vital one (if ABI stability is 
important to you).

Unfortunately, hiding the implementation from external code also carries 
a downside, as simple get/set methods can no longer be inlined in the 
public headers and the compiler is given fewer opportunities to 
optimise.  Hence "imposing costs on everybody".