Re: [boost] [Booster] Or boost is useless for library developers

Subject: Re: [boost] [Booster] Or boost is useless for library developers
From: Peter Dimov (pdimov_at_[hidden])
Date: 2010-05-18 08:30:58

Artyom wrote:
> Take a look on following two files:
>
> sp_counted_base.h
>
> http://cppcms.svn.sourceforge.net/viewvc/cppcms/framework/branches/refactoring/booster/booster/smart_ptr/sp_counted_base.h?revision=1172&view=markup

You don't need the mutex. Look at the recent sp_counted_base_spin.hpp.

> sp_counted_base.cpp
> http://cppcms.svn.sourceforge.net/viewvc/cppcms/framework/branches/refactoring/booster/lib/smart_ptr/src/sp_counted_base.cpp?revision=1185&view=markup

No need to use atomic_set in the constructor. If your constructor races with 
another member, atomics won't save you.

> It is the only class I had to change in boost-shared ptr to make it
> ABI stable.
>
> How did I this?
>
> 1) I moved implementation to library
> 2) I added pthread_mutex_t to the class even if I do not use it.
> 3) I removed debug hooks.
>
> Now what happens:
>
> 1. I can change the implementation of the counter - use atomic-ops
>   assembly or OS specific operations without warring about compatibility
>   because all operations are done in single library.
> 2. I can add atomic operations on platforms where they were not availible
>   before without changing layout of class - just by stopping
>   using pthread_mutex_t.
>
> What does it requires?
>
> 1. Linking against some core library.
> 2. Little thinking about what can happen.

With all due respect, may I submit that the reason it was so easy to make 
shared_ptr ABI-stable was that it was designed with this in mind?

shared_ptr was frequently criticised for having too much overhead and being 
too slow, and it was an intentional design decision to allow add_ref/release 
to be inlined. I knew that on most relevant architectures this didn't 
provide any significant performance benefit (even making them virtual is 
affordable - and this is another interesting tradeoff, allowing 
ABI-incompatible shared_ptr instances to coexist), but C++ programmers are 
notoriously picky. Besides, Boost was entirely header-only at the time 
shared_ptr was first designed and implemented, and had no build system.

In any event, the ABI-stable shared_ptr is spelled std::shared_ptr.