Re: [Boost-users] shared_ptr and weak_ptr concurrency

Subject: Re: [Boost-users] shared_ptr and weak_ptr concurrency
From: Stefan Strasser (strasser_at_[hidden])
Date: 2009-09-04 14:28:47

Am Friday 04 September 2009 18:17:36 schrieb Chris Uzdavinis:
> > the shared_ptr doc says that you can expect the same thread safety from
> > shared_ptr as you can from built-in types.
>
> This is true, but you're drawing the wrong conclusion from it.
> You need a lock around built-in types as well.  You need to use
> an "atomic" type to safely do what you're saying.
>
> In the same documentation, they give examples which are
> enlightening:
>
> //--- Example 3 ---
>
> // thread A
> p = p3; // reads p3, writes p
>
> // thread B
> p3.reset(); // writes p3; undefined, simultaneous read/write

I believe this means that the contents of p are undefined, not that there is 
an undefined state within a shared_ptr (on a platform that guarantees atomic 
pointers)

>
> > you can use multiple-readers-single-writer without any locks on built-in
> > types.
>
> Not true.  You can use multiple-readers, NO-writers without any locks,
> however.

you can on platforms with atomic builtin types, as the c++ standard doesn't 
say anything about that (yet).

so if it is really true that that's not the case for shared_ptr, then the 
statement of the shared_ptr doc would be wrong. but as far as I can see from 
the shared_ptr implementation there is nothing that would indicate that.

could someone with more insight into the implementation clear this up please? 
(and maybe the documentation)