Re: [Boost-users] shared_ptr and thread safety 2

From: Ovanes Markarian (om_boost_at_[hidden])
Date: 2007-08-06 13:32:24

On Mon, August 6, 2007 18:08, Jonathan Biggar wrote:
> Ovanes Markarian wrote:
>>>>> void g(boost::shared_ptr<const B> ptr)
>>>> {
>>>>   //...
>>>> }
>>>>
>>>> void f()
>>>> {
>>>>  boost::shared_ptr<const B> ptr(new B);
>>>>
>>>>  //give ptr to threads
>>>>  boost::thread thrd1(boost::bind(&g, ptr));
>>>>  boost::thread thrd2(boost::bind(&g, ptr));
>>>>
>>>>  //fire and forget
>>>>  ptr.reset();
>>>> }
>>> This example is OK. You have three distinct shared_ptr variables, all named
>>> 'ptr' (one per thread). Despite them having the same name, they are still
>>> different objects, not shared among threads, so you can assign/reset/destroy
>>> them at will.
>>
>> Yes, just in addition, your code would be broken, if g would expect a shared_ptr by reference,
>> i.e. void g(boost::shared_ptr<const B>& ptr). Then all your threads would own the same
>> shared_ptr
>> instance and the thread in which runs the function f would reset the pointer which might be used
>> by thrd1 or thrd2 and that might cause the scenario from the docs example.
>
> I wouldn't think so in this case, since boost::bind makes a copy of the
> shared_ptr before it gets passed to g().  If the code used
> boost::bind(&g, boost::ref(ptr)), then there would be a race condition.
>

Yes, sorry! You are right...

With Kind Regards,

Ovanes Markarian