Re: [Boost-users] [system] local static and threading in C++03

Subject: Re: [Boost-users] [system] local static and threading in C++03
From: Felipe Magno de Almeida (felipe.m.almeida_at_[hidden])
Date: 2012-12-27 07:26:36

On Thu, Dec 27, 2012 at 9:30 AM, Vicente Botet <vicente.botet_at_[hidden]> wrote:
> Felipe Magno de Almeida wrote
>> Hello Vicente,
>>
>> On Tue, Dec 25, 2012 at 6:28 PM, Vicente Botet &lt;
>
>> vicente.botet@
>
>> &gt; wrote:
>>> Felipe Magno de Almeida wrote
>>>> Hello,
>>
>> [snip - source code]
>>
>>>> But couldn't this make double construction/double free in C++03 systems
>>>> if system_category is ran concurrently?
>>>
>>> Yes sure, this is always possible zith c++0·.
>>
>> Not always possible. It is only possible for non-PODs. AFAIK.
>>
>>> Do you see a problem with this implementation?
>>
>> Since it has a race-condition, I would say I see a problem, yes.
>> More so since it is not documented.
>
> Form a theoretical point of view yes. But in practice, which observable
> problems could induce the double construction?
> I would expect only one destruction, don`t you?

No, I wouldn't. If it is constructed twice, I would expect atexit to be called
twice as well. So double destruction.

>> AFAIK, the normal solution
>> for this is to use call_once.
>>
>> system_category const& create_system_category()
>> {
>>   system_category const category;
>>   return category;
>> }
>> void init_system_category() { create_system_category(); }
>> once_flag init_flag = BOOST_ONCE_INIT;
>>
>> system_category const& get_system_category()
>> {
>>   boost::call_once(init_flag, &init_system_category);
>>   return create_system_category();
>> }
>
> This approach would make both libraries interdependent.

I don't understand what you mean.

> Best,
> Vicente

[snip]

Regards,
-- 
Felipe Magno de Almeida