Re: [Boost-users] [LockFree] a queue of std::shared_ptr?

Subject: Re: [Boost-users] [LockFree] a queue of std::shared_ptr?
From: Krzysztof Czainski (1czajnik_at_[hidden])
Date: 2013-02-22 12:19:42

2013/2/22 Jeffrey Lee Hellrung, Jr. <jeffrey.hellrung_at_[hidden]>

> On Fri, Feb 22, 2013 at 8:14 AM, Krzysztof Czainski <1czajnik_at_[hidden]>wrote:
>
>> 2013/2/12 Michael Caisse <mcaisse-lists_at_[hidden]>
>>
>>> On 02/11/2013 04:04 PM, Scott Smedley wrote:
>>>
>>>> Hi all,
>>>>
>>>> Is it possible to store std::shared_ptr objects in a
>>>> boost::lockfree::queue?
>>>>
>>>> eg. boost::lockfree::queue<std::**shared_ptr<Message>> q;
>>>>
>>>> Hi Scott -
>>>
>>> Lockfree queues require that the queued type (T) meets the following
>>> requirements:
>>>
>>>  o T must have a copy constructor
>>>  o T must have a trivial assignment operator
>>>  o T must have a trivial destructor
>>>
>>>
>>> shared_ptr has neither a trivial assignment operator or destructor and
>>> cannot be used in the lockfree containers.
>>>
>>> michael
>>>
>>
>> Hello,
>>
>> I'm curious: suppose T is a type, that meets requirements for lockfree
>> queues. Does optional<T> meet those requirements?
>>
>> a) Does optional<T> have a copy constructor? Yes.
>> b) Does optional<T> have a trivial assignment operator? Well, the
>> assignment is user-written in optional, but it's observable effect is
>> equivalent to memcpy, isn't it?
>> c) Does optional<T> have a trivial destructor? Well, optional's
>> destructor conditionally calls T's destructor, which is trivial. So is a
>> destructor trivial, if all it does is conditionally call a trivial
>> destructor?
>>
>
> This is true in theory for an optional implementation like
> boost::optional, which uses a boolean flag to indicate initialization
> state, though I'd doubt all the necessary type traits are specialized for
> boost::optional. However, an optional implementation that uses a pointer to
> indicate initialization state (null pointer indicates uninitialized,
> otherwise pointer points to internal buffer; thus, get_ptr() just returns
> the pointer) would not have a trivial copy constructor and assignment
> operator, and I think technically, whether boost::optional uses a boolean
> flag or a pointer is an implementation detail.
>
> - Jeff
>

Ok, that makes sense.

Cheers,
Kris