2013/2/12 Michael Caisse <mcaisse-lists@ciere.com>

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:

䰄 T must have a copy constructor
䰄 T must have a trivial assignment operator
䰄 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