Anthony Williams <anthony_w.geo <at> yahoo.com> writes:
Phil Endecott <spam_from_boost_dev <at> chezphil.org> writes:
Anthony Williams wrote:
The C++0x atomic<> template will work with any type that is "trivially copy assignable and bitwise equality comparable", and it is intended that operations are protected with a mutex where they cannot be done with atomic ops. The is_lock_free() member distinguishes the two cases. I would expect this to cover many uses, though obviously you can't write atomic<list<int>>, since list<> is definitely not bitwise equality-comparable.
Why have the "trivially copy assignable and bitwise equality comparable" restriction, if using a mutex is an acceptable solution?
I don't know. I've been focusing on the use of specializations that really are atomic when I've been reading the papers. I'll raise the issue.
The response from Lawrence is that firstly, types that do satisfy these restrictions can be lock-free on more platforms, and secondly if a type doesn't satisfy these constraints then you're calling user code whilst holding a lock. I think this is an important point, and it might be worth having a separate type to highlight the distinction. For types that satisfy the current requirements, you can't tell by use whether or not a lock is used, since the externally-visible effects are the same, so users might not think about the possible presence of a mutex, just that the ops are atomic (which is the idea). For types that had user-written copy constructors or comparison operators, you could potentially end up with deadlock, and race conditions, even though there's a mutex --- e.g. if the comparison operator referenced global state which could be accessed from elsewhere without the mutex. If we have a separate synchronisable<T> type that always uses a mutex, the issue is much more explicit, and people are more likely to check their code for such problems. Of course, people will still make mistakes sometimes, but these will be less common. Anthony -- Anthony Williams Just Software Solutions Ltd - http://www.justsoftwaresolutions.co.uk Registered in England, Company Number 5478976. Registered Office: 15 Carrallack Mews, St Just, Cornwall, TR19 7UL