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New C++ Working Draft and Concurrency Papers Now Available

Friday, 04 July 2008

The post-meeting mailing following June's C++ Standards committee meeting in France is now available. This includes a new Working Draft for the C++0x standard, and a few concurrency-related papers.

From a concurrency point of view, there are several papers of interest. Firstly, a few have been accepted into the working draft, notably:

N2661: A Foundation to Sleep On
This paper provides a generalised time point and duration library, which is used by the thread functions that take times or durations. These have been updated to use these new types and renamed to make their purpose clearer: functions that wait for a duration are now called xxx_for, and take a value of type std::chrono::duration<Rep,Period>, whereas those that take absolute time points are now called xxx_until and take a value of type std::chrono::time_point<Clock,Duration>.
N2668: Concurrency Modifications to Basic String
Update: This paper has not actually been approved, I was mistaken. Though the majority of the committee were in favour, there was not consensus, so this paper will be discussed at a future meeting. Thanks to Herb Sutter for picking me up on this.
The changes in this paper ensure that it is safe for two threads to access the same std::string object at the same time, provided they both perform only read operations. They also ensure that copying a string object and then modifying that copy is safe, even if another thread is accessing the original. This essentially disallows copy-on-write implementations since the benefits are now severely limited.
N2660: Dynamic Initialization and Destruction with Concurrency
With the changes from this paper, if an application uses multiple threads then the initialization and destruction of objects with static storage duration (such as global variables) may run concurrently on separate threads. This can provide faster start-up and shut-down times for an application, but it can also introduce the possibility of race conditions where none existed previously. If you use threads in your application, it is now even more important to check the initialization order of objects with static storage duration.
N2514: Implicit Conversion Operators for Atomics
With this change, the atomic types such as std::atomic_int are implicitly convertible to their corresponding fundamental types. This means, for example, that:
std::atomic_int x;
int y=x;
is well-formed where it wasn't previously. The implicit conversions are equivalent to calling the load() member function, and have memory_order_seq_cst ordering semantics.
N2674: Shared_ptr atomic access, revision 1
This paper introduces a new set of overloads of the free functions for atomic operations (such as atomic_load and atomic_store), which operate on instances of std::shared_ptr<>. This allows one thread to read an instance of std::shared_ptr whilst another thread is modifying that same instance if they both use the new atomic functions.
This paper also renames atomic_swap operations to atomic_exchange (and likewise for atomic_compare_swap and the corresponding member functions) for all atomic types, in order to avoid confusion with other types that provide swap functions. The atomic exchange operations only alter the value of a single object, replacing the old value with a new one, they do not exchange the values of two objects in the way that std::swap does.
N2664: C++ Data-Dependency Ordering: Atomics and Memory Model
With the adoption of this paper the memory model gets a new ordering option: memory_order_consume. This is a limited form of memory_order_acquire which allows for data-dependent ordering. If a thread uses memory_order_consume, then it is not guaranteed to see modifications to other variables made by the thread that performed the releasing operation unless those variables are accessed in conjunction with the consumed variable. This means, for example, that member variables of an object are visible if the consumed value is a pointer to that object, but that values of independent objects are not necessarily visible. This allows the compiler to perform some optimizations that are forbidden by memory_order_acquire, and reduces the synchronization overhead on some hardware architectures.
N2678: Error Handling Specification for Chapter 30 (Threads)
This paper brings the exceptions thrown by the thread under the new system_error umbrella, with corresponding error codes and error categories.
N2669: Thread-Safety in the Standard Library (Rev 2)
Now the standard supports threads, we need to say which standard library operations are thread-safe, and which are not. This paper basically says that non-modifying operations on the same object are safe, and any operations on separate objects are also safe. Also, separate threads may call the same library functions on separate objects without problems. As you might expect, concurrent modifications to the same object are data races and undefined behaviour.

The committee also voted to include N2659: Thread-Local Storage in C++0x, but it doesn't appear to be in the current draft. This paper introduces the thread_local keyword to indicate that each thread should have its own copy of a given object.

Finally, N2657: Local and Unnamed Types as Template Arguments has been incorporated in the working paper. Though this isn't directly concurrency related, it is something I've been campaigning for since N1427 back in 2003.

Apart from N2657, I've only listed the concurrency changes: check out the Working Draft for the C++0x standard, and the State of C++ Evolution for more details on the changes.

Posted by Anthony Williams
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