I have introduced In the post Sequential Consistency the default memory model. This model, in which all operations in all threads take place in a global time clock, has a big advantage but also a significant disadvantage.
In this post, our tour through the c++ memory model goes one step deeper. Until now, the posts were only about the atomicity of the atomic data types, but now we deal with the synchronization and ordering constraints of the operations.
In addition to booleans, there is atomics for pointers, integrals, and user-defined types. The rules for user-defined types are special.
The remaining atomics – in contrast to std::atomic_flag – are partial or full specializations of the class template std::atomic. Let’s start with std::atomic<bool>.
The atomics are the base of the C++ memory model. Per default, sequential consistency is applied.
I created the facebook group Modernes C++. I see one big advantage in this group.
Atomics guarantee two characteristics. On the one hand, they are atomic, on the other, they provide synchronization and order constraints on the program execution.
In case you use promise and future to synchronize threads, they have much in common with condition variables. But most of the time, tasks are the better choice.
Since C++11, C++ has a memory model. It is the foundation for multithreading. Without it, multithreading is not well defined.
The parent of a thread has to take care of their child. The parent can wait until his child is done or detach himself from his child. But that is not new. But that will not hold for std::async. The big charm of std::async is that the parent has not taken care of his child.