java and C++11 volatile

Question

I'd like to port some piece of code from Java to C++11 and I'm a bit confused with volatile keyword.

I'm not familiar with Java language and I don't get what a volatile variable is. It guarantees that every thread has access to the up to date value of variable - it is the C++ volatile behaviour. But it is usually used to synchronize - are all actions performed on volatile variable atomic?

So I think thath the C++11 good replacement for Java volatile will be std::atomic. Or I'm totally wrong, cause I missed some additional Java volatile features?

Answer 1

Yes, they would be a good match, there is a good article on this at Dr Dobbs.

In a nutshell, ordered atomic variables are safe to read and write on multiple threads at the same time without doing any explicit locking because they provide two guarantees: their reads and writes are guaranteed to be executed in the order they appear in your program's source code; and each read or write is guaranteed to be atomic, all-or-nothing.

Java provides this type of variable as volatile, C++ as std::atomic.

Answer 2

This page has a pretty nice explanation on Java's volatile keyword: http://www.javamex.com/tutorials/synchronization_volatile.shtml. It looks to me that C++11 std::atomic<> on primitive types (e.g., integers) indeed is a good replacement. Note that std::atomic<> provides support for read-modify-write operations (e.g., compare_exchange_strong and fetch_add).