Is pointer assignment atomic in C++?

Question

I've actually heard claims both ways. I suspect they are not, but I wanted to get the topic settled.

Answer 1

C++03 does not know about the existance of threads, therefore the concept of atomicity doesn't make much sense for C++03, meaning that it doesn't say anything about that.

C++11 does know about threads, but once again doesn't say anything about the atomicity of assigning pointers. However C++11 does contain std::atomic<T*>, which is guaranteed to be atomic.

Note that even if writing to a raw pointer is atomic on your platform the compiler is still free to move that assingment around, so that doesn't really buy you anything.

If you need to write to a pointer which is shared between threads use either std::atomic<T*> (or the not yet official boost::atomic<T*>, gccs atomic intrinsics or windows Interlocked*) or wrap all accesses to that pointer in mutexes.

Answer 2

The C++ norm does not define specific threading behavior. Depending on the compiler and the platform, the pointer assignment may or may not be atomic.