C++ std::atomic and shared memory?

I have 4096 bytes of shared memory allocated. How to treat it as an array of std::atomic<uint64_t> objects?

The final goal is to place array of 64-bit variables to shared memory and perform __sync_fetch_and_add (GCC built-in) on these variables. I would prefer using native C++11 code instead of using GCC build-ins. So how to treat the allocated memory as an std::atomic objects? Should I invoke placement new on 512 counters? What If std::atomic's constructor require additional memory allocations in some imeplementations? Should I consider the aligning of std::atomic object in shared memory?