Does the C++ 11 standard guarantees that std::atomic<> is implemented as a lock-free operation?

The C++ standard makes no guarantee that std::atomic<T> operations are lock-free. However, you can use std::atomic<T>::is_lock_free() to find out if the operation of std::atomic<T> is lock free 29.6.5 [atomics.types.operations.req] paragraph 7:

Returns: True if the object’s operations are lock-free, false otherwise.

If it is not lock-free it will still do the required synchronization but it uses some lock to do so.

If by atomic you mean, using hardware support without locks, then yes, the standard doesn't give you a guarantee for that. Why? Well, because different architectures support different kind of hardware atomicity. std::atomic<> has the handy is_lock_free() method which can be used to check wether the given object is actually lock free, or uses a lock internally to guarantee atomic operations. You can use that and check on your target hardware wether it would be lock free or not, and then decide what data structure to go for.

However, that being said, if the target architecture has hardware support for atomic operations for the fixed width integrals you are interested in, and you didn't obtain your copy of the standard library from the the shady software shop in the ghetto, it's probably going to use the hardware instead of a full blown lock.