Atomicity of certain types depends exclusively on the underlying hardware. Each processor architecture has different guarantees about atomicity of certain operations. For example:
The Intel486 processor (and newer processors since) guarantees that the following basic memory operations will always be carried out atomically:
- Reading or writing a byte
- Reading or writing a word aligned on a 16-bit boundary
- Reading or writing a doubleword aligned on a 32-bit boundary
Other architectures have different specifications on which operations are atomic.
C++ is a high-level programming language that strives to abstract you from the underlying hardware. For this reason standard simply cannot permit one to rely on such low-level assumptions because otherwise your application wouldn't be portable. Accordingly, all the primitive types in C++ are provided with atomic counterparts by C++11 compliant standard library out-of-the-box.
