[intro.memory] (original) (raw)

:

Thus a bit-field and an adjacent non-bit-field are in separate memory locations, and therefore can be concurrently updated by two threads of execution without interference.

The same applies to two bit-fields, if one is declared inside a nested struct declaration and the other is not, or if the two are separated by a zero-length bit-field declaration, or if they are separated by a non-bit-field declaration.

It is not safe to concurrently update two bit-fields in the same struct if all fields between them are also bit-fields of nonzero width.

— end note