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At sequence points, volatile objects are stable in the sense that previous accesses are complete and subsequent accesses have not yet occurred.
In Section 7.1.6.1 (The cv-qualifiers) of the C++ standard:
A pointer or reference to a cv-qualified type need not actually point or refer to a cv-qualified object, but it is treated as if it does.
Volatile is a hint to the implementation to avoid aggressive optimization involving the object because the value of the object might be changed by means undetectable by an implementation. See 1.9 for detailed semantics. In general, the semantics of volatile are intended to be the same in C++ as they are in C.