Maybe? There is wording in the C standard (since C03 TC3 I think) that allow reading accessing from non active members of the union, but the wording is unclear and implementations have taken a very narrow interpretation of it (i.e. the access path need to be explicitly through an union type and the actual object needs to be an union). See the GCC documentation on union and aliasing for example.
It's explicitly legal in both C99 and C11, but C99 forgot to remove it from its master (non-normative) list of undefined behavior. It's up for debate whether or not it's legal in C++ (ISTR a C++0x draft that copied the C99 changes to unions, but then they added unrestricted unions and completely changed the verbiage to have almost no relation to the C99 text).
It mostly isn't legal, though gcc explicitly promises to support it and Microsoft's compiler doesn't even do the optimizations that would cause trouble.
