reinterpret_cast from object to first member

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Yes, because both classes here are standard-layout types, you can convert between &b and &b.a.

reinterpret_cast<A*>(p) is defined to be the same as static_cast<A*>(static_cast<void*>(p)), (5.2.10p7) so both your questions are equivalent.

For standard-layout classes, the address of the struct/class is the same as the address of its first non-static member (9.2p19). And static_cast to/from void* will preserve the address (5.2.9p13), meaning the result will be valid.

If the classes were not standard-layout, you could not rely on this behavior.

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