C++ POD struct inheritance? Are there any guarantees about the memory layout of derived members

No, the layout is not guaranteed. The only guarantees are for standard-layout classes; and one of the conditions of such a class is that it

either has no non-static data members in the most derived class and at most one base class with non-static data members, or has no base classes with non-static data members

In other words, all data members must be in the same class, not in more than one.

There is NO guarantees about the memory layout of derived members and the cast is NOT safe.

As you have inheritance, also there could be padding, this is NOT trivial.

§ 9 Classes

1 A POD struct109 is a class that is both a trivial class and a standard-layout class, and has no non-static data members of type non-POD struct, non-POD union (or array of such types). Similarly, a POD union is a union that is both a trivial class and a standard layout class, and has no non-

Also std::is_pod<RGBA> is not a POD

std::cout << std::boolalpha;
std::cout << std::is_pod<RGBA>::value << '\n';

result is false. see live demo

It's easy to check for padding: Print sizeof(RGB) and sizeof(RGBA). If it's not 3 respective 4 then the structures are padded, and you need to remove it.

As for if the member a comes after b, you can use offsetof to check each members offset. If the offset for a is one larger than the offset of b then a comes directly after b.