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

Question

Let\'s say, I have a struct RGB and I want to create struct RGBA, which inherits RGB:

struct RGB {
    unsigned char r         


        

Answer 1

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

Answer 2

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.

Answer 3

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.