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;     unsigned char g;     unsigned char b; };  struct RGBA: RGB {     unsigned char a; }; 

Both will be used for reading uncompressed image data:

RGBA *pixel=static_cast<RGBA *>(image->uncompressed_data); 

Question: Is this safe, regarding the memory layout of struct RGBA? Does anyone guarantee, that:

• unsigned char a comes after the RGB struct (not before)
• There is no padding between struct RGB and the a parameter from struct RGBA?

will #pragma pack help here? It's all about memory layout during inheritance.

Answer 1

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.