C vs C++ struct alignment

Question

I've been asked in a recent interview about C++ struct fields alignment and theoretized that C and C++ follows the same strategy in struct packing.

Hovewer, it was the wrong assumption. The interviewer said that in general C and C++ are packing structs in different ways and we should never expect the opposite. IMHO it's strange statement. There is no pack "C" qualifier for structs in C++ for using in bilingual C/C++ header files.

So in practice it could mean that you can't create a struct in C++ and pass it to a C library because in general its fields will be aligned in a different way and have different offsets. But, in fact, most programmers seriously rely on this interoperability up to the point when they convert a pointer to a C POD struct to a reference to C++ wrapper around this struct with some helper methods. Can you please clarify this matter?

Answer 1

Both the C and C++ language standards make no requirements of struct padding and leave it to be a compiler implementation detail. A strict interpretation of this would mean that there is no guarantee a struct would be the same between the two.

In practice, however, a given version of a toolchain capable of both C and C++ (such as GCC or Clang) can pack an identical struct in the same manner, if needed. Without this, a lot of production code in the world simply wouldn't work. This is a guarantee given by the toolchain, however, and not the language.

It is worth noting that if you were to declare a similar struct to the C original, but added access specifiers (private, public and protected), that the layout would change, but that's a bit of a stretch since the struct is no longer identical.

Answer 2

This was most obviously wrong (on the interviewer side). It is clear to see that struct packing is the same for C and C++ for anybody who worked with any low-level API dealing with structs - for instance, a network API. All those are C functions, which accept 'C' structs, yet they are safely called millions of millions times a single day from C++ code.

You should be lucky you had this question. It makes it clear you should not be working there.