Is it possible to subclass a C struct in C++ and use pointers to the struct in C code?

Question

Is there a side effect in doing this:

C code:

struct foo {
      int k;
};

int ret_foo(const struct foo* f){ 
    return f.k; 
}

C++ c

Answer 1

This is entirely legal. In C++, classes and structs are identical concepts, with the exception that all struct members are public by default. That's the only difference. So asking whether you can extend a struct is no different than asking if you can extend a class.

There is one caveat here. There is no guarantee of layout consistency from compiler to compiler. So if you compile your C code with a different compiler than your C++ code, you may run into problems related to member layout (padding especially). This can even occur when using C and C++ compilers from the same vendor.

I have had this happen with gcc and g++. I worked on a project which used several large structs. Unfortunately, g++ packed the structs significantly looser than gcc, which caused significant problems sharing objects between C and C++ code. We eventually had to manually set packing and insert padding to make the C and C++ code treat the structs the same. Note however, that this problem can occur regardless of subclassing. In fact we weren't subclassing the C struct in this case.