inheritance and attribute packed

Question

Consider this code in C++

struct Base
{
   std::int64_t x;
   std::int64_t y;
};
static_assert(sizeof(Base) == 16, "Base not of size 16!");

struct Derived : Base
{
   std::int32_t z;
}__attribute__((packed));
static_assert(sizeof(Derived) == 20, "Derived not of size 20!");

clang considers this code valid, where as gcc results in firing of the second static_assert. ("Derived not of size 20!). If I add attribute packed in Base as well, then it is fine in both the compilers. Does anyone know which one is right and why?

Answer 1

Both are right.

The compiler is allowed to add padding to your class (between members and at the end) and it is implementation defined whether it does or not, so your static_assert's are basically testing something that depends on the compiler (and platform) used - thus they are not portable and you can't expect the same result everywhere.

Your use of __attribute__((packed)) basically tells the compiler(s) who understand the attribute, that you want a specific behaviour rather than what it would do by default. This may generate the result you want in this case, but it also has implications (memory layout/use and on some CPUs performance). And you still can't be certain that all compilers will do the same.