I am seeing different behaviours when it comes to multiple inheritance of empty classes in gcc/clang vs msvc2015. I was wondering if somebody would know what in the standard allows for such differences.
#include <cstdint>
using namespace std;
class Empty1 {};
static_assert(sizeof(Empty1) == 1, "Expected size of 1 for Empty1");
class Empty2 {};
static_assert(sizeof(Empty2) == 1, "Expected size of 1 for Empty2");
class Empty3 : Empty2, Empty1 {};
static_assert(sizeof(Empty3) == 1, "Expected size of 1 for Empty3");
class Int1 { uint32_t i; };
static_assert(sizeof(Int1) == 4, "Expected size of 4 for Int1");
class Int2 : Empty1 { uint32_t i; };
static_assert(sizeof(Int2) == 4, "Expected size of 4 for Int2");
class Int3 : Empty2 { uint32_t i; };
static_assert(sizeof(Int3) == 4, "Expected size of 4 for Int3");
class Int4 : Empty3 { uint32_t i; };
static_assert(sizeof(Int4) == 8, "Expected size of 8 for Int4");
static_assert(sizeof(Int4) == 4, "Expected size of 4 for Int4");
This codes, on msvc2015 generates:
error C2338: Expected size of 4 for Int4
While gcc and clang generate this instead:
error: static_assert failed "Expected size of 8 for Int4"
In other words, msvc2015 does not add any byte when inheriting from an empty class, but it does when inheriting from multiple ones. Is undefined behaviour in C++?
By default, MSVC doesn't do this optimization so that code it compiles can be ABI compatible with older versions of the compiler. However, if you use __declspec(empty_bases), you can tell MSVC to enable this optimization:
#ifdef _MSC_VER
#define EBO_ENABLE __declspec(empty_bases)
#else
#define EBO_ENABLE
#endif
class EBO_ENABLE Empty3 : Empty2, Empty1 {};
static_assert(sizeof(Empty3) == 1, "Expected size of 1 for Empty3");