在gcc / clang vs msvc2015中,当涉及到多个空类继承时,我看到了不同的行为。我想知道是否有人会知道标准中允许这种差异的原因。
#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");
此代码在msvc2015上生成:
error C2338: Expected size of 4 for Int4
虽然gcc和clang生成了这个:
error: static_assert failed "Expected size of 8 for Int4"
换句话说,msvc2015在从空类继承时不会添加任何字节,但在从多个继承时会这样做。在C ++中是未定义的行为吗?
答案 0 :(得分:2)
默认情况下,MSVC不会执行此优化,因此它编译的代码可以与旧版本的编译器兼容。但是,if you use __declspec(empty_bases)
,您可以告诉MSVC启用此优化:
#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");