c++ - 继承多个空类时，了解gcc/clang与msvc2015之间的不同填充规则

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Why does multiple inheritance increase the size of the object despite the bases being empty?

(2个答案)


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当在gcc / clang与msvc2015中对空类进行多重继承时，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内容。

#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生成了此代码:

error: static_assert failed "Expected size of 8 for Int4"

换句话说，从空类继承时，msvc2015不会添加任何字节，但是从多个类继承时，msvc2015不会添加任何字节。 C++中是否存在未定义的行为？

最佳答案

默认情况下，MSVC不会进行此优化，因此它编译的代码可以与较早版本的编译器ABI兼容。但是，if you use __declspec(empty_bases) 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");

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