在匿名结构中包装灵活数组时MSVC结构布局会发生变化吗?

我正在查看以下结构的布局信息 using godbolt :

struct Foo1 {
    int size;
    void *data[];
};

struct Foo2 {
    int size;
    struct {
        void *data[];
    };
};

Foo1 和 Foo2 还是一样。据我所知,匿名嵌套结构的任何字段都只是“折叠”到父结构中。那么 食物2 结果应该和 .

但是,由MSVC 19.16生成并在使用标志时显示的布局 /d1reportSingleClassLayoutFoo 不同之处:

class Foo1  size(8):
    +---
 0  | size
    | <alignment member> (size=4)
 8  | data
    +---
class Foo2  size(16):
    +---
 0  | size
    | <alignment member> (size=4)
    | <anonymous-tag> <alignment member> (size=8)
 8  | data
    | <alignment member> (size=7)
    +---

食物2 是我的两倍大 Foo1 . 和 data

-Wall :

warning C4200: nonstandard extension used: zero-sized array in struct/union
note: This member will be ignored by a defaulted constructor or copy/move assignment operator
warning C4820: 'Foo1': '4' bytes padding added after data member 'Foo1::size'
warning C4200: nonstandard extension used: zero-sized array in struct/union
note: This member will be ignored by a defaulted constructor or copy/move assignment operator
warning C4820: 'Foo2::<anonymous-tag>': '7' bytes padding added after data member 'Foo2::data'
warning C4201: nonstandard extension used: nameless struct/union
warning C4820: 'Foo2': '4' bytes padding added after data member 'Foo2::size'

但这些似乎都不能解释布局上的差异,也不能暗示未定义的行为。此外,该文件也没有: Anonymous structs .

warning C4200: nonstandard extension used: zero-sized array in struct/union
warning C4201: nonstandard extension used: nameless struct/union

“零尺寸阵列” 数据 size 字段抛出一个错误。

Foo1 和 食物2