访问位域联合是 C++ 标准中常见的初始数据未定义行为

Question

类似于 c union and bitfields 但在 C++ 中并且包括对初始序列的访问

类似于Union common initial sequence with primitive但使用bitfields

我想做的是：

struct A
{
   short common : 1;
   short a1: 5;
   short a2: 8;
};

struct B
{
   short common : 1;
   short b1: 3;
   short b2: 4;
   short b3: 6;
};

union C
{
    A a;
    B b;
};

然后使用如下：

short foo(C data)
{
   if (data.a.common)
   {
       return data.a.a1*data.a.a2;
   }
   return data.b.b1*data.b.b2*data.b.b3;
}

问题似乎是，如果 data.a.common 为假，代码将通过 A

的成员访问作为 B 的数据集

标准中似乎有关于并集和类似初始序列的语言，但不确定 common 因为 bitfield 会算

9.5.1 it is permitted to inspect the common initial sequence of any of standard-layout struct members;

任何澄清将不胜感激

Answer 1

来自 the definition of common initial sequence:

The common initial sequence of two standard-layout struct ([class.prop]) types is the longest sequence of non-static data members and bit-fields in declaration order, starting with the first such entity in each of the structs, such that corresponding entities have layout-compatible types, either both entities are declared with the no_unique_address attribute ([dcl.attr.nouniqueaddr]) or neither is, and either both entities are bit-fields with the same width or neither is a bit-field.

struct A 和 struct B 的公共初始序列由第一个位字段 common.

组成

访问位域联合是 C++ 标准中常见的初始数据未定义行为

Is accessing bitfield unions common initial data undefined behavior in C++ standards

c++

unions

language-lawyer

bit-fields