PODs 的 WinAPI 联合双关语到基本类型：依赖于实现还是符合标准？

Question

在处理 Windows API 时，人们经常在需要大量数据时偶然发现结构。 MSDN 文档鼓励对这些结构使用联合双关语以将它们转换为基本类型。这些转换是否符合标准或 MSDN 是否建议可能导致未定义行为的技术？

例如，在处理FILETIME结构体时需要进行运算，the MSDN documentation建议：

It is not recommended that you add and subtract values from the FILETIME structure to obtain relative times. Instead, you should copy the low- and high-order parts of the file time to a ULARGE_INTEGER structure, perform 64-bit arithmetic on the QuadPart member, and copy the LowPart and HighPart members into the FILETIME structure.

ULARGE_INTEGER 声明

typedef union _ULARGE_INTEGER {
  struct {
    DWORD LowPart;
    DWORD HighPart;
  };
  struct {
    DWORD LowPart;
    DWORD HighPart;
  } u;
  ULONGLONG QuadPart;
} ULARGE_INTEGER, *PULARGE_INTEGER;

那么文档的建议是什么，例如计算时间跨度，是这样的：

FILETIME filetime[2];
ULARGE_INTEGER large[2];
// ...
large[0].LowPart  = filetime[0].dwLowDateTime;
large[0].HighPart = filetime[0].dwHighDateTime;
large[1].LowPart  = filetime[1].dwLowDateTime;
large[1].HighPart = filetime[1].dwHighDateTime;

auto diff = large[1].QuadPart - large[0].QuadPart;

标准有这个臭名昭著的部分，它总是在类似的问题中提出（引用 N3337）：

In a union, at most one of the non-static data members can be active at any time, that is, the value of at most one of the non-static data members can be stored in a union at any time. [ Note: One special guarantee is made in order to simplify the use of unions: If a standard-layout union contains several standard-layout structs that share a common initial sequence (9.2), and if an object of this standard-layout union type contains one of the standard-layout structs, it is permitted to inspect the common initial sequence of any of standard-layout struct members; see 9.2. — end note ]

9.2特指结构体

由于 ULONGLONG 解析为 __int64 或 double，因此 ULARGE_INTEGER 联合包含两个结构和一个基本类型。在我阅读它时，引用中的特殊保证不适用，因为设置 HighPart 和 LowPart 并随后读取 QuadPart 将设置结构的两个成员，然后读取基本类型。引用要求所有包含的可以互换读取/写入的实体是 structs.

那么 MSDN 是否建议在这种特殊情况下违反标准的技术，还是我理解错了？

Answer 1

严格按照 C++ 标准，行为确实是未定义的（如您所想）。但是，请记住，不是由标准定义的，仍然可以由实现定义。如果 MSDN 是用 MSVC 编译器编写的，我不会感到惊讶，在这种情况下它可以很好地提供定义的行为。我相信 GCC 声明 union-based 类型双关语也被定义。

PODs 的 WinAPI 联合双关语到基本类型：依赖于实现还是符合标准？

WinAPI union punning of PODs to fundamental types: Implementation dependent or standard compliant?

c++

winapi

visual-c++

language-lawyer

type-punning