无法理解 LARGE_INTEGER 结构

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问题描述

使用 C++ 和一些 Winapi 的东西，我遇到了这个家伙:

With C++ and some Winapi things, I encountered this guy:

#if defined(MIDL_PASS)
typedef struct _LARGE_INTEGER {
#else // MIDL_PASS
typedef union _LARGE_INTEGER {
    struct {
        DWORD LowPart;
        LONG HighPart;
    };
    struct {
        DWORD LowPart;
        LONG HighPart;
    } u;
#endif //MIDL_PASS
    LONGLONG QuadPart;
} LARGE_INTEGER;

所以，在我看来，取决于 MIDL_PASS 是否设置，这要么是一个非常紧凑的结构，其中只有一个 LONGLONG，要么更有趣的是，这变成了一个联合.

So, the way I see it, depending on MIDL_PASS being set or not, this is either a very compact struct with only a LONGLONG in it, or the much more interesting case, this becomes a union.

如果这是一个联合，对我来说仍然有意义，有两种访问可能性，一次是 LONGLONG 在一个块中，一次是具有 Low 和 Highpart 的结构.到目前为止一切顺利.

In case this is a union, it still makes sense to me, to have two possibilites of access, once the LONGLONG in one chunk, and once the struct with Low and Highpart.So far so good.

但是我无法理解结构被声明两次的事实，同样的.似乎他们都是匿名的，但后一个可以通过u"获得.

But I cannot make any sense out of the fact that the struct is declared twice, identically. It seems they are both anonymous, but the latter one is available via "u".

现在我的问题:

为什么定义了两个结构(冗余?)，第一个的目的是什么，如果我什至无法访问它，因为没有绑定到任何类型/变量名.

Why are the two structs defined (redundantly?), what is the purpose of the first one, if I cannot even access it, due to not being bound to any type / variable name.

推荐答案

Microsoft 提供匿名结构作为 extension(他们的示例显示了另一个结构中的一个结构，但联合中的结构是相似的).如果您不介意基于其扩展名的不可移植代码，您可以使用以下内容:

Microsoft provides anonymous structs as an extension (their example shows one struct inside another struct, but a struct in a union is similar). If you don't mind non-portable code based on their extension, you can use things like:

LARGE_INTEGER a;
a.LowPart = 1;

但是如果你想要可移植的代码，你需要:

but if you want portable code, you need:

a.u.LowPart = 1;

联合让你可以使用.

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ONCE