命名数组元素或联合中的结构和数组

问题描述

请考虑以下结构:

struct Vector4D
{
   union
   {
      double components[4];
      struct { double x, y, z, t; } Endpoint;
   };
};

在我看来，我在WinApi的IPAddress结构中看到了类似的内容.这个想法是让我可以同时使用索引和名称来使用数组组件，例如:

It seems to me that I have seen something similar in WinApi's IPAddress struct. The idea is to give me the possibility to use the array components both by index and by name, for example:

Vector4D v;
v.components[2] = 3.0;
ASSERT(v.Endpoint.z == 3.0) //let's ignore precision issues for now

在C ++标准中，可以保证POD结构的开头没有空"空间，也就是说，元素x恰好位于Endpoint结构的beginnig中.目前很好.但是我似乎找不到任何保证，如果有的话，在x和y之间，或者在y和z之间将没有空白或填充.我尚未签出不过是C99标准.

In the C++ standard there is a guarantee that there will be no "empty" space at the beginning of a POD-struct, that is, the element x will be situated right in the beginnig of the Endpoint struct. Good so far. But I don't seem to find any guarantees that there will be no empty space or padding, if you will, between x and y, or y and z, etc. I haven't checked out the C99 standard though.

问题在于，如果Endpoint结构元素之间没有空格，那么该想法将行不通.

The problem is that if there is an empty space between Endpoint struct elements, then the idea will not work.

问题:

Questions:

1. 我是对的，实际上并不能保证它可以在C或C ++中正常工作.

1. Am I right that there indeed is no guarantee that this will work either in C or C++.

这将在任何已知的实现上切实可行吗?换句话说，您知道在其中不起作用的任何实现吗?

Will this practically work on any known implementation? In other words, do you know of any implementation where this doesn't work?

是否有任何标准的方法(我的意思不是特定于编译器的方法)来表达相同的想法?也许C ++ 0x对齐功能可能会有所帮助?

Is there any standard(I mean not compiler-specific) way to express the same idea? Maybe the C++0x alignment features might help?

顺便说一句，这不是我在生产代码中正在做的事情，不用担心，只是好奇.预先感谢.

By the way, this isn't something I am doing in production code, don't worry, just curious. Thanks in advance.

推荐答案

1. 是
2. 取决于架构的对齐需求和编译器策略
3. 否，但是您可以做一个对象包装器(但最终会得到.z()而不是仅.z)

1. yes
2. depends on the alignment needs of the architecture and the compilers strategy
3. no, but you could make a object wrapper (but you will end up with .z() instead of just .z)

大多数编译器应支持使用杂注或属性压缩结构.例如#pragma pack.

Most compilers should support squashing a structure using a pragma or an attribute. #pragma pack for example.

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