匿名联合和结构

问题描述

您将如何在标准C ++ 11/14中执行此操作？因为如果我没记错的话，这不是具有匿名结构的标准兼容代码。

How would you go about doing this in standard C++11/14 ? Because if I'm not mistaken this isn't standard compliant code with the anonymous structs.

我希望以与您相同的方式访问成员。 / p>

I wish to access the members the same way as you would with this.

template <typename some_type>
struct vec
{
    union {
        struct { some_type x, y, z; };
        struct { some_type r, g, b; };

        some_type elements[3];
    };
};

推荐答案

是的，C ++ 11和C ++都不14允许匿名结构。 包含这种情况的一些原因。您需要命名结构，它们也不能在匿名联合中定义。

Yes, neither C++11 nor C++14 allow anonymous structs. This answer contains some reasoning why this is the case. You need to name the structs, and they also cannot be defined within the anonymous union.

§9.5/ 5 [class.union]

因此将结构定义移到并集之外。

So move the struct definitions outside of the union.

template <typename some_type>
struct vec
{
    struct xyz { some_type x, y, z; };
    struct rgb { some_type r, g, b; };

    union {
        xyz a;
        rgb b;
        some_type elements[3];
    };
};

现在，我们要求 some_type 为 standard-layout ，因为这使匿名联合的所有成员 layout兼容。 。在标准的第§9/ 7节中对此进行了描述。

Now, we require some_type to be standard-layout because that makes all the members of the anonymous union layout compatible. Here are the requirements for a standard layout type. These are described in section §9/7 of the standard.

然后，来自§9.2[class.mem]

对于数组成员，来自§3.9/ 9 [basic.types]

要确保 some_type 是标准布局，请在 vec

To ensure that some_type is standard layout, add the following within the definition of vec

static_assert(std::is_standard_layout<some_type>::value, "not standard layout");

在 type_traits 标头中定义。现在，联合的所有3个成员都是标准布局，两个结构和数组都与布局兼容，因此这3个联合成员共享一个公共的初始序列，这使您可以编写然后检查（读取）属于该公共的任何成员初始序列（整个情况）。

std::is_standard_layout is defined in the type_traits header. Now all 3 members of your union are standard layout, the two structs and the array are layout compatible, and so the 3 union members share a common initial sequence, which allows you to write and then inspect (read) any members belonging to the common initial sequence (the entire thing in your case).

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