c++ - 匿名 union 和结构

Question

这个问题在这里已经有了答案:

Why does C++ disallow anonymous structs? (7 个回答)

关闭 8年前。

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。

我希望以与您相同的方式访问成员。

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

        some_type elements[3];
    };
};

Answer 1

是的，C++11 和 C++14 都不允许匿名结构。 This answer包含为什么会出现这种情况的一些推理。您需要命名结构，并且它们也不能在匿名 union 中定义。

§9.5/5 [class.union]

... The member-specification of an anonymous union shall only define non-static data members. [ Note: Nested types, anonymous unions, and functions cannot be declared within an anonymous union. —end note ]

所以将结构定义移到 union 之外。

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，因为这使得匿名 union 的所有成员 layout 兼容。 Here are the requirements对于标准布局类型。这些在标准的第 9/7 节中进行了描述。

然后，从 §9.2 [class.mem]

16   Two standard-layout struct (Clause 9) types are layout-compatible if they have the same number of non-static data members and corresponding non-static data members (in declaration order) have layout-compatible types (3.9).
18   If a standard-layout union contains two or more standard-layout structs that share a common initial sequence, and if the standard-layout union object currently contains one of these standard-layout structs, it is permitted to inspect the common initial part of any of them. Two standard-layout structs share a common initial sequence if corresponding members have layout-compatible types and either neither member is a bit-field or both are bit-fields with the same width for a sequence of one or more initial members.

对于数组成员，来自 §3.9/9 [basic.types]

... Scalar types, standard-layout class types (Clause 9), arrays of such types and cv-qualified versions of these types (3.9.3) are collectively called standard-layout types.

要确保 some_type 是标准布局，请在 vec

的定义中添加以下内容

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

std::is_standard_layout在 type_traits header 中定义。现在您的 union 的所有 3 个成员都是标准布局，两个结构和数组是布局兼容的，因此 3 个 union 成员共享一个共同的初始序列，这允许您写入然后检查(读取)任何属于共同的成员初始序列(在你的情况下整个事情)。