c - 结构末尾未命名位域的用途是什么

Question

我正在学习C。在C Primer Plus中，我看到了一个位域示例，如下所示:

struct box_props {
    bool opaque               : 1;
    unsigned int fill_color   : 3;
    unsigned int              : 4;
    bool show_border          : 1;
    unsigned int border_color : 3;
    unsigned int border_style : 2;
    unsigned int              : 2;
};

我确实理解中间的 4 位未命名位字段用于让后续位从新字节开始。但是，我不明白为什么在结构的末尾还有另一个未命名的位域。它的目的是什么？有必要吗？

Answer 1

Is it necessary?

不，它是可选的。

What's the purpose of it?

这是标准在 §9.6.2，C++11(草案 N3337，强调我的)中所说的内容:

A declaration for a bit-field that omits the identifier declares an unnamed bit-field. Unnamed bit-fields are not members and cannot be initialized. [Note: An unnamed bit-field is useful for padding to conform to externally-imposed layouts. — end note ] As a special case, an unnamed bit-field with a width of zero specifies alignment of the next bit-field at an allocation unit boundary. Only when declaring an unnamed bit-field may the value of the constant-expression be equal to zero.

所以这是对编译器的一个提示，将 struct 的所有成员相加得到 2 个八位字节，因此希望编译器将 struct 设为 2 个八位字节长。但是，根据标准，没有这样的要求。这是前一点 §9.6.1 的摘录:

extra bits are used as padding bits and do not participate in the value representation of the bit-field. Allocation of bit-fields within a class object is implementation-defined. Alignment of bit-fields is implementation-defined. Bit-fields are packed into some addressable allocation unit.

因此，对于使用位域的 struct/class 的大小或对齐方式，标准不提供任何进一步的保证。