What's the purpose of unnamed bit field at the end of structure

Question

I am learning C. In C Primer Plus, I saw an bit field example as follows:

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;
};

I do understand the 4-bit unnamed bit field in the middle is used for letting the following bits start at a new byte. However, I don't understand why there is another unnamed bit field at the end of the structure. What's the purpose of it? Is it necessary?

Answer 1

Is it necessary?

Nope, it's optional.

What's the purpose of it?

Here's what the standard says in §9.6.2, C++11 (draft N3337, emphasis mine):

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.

So it's a hint to the compiler that summing up all the members of the struct leads to 2 octects and thus is done hoping the compiler would make the struct 2 octects long. However, as per the standard there's no such requirement. Here's the excerpt from the previous point, §9.6.1:

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.

Hence the standard does not guarantee any further than this regarding the size or alignment of a struct/class using bit-fields.