Bit-Fields

D also permits the definition of integer struct and union members of arbitrary numbers of bits, known as bit-fields. A bit-field is declared by specifying a signed or unsigned integer base type, a member name, and a suffix indicating the number of bits to be assigned for the field, as shown in the following example:

struct s {
    int a : 1;
    int b : 3;
    int c : 12;
};

The bit-field width is an integer constant separated from the member name by a trailing colon. The bit-field width must be positive and must be of a number of bits not larger than the width of the corresponding integer base type. Bit-fields larger than 64 bits may not be declared in D. D bit-fields provide compatibility with and access to the corresponding ANSI-C capability. Bit-fields are typically used in situations when memory storage is at a premium or when a struct layout must match a hardware register layout.

A bit-field is a compiler construct that automates the layout of an integer and a set of masks to extract the member values. The same result can be achieved by simply defining the masks yourself and using the & operator. C and D compilers try to pack bits as efficiently as possible, but they are free to do so in any order or fashion they desire, so bit-fields are not guaranteed to produce identical bit layouts across differing compilers or architectures. If you require stable bit layout, you should construct the bit masks yourself and extract the values using the & operator.

A bit-field member is accessed by simply specifying its name in combination with the “.” or -> operators like any other struct or union member. The bit-field is automatically promoted to the next largest integer type for use in any expressions. Because bit-field storage may not be aligned on a byte boundary or be a round number of bytes in size, you may not apply the sizeof or offsetof operators to a bit-field member. The D compiler also prohibits you from taking the address of a bit-field member using the & operator.