C++ Bit Fields

C++ Bit Fields

 

Classes and structures can contain members that occupy less storage than an integral type. These members are specified as bit fields. The syntax for bit-field member-declarator specification follows:


declarator  : constant-expression

The (optional) declarator is the name by which the member is accessed in the program. It must be an integral type (including enumerated types). The constant-expression specifies the number of bits the member occupies in the structure. Anonymous bit fields — that is, bit-field members with no identifier — can be used for padding.

System_CAPS_noteNote

An unnamed bit field of width 0 forces alignment of the next bit field to the next type boundary, where type is the type of the member.

The following example declares a structure that contains bit fields:

// bit_fields1.cpp
// compile with: /LD
struct Date {
   unsigned short nWeekDay  : 3;    // 0..7   (3 bits)
   unsigned short nMonthDay : 6;    // 0..31  (6 bits)
   unsigned short nMonth    : 5;    // 0..12  (5 bits)
   unsigned short nYear     : 8;    // 0..100 (8 bits)
};

The conceptual memory layout of an object of type Date is shown in the following figure.

Memory layout of a date object

Memory Layout of Date Object

Note that nYear is 8 bits long and would overflow the word boundary of the declared type, unsigned short. Therefore, it is begun at the beginning of a new unsigned short. It is not necessary that all bit fields fit in one object of the underlying type; new units of storage are allocated, according to the number of bits requested in the declaration.

Microsoft Specific 

The ordering of data declared as bit fields is from low to high bit, as shown in the figure above.

END Microsoft Specific

If the declaration of a structure includes an unnamed field of length 0, as shown in the following example,

// bit_fields2.cpp
// compile with: /LD
struct Date {
   unsigned nWeekDay  : 3;    // 0..7   (3 bits)
   unsigned nMonthDay : 6;    // 0..31  (6 bits)
   unsigned           : 0;    // Force alignment to next boundary.
   unsigned nMonth    : 5;    // 0..12  (5 bits)
   unsigned nYear     : 8;    // 0..100 (8 bits)
};

the memory layout is as shown in the following figure.

Layout of Date object with zero-length bit field

Layout of Date Object with Zero-Length Bit Field

The underlying type of a bit field must be an integral type, as described in Fundamental Types.

The following list details erroneous operations on bit fields:

  1. Taking the address of a bit field.

  2. Initializing a reference with a bit field.

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