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C++ Bit Fields

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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:

Syntax

declarator : constant-expression

Remarks

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.

Note

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:

Diagram of the memory layout of a date object, showing where the n WeekDay, n MonthDay, n Month, and n Year bit fields are located.

nYear is 8 bits long, which would overflow the word boundary of the declared type, unsigned short. Therefore, it starts at the beginning of a new unsigned short. It isn't 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 previous figure.

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)
};

Then the memory layout is as shown in the following figure:

Diagram of the layout of a Date object, with a zero length bit field, which forces alignment padding.

The underlying type of a bit field must be an integral type, as described in Built-in types.

If the initializer for a reference of type const T& is an lvalue that refers to a bit field of type T, the reference isn't bound to the bit field directly. Instead, the reference is bound to a temporary initialized to hold the value of the bit field.

Restrictions on bit fields

The following list details erroneous operations on bit fields:

  • Taking the address of a bit field.

  • Initializing a non-const reference with a bit field.

See also

Classes and Structs