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:
declaratoropt : constant-expression
The 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
unsigned nWeekDay : 3; // 0..7 (3 bits)
unsigned nMonthDay : 6; // 0..31 (6 bits)
unsigned nMonth : 5; // 0..12 (5 bits)
unsigned nYear : 8; // 0..100 (8 bits)