Question

I just saw a code snippet with a piece of syntax that I have never seen before.
What does bool start : 1; mean? I found it inside a class definition in a header file.

Answer 1

struct record {
    char *name;
    int refcount : 4;
    unsigned dirty : 1;
};

Those are bit-fields; the number gives the exact size of the field, in bits. (See any complete book on C for the details.) Bit-fields can be used to save space in structures having several binary flags or other small fields, and they can also be used in an attempt to conform to externally-imposed storage layouts. (Their success at the latter task is mitigated by the fact that bit-fields are assigned left-to-right on some machines and right-to-left on others).

Note that the colon notation for specifying the size of a field in bits is only valid in structures (and in unions); you cannot use this mechanism to specify the size of arbitrary variables.

• References: K&R1 Sec. 6.7 pp. 136-8
• K&R2 Sec. 6.9 pp. 149-50
• ISO Sec. 6.5.2.1
• H&S Sec. 5.6.5 pp. 136-8