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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.
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Unlike all other operators in Objective-C—which are either unary or binary operators—the conditional operator is a ternary operator; that is, it takes three operands. The two symbols used to denote this operator are the question mark ( ? ) and the colon ( : ).
It's commonly used to pack lots of values into an integral type. In your particular case, it defining the structure of a 32-bit microcode instruction for a (possibly) hypothetical CPU (if you add up all the bit-field lengths, they sum to 32). This prints out 7, which is the three bits making up the alu bit-field.
As everyone referred that, It is a way of representing conditional operator if (condition){ true } else { false }
They can be used as "dummy" fields, for alignment purposes. An unnamed bit field whose width is specified as 0 guarantees that storage for the member following it in the struct-declaration-list begins on an int boundary.
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.
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