Wincent Colaiuta's weblog: Things you learn from looking at the HexFiend source code

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September 24, 2007

Things you learn from looking at the HexFiend source code

static BOOL isHex(unichar c) {
    switch (c) {
	case '0' ... '9':
	case 'a' ... 'f':
	case 'A' ... 'F':
	    return YES;
	default: return NO;
    }
}

Well that one is an eye-opener. I have often wished for the "range" shorthand provided here by the ... (three dots) operator. The thing is, I'd never seen this operator in C code before and my trusty C Pocket Reference only makes one mention of the operator as an "optional argument indicator". In its section on case labels it only says:

Every case label has the form case const:; where const is a constant integer expression.

How could I have missed such a useful thing? The Wikipedia page on page on C and C++ operators makes no reference to it, nor does the main article on C.

The 550-page C99 standard only makes references to ... as the "ellipsis punctuator" and describes how it is used to indicate an unspecified number of additional arguments.

Finally I found the answer: it's a GNU GCC extension. As noted by the documentation:

Write spaces around the ..., for otherwise it may be parsed wrong when you use it with integer values.

Very useful indeed.