On 16 March 2018 at 19:30, Garreau, Alexandre wrote: > Hi, > > I was reading the “C Extensions” section of GCC manual [0] about GNU > extensions, and going through the part about conditional expression > operand omission, I noticed it said it allowed the omission of the > *middle* operand (then nicely allowing some construct really similar to > the lisp or) > > But I don’t understand why the analogous omission of the last operand > with the same semantics (returning the first operand if we need to > return the omitted one) is not permitted then. It’s not already standard > nor then supported by gcc, and though the same effect can be obtained by > negating the first operand, I find usually more expressive to let a > positive test expression. Also omitting the last instead of the middle > operand would have seemed more natural to write (as it maybe would have It seems less natural to me. "return val ?: alt;" means "if val is good, return it, otherwise some fallback alternative". This is useful e.g. when val is a pointer and you want to avoid returning NULL. "return val ? alt :;" would mean "if val is good, return something else, otherwise return the value we just decided is not good" The other reason that "x ? : y" makes more sense is that for scalar values in C there are many true values (anything non-zero) but only one false value (zero). If it's any of the true values, return it, otherwise return false. Whereas with "x ? y :" it would mean if it's any true value, return a specific value (losing the original value of x) otherwise return x (which we know must be zero in this case). When you're using the extension to avoid evaluating an operand twice (as explained in the manual) "x ? : y" serves a useful purpose. Your suggestion doesn't add anything, because you can just write "x ? y : 0" to avoid evaluating x twice. > allowed to omit the colon too, if I’m not wrong about something > conflicting with it in C syntax, as it would simplify syntax and allow > more non-simply-erroneous combinations of syntax). > > Is there any justification to that? was it just considered as > unnecessary? Was this form considered less useful? was a feature > looking alike lisp or considered more evidently useful? > > Should I ask on the main gcc list? > > [0] (info "(gcc-6) C Extensions") > [1] (info "(gcc-6) Conditionals")
