?: grouping

[Steve Summit]

I'm usually pretty bored with these nitty-gritty compiler
questions, because I don't want to think that hard, but now I'm
working on a compiler myself, and I came up with one.

How should ?: group?  The C Reference Manual says it groups left
to right, just like all the binary operators.  In the case of a
ternary operator it's not as obvious what "left to right" means,
but I would think that a?b:c?d:e should be evaluated as
(a?b:c)?d:e (do the leftmost one first).  The first time I
implemented it, I ended up with something that would interpret it
as a?b:(c?d:e), for no other reason than because it was easier to
implement that way.  Lo and behold, that's what the 4.1 compiler
does, too!  As a quick example, the program

	main()
	{
	printf("%d\n", 1?1:0?3:4);
	}

prints 1.  (1?1:0)?3:4 would be 3.

I'm not too worried about this, because the "compiler" I'm working
on is actually just a stripped-down version of cpp, and I'm
reasonably sure that nobody is ever going to use a ?: on an #if
line in this application.  Still, it is a curious question.

                                         Steve Summit
                                         tektronix!tekmdp!stevesu