Re: How to handle operator of undefined associativity

glen herrmannsfeldt <gah@ugcs.caltech.edu> wrote:
> BGB / cr88192 <cr88192@hotmail.com> wrote:
(snip)


>> but, with a unary operator, there is only a single possible parsing:
>> the operator and the expression it operates on.


> That is true if you allow only prefix operators, or only
> postfix operators, but not if you allow both. Consider C:


> *x++ is *(x++) or (*x)++


> Where * and ++ are both unary operators.


(snip)


> [This isn't associativity, it's precedence. Humph. -John]


So now I have to actually get out my K&R2...


I know that it always works as *(x++), but why?


It seems that * and ++ are at the same precedence level, but associate
right to left. I haven't actually looked at the table in many years,
yet haven't had much problem with precedence and associativity.
(There is, of course, the well known low precedence of << and >>.)


This could be avoided by not having postfix and prefix
operators at the same precedence level. It seems that
C didn't do that.


(I haven't looked at the C99 precedence table, but I will
presume that they didn't change this.)


-- glen
[I happen to have C99 here, and they did change it. The grammar separates out
postfix-expression and unary-expression, with postfix-expression coming first.
Offhand, I can't think of any plausible examples where the C99 grammar would
parse an expression differently. -John]

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