Re: warning: operation on 'zero' may be undefined

[Robert Dewar <dewar at gnat dot com>]

Paul Schlie wrote:

> For my own edification, it's not clear to me that the instruction sequence:
>
>   int zero = 125;
>   zero = (++zero)%3;
>
> is ambiguous in any way, as I was under the impression that the left hand
> side of an assignment expression will be assigned the resulting value of the
> evaluation of it's right hand side, which unambiguously logically equivalent
> to:
>
>  int zero = 125;
>  zero = ((zero = zero + 1) % 3) ;
>  
It's a matter of language definition, there simply is a requirement in 
the C definition
that the increment of zero take place before or after the assignment. 
You are missing
a required sequence point. Basically the rule in C is that if you have 
an expression
(remember that an assignment is just an expression), and somewhere in 
that expression,
you use an increment operator on a variable, you can't mention that 
variable elsewhere
in the expression. End of story.

Yes, as you analyze, it is likely the compiler will behave as you 
expect, but it is
right to warn you that you have undefined code and you have no right to
expect that behavior.