[Coco] Found strangeness in C compiler (bug)

[Mathew Boytim]

 I agree the result is undefined.  This is what gcc has to say
 main.c:7:4: warning: operation on ‘da’ may be undefined [-Wsequence-point] *da++ = *dN++ * v - *da; // this is correct as the increment should happen after the assignment  ~~^~

The standard doesn't say when the pointer is incremented, it only says where the result will be stored so you can't say "the increment will happen after the assignment".  I have to agree that the behavior is not what I would expect but undefined is undefined.  Bottom line is that you can't do that and expect it to be predictable.
Matt



    On Tuesday, May 21, 2019, 2:06:14 AM EDT, Mark McDougall <msmcdoug at iinet.net.au> wrote:  
 
 On 21/05/2019 3:32 pm, Walter Zambotti wrote:

> *da = *dN++ * v - *da++;  // this is wrong as the expression on the right
> increments da before it is assigns the value to where da points.

I think you'll find this behaviour is undefined by the standard, which 
is why you have inconsistent results depending on type.

It's not something I'd code that way in any case. I think your "work 
around" is better (more readable/maintainable) code.

Regards,

---8<------8<------8<------8<------8<------8<------8<------8<------8<------8<---

2) Between the previous and next sequence point, the prior value of a 
scalar object that is modified by the evaluation of the expression, must 
be accessed only to determine the value to be stored. If it is accessed 
in any other way, the behavior is undefined.

cout << i << i++; // undefined behavior (until C++17)
a[i] = i++; // undefined behavior (until C++17)



-- 
Mark McDougall
<http://retroports.blogspot.com.au>

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