Re: undefined compound chaining behavior

[Rik]

On 06/06/2014 08:06 AM, address@hidden wrote:
> Message: 5
> Date: Fri, 6 Jun 2014 17:20:08 +0330
> From: Hossein Sajjadi <address@hidden>
> To: address@hidden
> Subject: chaining compound assignment results in undefined behavior
> Message-ID:
>       <address@hidden>
> Content-Type: text/plain; charset=UTF-8
>
> Hi
>
> There is a problem with chaining compound assignment.
> For example the expression a=1;a+=a+=4 will result 10.
> I think such a behavior is borrowed from C language but this behavior
> is known as "undefined behavior".
>  This issue have been resolven in Java.
>  Java language guarantees that the operands of operators appear
> to be evaluated in a specific evaluation order , namely, from left to
> right. ?15.7
> Other specifications about evaluation of expressions are in
> "The Java? Language Specification"
>
>  also this approach is followed by c# language
>  so result of above expression in Java is 6

I don't think we need to do anything here.  First, code should
self-documenting and easy to understand.  When I first looked at the
statement I couldn't figure out what the intent was, and so I'm not
surprised that the compiler also has a hard time deciding what to do.  So,
I would suggest that rather than introduce complexities in to the parser,
the programmer should simply write what they mean.

If the programmer wants 6 as an answer, then code
a = 1; a += a; a+= 4

If the programmer wants 10 as an answer, then code
a = 1; a +=4; a += a

I tried the example in C++ and the result with the gcc compiler is 10 so
I'm perfectly happy to define the behavior that multiple in-place operators
are evaluated in a specific evaluation order which happens to be "right to
left".

Cheers,
Rik