Why no (auto foo = bar) in while loops?

[Jonathan M Davis]

On Thursday, August 25, 2011 11:31:56 Steven Schveighoffer wrote:
> On Thu, 25 Aug 2011 11:15:44 -0400, Jonathan M Davis <jmdavisProg at gmx.com>
> 
> wrote:
> > On Thursday, August 25, 2011 07:11:31 Steven Schveighoffer wrote:
> >> > On 08/25/2011 12:47 AM, Mafi wrote:
> >> >> I'm not really sure if it's good for 'while'.
> >> >> I'm unsure because there are two somewhat natural semantics for
> >> >> such
> >> 
> >> a
> >> 
> >> >> construct.
> >> >> 
> >> >> //example
> >> >> //more to the nature of while
> >> >> while(auto obj = init) { work(obj); }
> >> >> 
> >> >> //1 (your interpretation)
> >> >> typeof(init) obj;
> >> >> while(obj = init) { work(obj); }
> >> >> 
> >> >> //2
> >> >> /*
> >> >> seems more natural for me because init runs only once (like any
> >> >> other
> >> >> init and like in 'for' or 'if')
> >> >> */
> >> 
> >> I'd say just leave the auto x = y for if and for loops, where there is
> >> a
> >> clear "run once" section.
> >> 
> >> Also, what do you do with a do while loop if the while contains this
> >> construct?
> >> 
> >> I just find the whole thing confusing.
> > 
> > I don't see why it would be confusing. Because it's unclear whether it's
> > set
> > every time or whether it's a new variable every time? It's actually one
> > of
> > those things that I do periodically thinking that it works and then get
> > frustrated when it doesn't, so I'd love to see it added.
> 
> See Mafi's response above.
> 
> What it boils down to is, I usually replace code like this:
> 
> auto x = condition;
> if(x)
> {
>     // use x
> }
> 
> with this:
> 
> if(auto x = condition)
> {
>     // use x.
> }
> 
> so the logical mapping to while would map this:
> 
> auto x = condition;
> while(x)
> {
>     // use x.
> }
> 
> to this:
> 
> while(auto x = condition)
> {
>     // use x.
> }
> 
> But it looks like every time through the loop x gets reassigned to
> condition.  That's not what I'd want.  At the very least, it's confusing
> semantics one way or the other.
> 
> If you use the construct in the *init* section of a for loop, it reads the
> same as the if statement, since that part is only executed once.
> 
> The while loop just doesn't fit the model.

Well, I'd definitely want it so that x is the value of the condition every 
time, and it surprises me that anyone would have thought that that would do 
anything else. The whole point of creating the variable in the if or while 
condition like that IMHO is so that you can save what the exact condition was 
for that run of the if statement or while loop without having to have to run 
the condition twice or have the variable exist outside the scope of the if-
statement or while-loop. If you wanted it to just be the value of the 
condition on the first iteration, then for does that already quite explicitly. 
But then again, assuming that for allows you to assign in the condition as 
well, then at least it's more explicit there, but I would have thought that 
the semantics of the while loop would have been quite clear.

- Jonathan M Davis