[boost] Re: looping construct - testers wanted ;)

From: David Abrahams (dave_at_[hidden])
Date: 2004-05-07 20:04:20

"Eric Niebler" <eric_at_[hidden]> writes:

> David Abrahams wrote:
>> "Eric Niebler" <eric_at_[hidden]> writes:
>> 
>>>For all but the most trivial loops, the cost of reading and writing a
>>>bool is lost in the noise. IMO, being able have reference loop
>>>variables is worth the very small price.
>> "There's gotta be a better way" ;-)
>> 
>
> There isn't, and it's easy to see why. If you want to support
> reference loop variables, you need to be able to rebind the reference
> at each iteration. References aren't rebindable, so you need a
> declaration. In C++, there is only one way to declare a varible,
> inject it into a subsequent scope and *not* evaluate it in boolean
> context: the for loop. Hence, the inner for loop is necessary, as is a
> mechanism for executing the loop only once and for handling break
> statements.

That isn't enough to convince me that a boolean is needed, yet.  Maybe
something could be done with "goto"?  I haven't looked at
BOOST_FOREACH carefully, but I'm imagining something like:

    for (C::iterator s = c.begin(), f = c.end(); s != f; ++s) // outer
    {
        for(C::reference r = *s;;) // inner
        {
            // Body goes here
            goto continue_;
        }
        break; // if there's a break in the body, break out all the way
        continue_:
    }

Not sure if you can generate those labels reliably, but it seems
possible as long as nobody wants to put two FOREACHes on a single
line.

-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com