Re: [boost] Yet another FOREACH

From: Eric Niebler (eric_at_[hidden])
Date: 2005-10-12 11:39:16

A. Azuma wrote:
> 2005/10/9, Daniel James <daniel_at_[hidden]>:
> 
>>Unfortunately, this technique is not standards compliant. The problem is
>>in this code:
>>[snip]
> 
> 
> Ah, I see. I overlooked this problem. I'm ashamed of my
> misinterpretation . After a
> little thought, I have concluded that there might not be any tricks to
> enable this
> kind of ADL. I would have to abandon my idea. Does anyone have good idea?
> 

No reason to be ashamed -- it was a clever idea. There was one other 
person during the FOREACH review who argued for a similar interface, 
using TYPEOF in the implementation. There are a few reasons IMO why it 
wasn't a good idea:

1) It would have required people to register types with TYPEOF in order 
to use FOREACH. Unacceptable, IMO. (Your idea doesn't suffer from this, 
but it's non-standard.)

2) The current signature of FOREACH (FOREACH(type var, sequence)) 
mirrors the foreach construct of other popular languages. Also, this is 
the signature likely to be adopted should a new foreach looping 
construct be added to C++.

3) FOREACH(iter, sequence) doesn't make as strong a guarantee as does 
FOREACH(type var, sequence). With the second, you can be sure that every 
element in the sequence wiil be visited exactly once, and we won't walk 
off the end of the sequence. But with FOREACH(iter, sequence), nothing 
is stopping people from burying a ++iter somewhere in the loop body, 
throwing off the looping mechanism. When you need an iterator, there is 
always the general for(;;) statement. Use FOREACH for the simple, common 
cases.

-- 
Eric Niebler
Boost Consulting
www.boost-consulting.com