[boost] Re: Formal Review: FOREACH macro

From: Eric Niebler (eric_at_[hidden])
Date: 2005-05-02 15:15:28

Stefan Strasser wrote:
> Eric Niebler schrieb:
> 
>> Even when writing loops by hand, it's a good idea to save 
>> the result of .end() in a local to avoid repeated function calls that 
>> are not necessary. It's called "hoisting". It's one of the 
>> side-benefits of using FOREACH and std:: algorithms.
>>
>>
> 
> I think it's not worth it here.
> besides that it isn't a performance overhead on most containers you 
> don't expect that from a keyword.

Huh, *I* would expect that from a keyword.

> 
> and it is not consistent:
> 
>   char str[]={'a','b',0,0};
>   foreach(char c,str){
>     str[2]='c';
>     std::cerr << c << std::endl;
>   }
> 
> output is "abc".

Right. When iterating a null-terminated string, there isn't any need to 
find the end of the sequence up front. I just quit at the first 
null-terminator.

> 
>   std::string str="ab";
>   foreach(char c,str){
>     if(str.length() == 2) str+='c';
>     std::cerr << c << std::endl;
>   }
> 
> 
> output is "ab".

See the discussion about iterator invalidation. This code has undefined 
behavior.

You have found an interesting inconsistency, though. In the case of 
null-terminated strings, I felt I could be smarter than Boost.Range 
(which uses std::strlen() to find the end). You can only detect the 
difference in this one scenario, though, and I consider the perf win to 
be worth it. But I don't have strong feelings on the issue.

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