Re: [Boost-users] Range_iterator - char* - reverse

Subject: Re: [Boost-users] Range_iterator - char* - reverse
From: Rao, Anant (Anant.Rao_at_[hidden])
Date: 2010-12-10 14:49:56

Hi Eric,
Yes, it was a typo  on my part to use ++ when I shd have had a --.
But, I was talking about the compile error. That is, even if I had a '--', it would still complain.
The offending line is:

while( cp != ire )
{
  //Do something
  cp--;
}

Please bear with me as I (hopefully) make my requirement statement more precise:

Fwd:
iterator_range<char*> ir( &s[0], &s[ strlen(s) ] );

for (cp = (char*)ir.begin(); cp != (char*)ir.end(); ++cp)
{
       cout << "cp in ri [" << *cp << "]\n";
}

I want similar code (using 'cp') to traverse backwards using 'ir' or another container-in-reverse mode.

Your help much appreciated.
Thanks a lot again,
Anant

-----Original Message-----
From: boost-users-bounces_at_[hidden] [mailto:boost-users-bounces_at_[hidden]] On Behalf Of Eric MALENFANT
Sent: Friday, December 10, 2010 11:23 AM
To: boost-users_at_[hidden]
Subject: Re: [Boost-users] Range_iterator - char* - reverse

De : Rao, Anant
> 
> So, there's absolutely no way to use the same ptr to traverse
> backwards? I tried the options based on my limited knowledge, but if
> there're other options, please let me know (using the same ptr).
> 
> It's ironic a simple pointer could go fwd, but not back.
> 

Well, it can go forward or back, but you have to ask it. "++" to go forward, "--" to go back.

>   char *cp;
>   boost::range_reverse_iterator< char*>::type irb = boost::rbegin( ir );
>   boost::range_reverse_iterator< char*>::type ire = boost::rend( ir );
>   for (; irb != ire; ++irb)
>   {
>      cout << "reverse cp [" << *irb << "]\n";
>   }
> 
>   This works like a charm - both in fwd and reverse traversal.

This works because irb is a reverse iterator, with operator++ defined to go "backwards". That behavior will not change because the initial value happens to come from a reverse_iterator. A char* is a char*, regardless of the source of its initial value.

> 	Option  1:
> 	for (cp = boost::rbegin(ir); cp != boost::rend(ir); ++cp)
> 	{
> 	      cout << "cp reverse in ri [" << *cp << "]\n";
> 	}

cp is a char* here, and "++" on a char* is defined to increment it, moving it "forward". 
To move it "backward", use "--"
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