On 17 Aug., 05:29, peter miller wrote:
>  [...]
>  because there's an off by one error in 25.3.10/4; the definition should
>  read:
>
>  *(result + (last - first) - 1 - i) = *(first + i)
>                             ^^^

I agree. Good catch. Hopefully, the committee will notice this as I
don't really know what the exact protocol is for reporting these
errors. Posting here is a good start, I guess.

Cheers!
SG


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On 2011-08-24 23:06, SG wrote:
>  On 17 Aug., 05:29, peter miller wrote:
>>    [...]
>>    because there's an off by one error in 25.3.10/4; the definition should
>>    read:
>>
>>    *(result + (last - first) - 1 - i) = *(first + i)
>>                               ^^^
>
>  I agree. Good catch. Hopefully, the committee will notice this

I have forwarded Peter's description to the LWG group, so they must have
noticed it ;-)

>  as I
>  don't really know what the exact protocol is for reporting these
>  errors. Posting here is a good start, I guess.

The description in

http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html

says (search for "How to submit an issue") that you can send an e-mail
to the author of the list and this e-email address is provided on the
top of the document (search for "Reply to").

But as you say, I generally recommend to post to this group (or
c.l.c.mod) first, because sometimes people believe that there is an
issue, but in fact there isn't.

Greetings from Bremen,

- Daniel Kr   gler



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Daniel (and SG)

> I have forwarded Peter's description to the LWG group, so they must
> have noticed it ;-)

Yeah, and I ought to have said "thanks" for that, by now. So thanks,
Daniel; you do great work.

(And I might get back to replying about iterators - eventually.
Some of it was me forgetting about uniform initializers...)

> But as you say, I generally recommend to post to this group (or
> c.l.c.mod) first, because sometimes people believe that there is an
> issue, but in fact there isn't.

And even standards' writers can make schoolboy errors... ;)

It also gives the C++ community a "heads up".  So it's a bit
more public spirited, too.

Cheers
Peter

(PS This is a resend, because of my ISP's flaky news server.)
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/*
Hi,

the output of the program below should be:

"three two one null \n"

But when std::reverse_copy is implemented as described in
N3291=11-0061 25.3.10 [alg.reverse] it's:

"null three two one \n"

because there's an off by one error in 25.3.10/4; the definition should
read:

*(result + (last - first) - 1 - i) = *(first + i)
                         ^^^

Hopefully, I've not made an embarrassing mistake of my own.

Peter
*/


#include <algorithm>
#include <iostream>


template <typename BiIterator, typename OutIterator>
auto
reverse_copy_as_described_in_N3291(
   BiIterator first, BiIterator last, OutIterator result )
-> OutIterator
{
  // 25.3.10/4 [alg.reverse]:
  // "...such that for any non-negative integer i < (last - first)..."
  for ( unsigned i = 0; i < ( last - first ); ++i )
     // "...the following assignment takes place:"
     *(result + (last - first) - i) = *(first + i);

  // 25.3.10/6
  return result + (last - first);
}

int main()
{
   using std::begin;
   using std::end;
   using std::cout;

   static const char*const in[3]  { "one", "two", "three" };
   const char*             out[4] { "null", "null", "null", "null" };

   reverse_copy_as_described_in_N3291( begin( in ), end( in ), out );


   for ( auto s : out )
       cout << s << ' ';

   cout << std::endl;

   return 0;

}


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On 2011-08-17 05:29, peter miller wrote:
>
> /*
> Hi,
>
> the output of the program below should be:
>
> "three two one null \n"
>
> But when std::reverse_copy is implemented as described in
> N3291=11-0061 25.3.10 [alg.reverse] it's:
>
> "null three two one \n"
>
> because there's an off by one error in 25.3.10/4; the definition should
> read:
>
> *(result + (last - first) - 1 - i) = *(first + i)

I agree with your problem analysis. Forwarded to the library working group.

Greetings from Bremen,

- Daniel Kr   gler


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