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### 2074. Off by one error in `std::reverse_copy`

Section: 25.7.10 [alg.reverse] Status: C++14 Submitter: Peter Miller Opened: 2011-08-17 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The output of the program below should be:

>     "three two one null \n"

But when `std::reverse_copy` is implemented as described in N3291 25.7.10 [alg.reverse] it's:

>     "null three two one \n"

because there's an off by one error in 25.7.10 [alg.reverse]/4; the definition should read:

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

Test program:

>     #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;
>     }

[2012, Kona]

Move to Ready.

[2012, Portland: applied to WP]

Proposed resolution:

This wording is relative to the FDIS.

Change 25.7.10 [alg.reverse] p4 as follows:

>     template<class BidirectionalIterator, class OutputIterator>
>       OutputIterator
>         reverse_copy(BidirectionalIterator first,
>                      BidirectionalIterator last, OutputIterator result);

> > -4- Effects: Copies the range [`first,last`) to the range [`result,result+(last-first)`) such that for any non-negative integer `i < (last - first)` the following assignment takes place: `*(result + (last - first) - 1 - i) = *(first + i)`.

> > -5- Requires: The ranges [`first,last`) and [`result,result+(last-first)`) shall not overlap.

> > -6- Returns: `result + (last - first)`.

> > -7- Complexity: Exactly `last - first` assignments.