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

**Section:** 25.6.10 [alg.reverse] **Status:** C++14
**Submitter:** Peter Miller **Opened:** 2011-08-17 **Last modified:** 2016-02-10

**Priority: **Not Prioritized

**View all other** issues in [alg.reverse].

**View all issues with** C++14 status.

**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 25.6.10 [alg.reverse]
it's:

"null three two one \n"

because there's an off by one error in 25.6.10 [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 25.6.10 [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.