Issue 2074: Off by one error in std::reverse_copy (original) (raw)

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2074. Off by one error in std::reverse_copy

Section: 26.7.10 [alg.reverse] Status: C++14 Submitter: Peter Miller Opened: 2011-08-17 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [alg.reverse].

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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 26.7.10 [alg.reverse]it's:

"null three two one \n"

because there's an off by one error in 26.7.10 [alg.reverse]/4; the definition should read:

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

Test program:

#include #include

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 charconst 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 26.7.10 [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.