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Thread: Basic <algorithm> (reverse_copy) doesn't work with QString

  1. #1
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    Default Re: Basic <algorithm> (reverse_copy) doesn't work with QString

    Following code returns empty string, why?

    Qt Code:
    1. QString reverseQString( const QString & str )
    2. {
    3. QString rev;
    4. reverse_copy( str.begin(), str.end(), rev.begin() );
    5. return rev;
    6. }
    To copy to clipboard, switch view to plain text mode 


    Added after 27 minutes:


    Is the problem that reverse_copy in fact needs functionality of std::back_inserter, not just an iterator?
    Last edited by AnthonyL; 27th August 2016 at 15:43.

  2. #2
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    Default Re: Basic <algorithm> (reverse_copy) doesn't work with QString

    Hello,

    the local object rev is an empty string. std::reverse_copy expects that the destination container (in your case rev) already has allocated memory to store the reverse copy. To achieve this, add
    Qt Code:
    1. rev.resize(str.size());
    To copy to clipboard, switch view to plain text mode 
    before calling reverse_copy.

    See also http://www.cplusplus.com/reference/a.../reverse_copy/
    Best regards
    ars

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    Default Re: Basic <algorithm> (reverse_copy) doesn't work with QString

    My guess would be that QString::begin() returns QString::end() when called on a null QString and one cannot assign to end().
    I.e. QString::begin() of a null QString might not qualify as an OutputIterator.

    You could try with a std::back_inserter.

    Cheers,
    _

  4. #4
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    Default Re: Basic <algorithm> (reverse_copy) doesn't work with QString

    Quote Originally Posted by ars View Post
    Hello,

    the local object rev is an empty string. std::reverse_copy expects that the destination container (in your case rev) already has allocated memory to store the reverse copy. To achieve this, add
    Qt Code:
    1. rev.resize(str.size());
    To copy to clipboard, switch view to plain text mode 
    before calling reverse_copy.

    See also http://www.cplusplus.com/reference/a.../reverse_copy/
    Best regards
    ars
    It then works, thanks

  5. #5
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    Default Re: Basic <algorithm> (reverse_copy) doesn't work with QString

    Using std::back_inserter makes use of a sequence of QString:ush_back() calls, which result in multiple memory reallocations (depending on the memory allocation strategy implemented in QString:ush_back()). In the scenario posted by OP we already know the size of the result, so I expect better performance using a single memory reallocation than a sequence of reallocations. Of course this will not give measurable differences for small containers/strings.

    Best regards
    ars

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