[SOLVED] operator [] or QList::at()?

  • Greetings.

    I wonder if using the operator [] on a constant QList (or QVector) causes a deep copy.

    I have a function that receives as a parameter a const QList< cv::Mat >& and l want to know if it is better access each element with the operator [] or the member function QList::at(). As far as I read this last function can be faster than operator [], but for a constant container I'm not sure if the performance is the same in both cases.

    On matters of style I prefer to use the operator [], but I'm not sure which is the better option in this situation?

    Thanks in advance for any help and/or suggestion.

  • Moderators

    Check out the documentation: the constant variant is the same as QList::at(). "Link":http://qt-project.org/doc/qt-5/qlist.html#operator-5b-5d-2.

  • Hi,
    When accessing QList members it's good to know that most Qt classes use 'copy at write' operations with a reference counter. This said the performance between [] or at() might be slim for const use (no writable). So Qt will give a reference to the QList member and not a deep copy!

  • Thank you all.

    So in summary for this case, both options (operator [] and QList::at()) have the same or similar performance... I am right?

  • Moderators

    Yes, you are, as long as we are talking about const containers. In non-const onces, operator[] can insert a new default-constructed value when an item at given index is not present, thus copying the data.

Log in to reply

Looks like your connection to Qt Forum was lost, please wait while we try to reconnect.