Important: Please read the Qt Code of Conduct -

[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":

  • 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