The Dangers of Copy On Write (COW)



  • I had the following function in my code:
    @ResultConstIterator getIterator(const ResultVector& checkArray, size_t argumentNumber)
    {
    if (argumentNumber < checkArray.size()) {
    return checkArray.begin()+argumentNumber;
    } else {
    return checkArray.end();
    }
    }
    @

    which was called like this:

    @
    ResultConstIterator entry = getIterator(checkArray, nr);
    if (entry != checkArray.end()) {
    doSomething(entry);
    }
    @

    When I changed the type of ResultVector from std::vector to QVector suddenly doSomething was called in every case. Fortunately I had a unit test that caught this. After some debugging and head scratching I found the cause of the changed behaviour.

    Do you also see it?



  • I am bit curious here.
    QVector::end () returns a pointer pointing to the imaginary item after the last item in the vector.
    And The vector::end returns iterator [Random Access Iterator] referring the last element .
    'entry' is of type ResultConstIterator .

    So when checkArray is QVector, @if (entry != checkArray.end()) @ means you are comparing a ResultConstIterator with an iterator?

    Just to make sure, if i am understanding correctly.

    And yea, Implicit sharing is the main difference between a std::vector and QVector.



  • You're mixing const iterators with non-const iterators. Don't :-)



  • The problem is indeed comparing an iterator with a const iterator, which is not easy to prevent in C++03. The correct code should be:

    @
    ResultConstIterator entry = getIterator(checkArray, nr);
    ResultConstIterator constEnd = checkArray.end();
    if (entry != constEnd) {
    doSomething(entry);
    }
    @

    QVector::end() does a detach() resulting in different end pointers.

    With QVector constEnd() can be used, and C++11 has cend().


Log in to reply
 

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