QList::contains and constness



  • Hi all

    Inside MainWindow I have an object nodeList of type QList<NodeDescritor*> and I have this simple code (NodeDescriptor is my own class derived from QObject):

    @
    State MainWindow::getNodeState(const NodeDescriptor *node) const
    {
    // some code here
    if (nodeList.contains(node))
    {
    // some code here
    }
    // some code here
    }
    @

    During compilation, the invokation of contains() produces the following error: invalid conversion from 'const NodeDescriptor*' to 'NodeDescriptor*' [-fpermissive].

    Looking at Qt source code, I discovered the declaration of QList::contains:

    @
    template <typename T>
    Q_OUTOFLINE_TEMPLATE bool QList<T>::contains(const T &t) const
    @

    In My case T is NodeDescriptor*. I think the template should be let's say expanded into the following:

    @
    bool QList::contains(const NodeDescriptor* &t) const
    @

    For curiosity, I declared a new method with the same signature of contains() within MainWindow:

    @
    bool test(const NodeDescriptor* &t) const
    @

    @
    State MainWindow::getNodeState(const NodeDescriptor *node) const
    {
    // some code here
    if (test(node))
    {
    // some code here
    }
    // some code here
    }
    @

    An invoking the latter does not produce any error.

    I really do not understand why. The two methods have the same signature and are invoked from within the same block statement. As far as I remember, this does not happen for STL list class.

    At the moment, I solved the problem using const_cast. But I don't like this solution cause in theory QList::contains should accept const NodeDescriptor *.

    Any idea ?

    Thanks
    Fabio



  • I believe the error is caused because the template parameter T that you give as input is const NodeDescriptor* where as the parameter T that contains uses is simply NodeDescriptor*.

    To see if it works you can try and change the signature of getNodeState to use NodeDescriptor*



  • Yes! I already did the test you mentioned and it works. Anyway, I want the sign of getNodeState have const NodeDescriptor * as input param type to be sure for the invokers.

    Thanks for your reply



  • I'm afraid you don't have many options in this case. I have tried using STL containers and I get the same compiler error.
    Maybe in this case a const_cast is warranted unless you find a better way :)


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