Question

I’d like to store in QHash elements that inherits from one class. So I’ve got:

class ImageInterface
{
public:
    ImageInterface();
    ImageInterface(const QString& path);
    virtual QString getName() const = 0;
};

And implementation:

class Image : public ImageInterface
{
public:
    Image();
    Image(const QString& path);
    QString getName() const { return name; }

private:
    QString name;
};

Then I want to use it in QHash:

QHash<QString, ImageInterface> *imageMap;
imageMap = new QHash<QString, ImageInterface>();
ImageInterface *im = new Image(var);
imageMap->insert(im->getName(), *im);
imageMap->take(name);

There are errors.

    ../EmulatorKamery/mainwindow.cpp:59:28: error: cannot allocate an object of abstract type ‘ImageInterface’
    ../EmulatorKamery/imageinterface.h:8:11: note:   because the following virtual functions are pure within ‘ImageInterface’:
    ../EmulatorKamery/imageinterface.h:13:25: note:     virtual QString ImageInterface::getName() const
    In file included from /usr/include/qt4/QtCore/qvariant.h:50:0,
                     from /usr/include/qt4/QtCore/qabstractitemmodel.h:45,
                     from /usr/include/qt4/QtGui/qabstractitemview.h:46,
                     from /usr/include/qt4/QtGui/qlistview.h:45,
                     from /usr/include/qt4/QtGui/qlistwidget.h:45,
                     from /usr/include/qt4/QtGui/QListWidget:1,
                     from ../EmulatorKamery/mainwindow.h:5,
                     from ../EmulatorKamery/mainwindow.cpp:1:
    /usr/include/qt4/QtCore/qhash.h: At global scope:
    /usr/include/qt4/QtCore/qhash.h: In instantiation of ‘QHashNode<QString, ImageInterface>’:
    /usr/include/qt4/QtCore/qhash.h:765:9:   instantiated from ‘QHash<K, V>::iterator QHash<K, V>::insert(const Key&, const T&) [with Key = QString, T = ImageInterface]’
    ../EmulatorKamery/mainwindow.cpp:46:48:   instantiated from here
    /usr/include/qt4/QtCore/qhash.h:221:7: error: cannot declare field ‘QHashNode<QString, ImageInterface>::value’ to be of abstract type ‘ImageInterface’
    ../EmulatorKamery/imageinterface.h:8:11: note:   since type ‘ImageInterface’ has pure virtual functions
    /usr/include/qt4/QtCore/qhash.h: In member function ‘T QHash<K, V>::take(const Key&) [with Key = QString, T = ImageInterface]’:
    ../EmulatorKamery/mainwindow.cpp:59:28:   instantiated from here
    /usr/include/qt4/QtCore/qhash.h:805:24: error: invalid abstract return type for member function ‘T QHash<K, V>::take(const Key&) [with Key = QString, T = ImageInterface]’
    ../EmulatorKamery/imageinterface.h:8:11: note:   since type ‘ImageInterface’ has pure virtual functions
    ../EmulatorKamery/mainwindow.cpp:59:28:   instantiated from here
    /usr/include/qt4/QtCore/qhash.h:808:18: error: cannot allocate an object of abstract type ‘ImageInterface’
    ../EmulatorKamery/imageinterface.h:8:11: note:   since type ‘ImageInterface’ has pure virtual functions
    /usr/include/qt4/QtCore/qhash.h:813:24: error: cannot allocate an object of abstract type ‘ImageInterface’
    ../EmulatorKamery/imageinterface.h:8:11: note:   since type ‘ImageInterface’ has pure virtual functions
    /usr/include/qt4/QtCore/qhash.h:813:11: error: cannot declare variable ‘t’ to be of abstract type ‘ImageInterface’
    ../EmulatorKamery/imageinterface.h:8:11: note:   since type ‘ImageInterface’ has pure virtual functions
    ../EmulatorKamery/mainwindow.cpp:59:28:   instantiated from here
    /usr/include/qt4/QtCore/qhash.h:819:16: error: cannot allocate an object of abstract type ‘ImageInterface’
    ../EmulatorKamery/imageinterface.h:8:11: note:   since type ‘ImageInterface’ has pure virtual functions
    /usr/include/qt4/QtCore/qhash.h:821:14: error: cannot allocate an object of abstract type ‘ImageInterface’
    ../EmulatorKamery/imageinterface.h:8:11: note:   since type ‘ImageInterface’ has pure virtual functions
    /usr/include/qt4/QtCore/qhash.h:822:1: warning: control reaches end of non-void function [-Wreturn-type]

How should I implemented something like this?

Answer 1

You have to store pointers to your abstract type. Otherwise it’s going to try to instantiate instances of that class, which it can’t since it’s abstract.

QHash<QString, ImageInterface*> *imageMap;