Q_PROPERTY with parameterized read and write accessors - c++

Question

I have a Configuration that holds some basic information about file locations like download, install, picture, music, document etc. location.
These are currently exposed to QML using Q_PROPERTY. They all have their own accessors:
Q_PROPERTY(QUrl download_location READ download_location WRITE set_download_location NOTIFY download_location_changed)
These accessors basically do all the same stuff and I'd like to get rid of all this redundant code I have to write.
My first idea is to have a little nested class FileLocation that provides get, set and validation functions. But then, how would I connect these to the Q_PROPERTY?
Also, if I had something like a static functions which took parameters, (e.g. check_validity( QUrl location )), how would I hand over this parameter from QML side?
I think I'm on the wrong path here, so my question is how to keep redundant code in the context of Q_PROPERTY within reasonable limits, avoiding to write loads and loads of get, set and changed functions for very similar objects?
Here's some more code:
class Configuration : public QObject
{
QObject
Q_PROPERTY(QUrl download_location READ download_location WRITE set_download_location NOTIFY download_location_changed)
Q_PROPERTY(QUrl music_location READ music_location WRITE set_music_location NOTIFY music_location_changed)
...
signals:
void download_location_changed();
void music_location_changed();
...
public slots:
void set_download_location(QUrl location)
{
download_location = location;
emit download_location_changed(download_location);
}
void set_music_location(QUrl location)
{
music_location = location;
emit music_location_changed(music_location);
}
...
private:
QUrl download_location,
music_location,
...;
}
So as you see there's a lot of recurring code that does the same, and I'd like to damp that down a little. How do I do that? I was thinking about some general functions set, get, changed etc. that get the member to work on was a parameter. But then I didn't know how to hand over which member to work on from qml.
I just found out about the possibility to expose C++ classes to qml via qmlRegisterType(...) - maybe this is the way to go here?

Answer 1

You could go for an evil macro:
#define IMPL(data, name) \
inline decltype(data) name() const { return data; } \
inline void set_##name(decltype(data) value) { if (value != data) { data = value; emit name##Changed();} }
Of course, if you don't need any extra stuff, you can simply use a MEMBER property and have Qt auto generate accessors for you.
This will however not work if you need to do custom things in the accessors, the macro will, just add your stuff to it.
Lastly, when you declare a Q_PROPERTY, you can right-click the property, go to refactoring, and select "generate missing members...", Qt will generate the default stubs for the accessors, and you will only have to add your custom stuff in. The downside to this is it has the nasty habit of putting the generated code in the silliest place possible, so if you want your class to look neat, you have to move it by hand.
qmlRegisterType() is when you want to register a type so it can be created in QML, you generally don't have to do anything to access a QObject derived object from QML - it works for QObject derivatives, you only need to register a metatype for types which do not have meta information generated.
Last but not least - I don't see any point in having your configuration as a C++ object, you can just as well do it in QML, and use Qt.labs.settings to make your settings persistent. C++ is optimal for performance critical parts only. If you do your configuration in QML, you don't need to worry about any of that boilerplate code, as it is all automatic in QML, no need to write accessors, no need to recompile your project on every tiny change.