Overriding a protected pure virtual function with a signal
-
@Christian-Ehrlicher My base class calls a virtual function which is meant to be overridden by platform specific code. That code could be Qt, or objective c++, or whatever. In the case of Qt, rather than overriding the virtual function to then just emit another signal, I wanted to make that override the actual signal.
@mooglus said in Overriding a protected pure virtual function with a signal:
rather than overriding the virtual function to then just emit another signal, I wanted to make that override the actual signal.
You know what you're going to be suggested/told! Safer to leave it as a normal method, like the other code expects, and do whatever you want Qt-wise, like
emitting a signal, inside that method :) You may be fine with what you're doing, but for the sake of one extra function call it doesn't seem worth it.On a separate matter/bug-bear. You look like you're a reasonable programmer/Qt-er. So why are you still using the unhelpful, old-style
SIGNAL/SLOT()macros when you could & should be using the new style ones, which at least have compile-time support? :) -
@mooglus said in Overriding a protected pure virtual function with a signal:
rather than overriding the virtual function to then just emit another signal, I wanted to make that override the actual signal.
You know what you're going to be suggested/told! Safer to leave it as a normal method, like the other code expects, and do whatever you want Qt-wise, like
emitting a signal, inside that method :) You may be fine with what you're doing, but for the sake of one extra function call it doesn't seem worth it.On a separate matter/bug-bear. You look like you're a reasonable programmer/Qt-er. So why are you still using the unhelpful, old-style
SIGNAL/SLOT()macros when you could & should be using the new style ones, which at least have compile-time support? :)@JonB It's undoubtedly safer, but in larger classes these extra functions can add up to lots of boiler plate.
Well spotted, on the old-school SIGNAL/SLOT() :D We have lots of legacy code that hasn't been had these calls weeded out. I did notice as I was pasting the example, but thought it didn't matter too much for this example. -
@JonB It's undoubtedly safer, but in larger classes these extra functions can add up to lots of boiler plate.
Well spotted, on the old-school SIGNAL/SLOT() :D We have lots of legacy code that hasn't been had these calls weeded out. I did notice as I was pasting the example, but thought it didn't matter too much for this example.@mooglus said in Overriding a protected pure virtual function with a signal:
but in larger classes these extra functions can add up to lots of boiler plate.
Nah, I'm talking about a one-liner. You can even add it in-line in the
.hfile. If you're not prepared to add one line of code to an interface between two different products/systems you're in trouble. Even I would do it, and believe me I like minimal code! Totally my opinion/your choice of course. I'll leave others to answer your specific approach. -
Multiple inheritance with QObject could be problematic. When combining functionality with QObject classes I usually use composition rather than inheritance. I have run into issues with being able to copy objects based upon QObject. It also eliminates the use of templates for those classes. If I need a signal on an object that is not QObject based, and I can't or don't want to inherit from QObject. I will often create a subclass that gets created in the constructor and have it call methods on the parent object. I call this object a signal mule.
-
Multiple inheritance with QObject could be problematic. When combining functionality with QObject classes I usually use composition rather than inheritance. I have run into issues with being able to copy objects based upon QObject. It also eliminates the use of templates for those classes. If I need a signal on an object that is not QObject based, and I can't or don't want to inherit from QObject. I will often create a subclass that gets created in the constructor and have it call methods on the parent object. I call this object a signal mule.
-
@mooglus said in Overriding a protected pure virtual function with a signal:
Wouldn't you need to define an interface for your mule?
How would that be bad? If I need functionality I code that functionality.
@fcarney It just seems like more typing to provide functionality that inheritance provides. However, if you're in a situation where inheritance is causing you problems then it makes sense. I've read many times that it's good to favour composition over inheritance, in practice I often find it inconvenient. I would upvote you, but your reputation is 666, I could never ruin that ;)
-
@fcarney It just seems like more typing to provide functionality that inheritance provides. However, if you're in a situation where inheritance is causing you problems then it makes sense. I've read many times that it's good to favour composition over inheritance, in practice I often find it inconvenient. I would upvote you, but your reputation is 666, I could never ruin that ;)
-
@fcarney Interesting reply. I've not run into trouble (yet) with multiple inheritance and Qt. Wouldn't you need to define an interface for your mule? That sounds like a bit of a hassle.
@mooglus said in Overriding a protected pure virtual function with a signal:
I've not run into trouble (yet) with multiple inheritance and Qt.
If you do mean from
QObject(perhaps you don't) then that is surprising. Inheriting from more than oneQObject-derived class is problematic. There are various versions of Qt here, maybe things have changed over the years.https://stackoverflow.com/questions/8578657/qobject-multiple-inheritance/8578921
https://www.ics.com/blog/multiple-inheritance-qt
https://forum.qt.io/topic/88295/with-qt-5-6-is-it-possible-to-avoid-the-diamond-problem-when-signals-and-slots-are-defined-in-two-brancheshttps://doc.qt.io/qt-5/moc.html#multiple-inheritance-requires-qobject-to-be-first
If you are using multiple inheritance, moc assumes that the first inherited class is a subclass of QObject. Also, be sure that only the first inherited class is a QObject.
-
I wanted to inherit a templated class with a QObject. I ran into issues. Also multiple QObject inheritance doesn't work either.
@fcarney said in Overriding a protected pure virtual function with a signal:
I wanted to inherit a templated class with a QObject.
Works fine as long as you don't need moc stuff in the template
I ran into issues. Also multiple QObject inheritance doesn't work either.
Can not work to the parent-child relatonsship
Qt Online Installer direct download: https://download.qt.io/official_releases/online_installers/
Visit the Qt Academy at https://academy.qt.io/catalog -
@fcarney said in Overriding a protected pure virtual function with a signal:
I wanted to inherit a templated class with a QObject.
Works fine as long as you don't need moc stuff in the template
I ran into issues. Also multiple QObject inheritance doesn't work either.
Can not work to the parent-child relatonsship
@Christian-Ehrlicher said in Overriding a protected pure virtual function with a signal:
Can not work to the parent-child relatonsship
Nor
moc, more specifically it confuses the hell out of the meta-object system (e.gQMetaObject). -
@fcarney It just seems like more typing to provide functionality that inheritance provides. However, if you're in a situation where inheritance is causing you problems then it makes sense. I've read many times that it's good to favour composition over inheritance, in practice I often find it inconvenient. I would upvote you, but your reputation is 666, I could never ruin that ;)
