Qt examples and the usage of pointers
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I've been reading a few example Qt has and I noticed that they use a lot of pointers and don't delete them after, so I was wondering if it's a magic or it actually leaks, for instance:
The example
echoplugin, on the fileechowindow.cppat line118it takes the instance of a plugin and after that it's not deleted. The same happens with a lot of other things, likenew QLineEdit.What is happening on the background here?
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@Defohin
Actually it is documented here.
Quoting a statement from it:Once loaded, plugins remain in memory until all instances of QPluginLoader has been unloaded, or until the application terminates. You can attempt to unload a plugin using unload()...
And then what unload says:
This happens automatically on application termination, so you shouldn't normally need to call this function.
Don't try to delete the root component. Instead rely on that unload() will automatically delete it when needed.So in the end all the instances will be deleted on program termination.
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@Defohin AFAIK they are reparented. You see these widgets are added into the gridlayout and is then set as the main layout for
EchoWindow. So when EchoWindow is deleted it deletes the layout which eventually deletes these widgets. -
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@Defohin said in Qt examples and the usage of pointers:
@p3c0 How do I know when a pointer is going to leak or not in Qt?
You cannot. Just like any other pointer.
But you can Design your app so all Widgets
have a Parent and all parents are owned by
Main Window.
So just normal care is needed. -
@mrjj That is kinda tricky, I tried to use a few softwares to detect leaking but they say that even if my pointer has a parent like
new SomeClass(this)it's "leaking", it's probably false positive, but they say it's a leak, so I have no idea when it's an actual leak or not. -
Yes its tricky.
Tools like valgrind needs tweaking to reduce the noise.
If the tool is too simple , it dont understand the delete by parent and
when it see no delete for it, flag as leak.On the bright side, its not that easy to make a leak as not assigning a parent will make it a
window and embedding a widget into something will (almost) surely own it. ( like layouts) -
@mrjj said in Qt examples and the usage of pointers:
On the bright side, its not that easy to make a leak as not assigning a parent will make it a
window and embedding a widget into something will (almost) surely own it. ( like layouts)What you mean with that? Sorry for my ignorance.
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@Defohin
Hi
Well when I first started Qt I was also a bit sceptical about never
calling delete on my widgets. ( as in pure c++ its a leak)However, if you try to design an application and forget to assign parent, you will find it as it becomes a window.
QLabel *mine= new QLabel();
mine->show()Will popup as a window and its very clear something its up.
Also then you want to place it into some other widget, you
use layouts.
and as soon as you go
somewidget->layout()->addWidget(mine)
its owned by the layout.So its actually a bit hard to create a widget you dont want as window and create leak
without assigning a parent as it will clearly show :)That leaves mostly QDialogs and windows to leak but there you have
diag->setAttribute(Qt::WA_DeleteOnClose);
So if closed, they clean up. -
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@Defohin
Yes that could happen.
http://doc.qt.io/qt-5/objecttrees.html -
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@Defohin
well, you should also look into using smart pointers.
Thats how its managed in plain c++.
http://stackoverflow.com/questions/31750689/how-to-use-smart-pointer-for-auto-clean-up
https://en.wikipedia.org/wiki/Smart_pointerSmall note. Do not put smart pointers on Widgets with parents as it get ugly :)
Also Qt comes with Qt versions of those.
