Pro and cons about creating QObjects components on the stack instead of creating them dynamically.
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@kshegunov
You haven't commented on my sugegstion, which probably means you don't like it but are too kind to me to say so ;-)The question, as I read/think of it, is: Imagine I have a big function, with the
new QToolbarat the top and lots of (potential)returns dotted all over the place, with the finaladdToolbar()near the end. I do not want to manually put in my owndelete/deleteLater()s prior to thereturns, because I'll make a mistake.In that case, wouldn't my suggestion of a "temporary stack-based parent till the
addToolbar()call" be a simple way of ensuring anewedQToolbargets deleted on function scope exit be acceptable?@JonB I liked your answer too much. I forgot to say that in some cases I could have objects that couldn't have a parent like a toolbar or other qt starndard objects. In that case I can't give them a temporary parent.
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@Jimmy-Loyola said in Pro and cons about creating QObjects components on the stack instead of creating them dynamically.:
its a little bit unusual listening about that heap allocation is more secure than stack allocation for memory management. But if this is the qt philosophy we must follow it.
I didn't say that, in fact I argued that both are valid approaches. As you can leak or double-delete a heap object, so you can double delete a stack object, it's just a fact of life. In any case Qt allows you both gracefully, so it's up to your preference which you're going to use. Me personally, I'm a stack-lover, I always put things in the stack if I can help it. It usually looks something like this:
class Dummy : public QObject { public: using QObject::QObject; }; class DummyParent : public QObject { public: DummyParent(QObject * parent = nullptr) : QObject(parent), owned(this) { } Dummy owned; };This construct is a FILO (i.e. stack) by itself, so it works gloriously without an issue.
@kshegunov Doesn't Dummy class have a constructor?
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@Jimmy-Loyola said in Pro and cons about creating QObjects components on the stack instead of creating them dynamically.:
its a little bit unusual listening about that heap allocation is more secure than stack allocation for memory management. But if this is the qt philosophy we must follow it.
I didn't say that, in fact I argued that both are valid approaches. As you can leak or double-delete a heap object, so you can double delete a stack object, it's just a fact of life. In any case Qt allows you both gracefully, so it's up to your preference which you're going to use. Me personally, I'm a stack-lover, I always put things in the stack if I can help it. It usually looks something like this:
class Dummy : public QObject { public: using QObject::QObject; }; class DummyParent : public QObject { public: DummyParent(QObject * parent = nullptr) : QObject(parent), owned(this) { } Dummy owned; };This construct is a FILO (i.e. stack) by itself, so it works gloriously without an issue.
@kshegunov I'm a stack-lover too so I'am traying to learn how to manage qt stacked objects properly. Could you link some good documentation about this topic? I would be glad. I see even qt recommend use dynamic memory to allocate gui objects but its docs doesn't specify how to deal with potential problems of this approach in a detailed way.
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@kshegunov I'm a stack-lover too so I'am traying to learn how to manage qt stacked objects properly. Could you link some good documentation about this topic? I would be glad. I see even qt recommend use dynamic memory to allocate gui objects but its docs doesn't specify how to deal with potential problems of this approach in a detailed way.
@Jimmy-Loyola said in Pro and cons about creating QObjects components on the stack instead of creating them dynamically.:
I'm a stack-lover too so I'am traying to learn how to manage qt stacked objects properly. Could you link some good documentation about this topic?
Not currently, but I'll try to muster something this evening. In the mean time the major rule is - make sure the child is freed (by the stack) before the parent, this is enough. Also be wary of ownership transfer in Qt (it's noted in the docs).
I would be glad. I see even qt recommend use dynamic memory to allocate gui objects but its docs doesn't specify how to deal with potential problems of this approach in a detailed way.
I wouldn't go as far as to say it "recommends" it, but yes, it's more usual.
Read and abide by the Qt Code of Conduct
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@Jimmy-Loyola said in Pro and cons about creating QObjects components on the stack instead of creating them dynamically.:
I'm a stack-lover too so I'am traying to learn how to manage qt stacked objects properly. Could you link some good documentation about this topic?
Not currently, but I'll try to muster something this evening. In the mean time the major rule is - make sure the child is freed (by the stack) before the parent, this is enough. Also be wary of ownership transfer in Qt (it's noted in the docs).
I would be glad. I see even qt recommend use dynamic memory to allocate gui objects but its docs doesn't specify how to deal with potential problems of this approach in a detailed way.
I wouldn't go as far as to say it "recommends" it, but yes, it's more usual.
@kshegunov said in Pro and cons about creating QObjects components on the stack instead of creating them dynamically.:
make sure the child is freed (by the stack) before the parent
What do you thinkg about make sure the children are freed before their parents in the all application classes could require big efforts?
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@kshegunov said in Pro and cons about creating QObjects components on the stack instead of creating them dynamically.:
make sure the child is freed (by the stack) before the parent
What do you thinkg about make sure the children are freed before their parents in the all application classes could require big efforts?
@Jimmy-Loyola
@kshegunov is talking about "freed by the stack". That just means scope exit for local variables. So long as you add stack children after parent, not set parent on a child, this is the natural way function calls will arrange, is it not? -
@kshegunov said in Pro and cons about creating QObjects components on the stack instead of creating them dynamically.:
make sure the child is freed (by the stack) before the parent
What do you thinkg about make sure the children are freed before their parents in the all application classes could require big efforts?
@Jimmy-Loyola said in Pro and cons about creating QObjects components on the stack instead of creating them dynamically.:
What do you thinkg about make sure the children are freed before their parents in the all application classes could require big efforts?
Not really. If you stick to the "child is a member of parent object" (as I'd shown) you wouldn't have much trouble. Still caution is advised if objects are given to Qt for say to be inserted in a tab widget, because usually these containers take ownership of the object you pass them.
@JonB said in Pro and cons about creating QObjects components on the stack instead of creating them dynamically.:
@kshegunov is talking about "freed by the stack". That just means scope exit for local variables. So long as you add stack children after parent, not set parent on a child, this is the natural way function calls will arrange, is it not?
Quite correct, yes. So like this:
QObject parent(nullptr); // Root object, no parent QObject child1(&parent); QObject child2(&child1);By the natural way the stack is constructed you destroy them backwards, so you always destroy the children before the parent and you're perfectly fine. Compare with:
QObject child1(nullptr); // Why no parent if it's a child QObject parent(nullptr); // First to go out of scope, the bells should be ringing in your ears already child.setParent(&parent); // Oh, no, no, I didn't mean to break the stack when we exit ... -
@Jimmy-Loyola
@kshegunov is talking about "freed by the stack". That just means scope exit for local variables. So long as you add stack children after parent, not set parent on a child, this is the natural way function calls will arrange, is it not?@JonB Right, the last one thing, why should I set a parent to a stacked QObject if I don't want that qt manages memory for it?
I mean, if I never set a parent for stacked qobject the order of the objects creation doesn't matter because they aren't in the qt object tree. I am right or not? Thanks -
@JonB Right, the last one thing, why should I set a parent to a stacked QObject if I don't want that qt manages memory for it?
I mean, if I never set a parent for stacked qobject the order of the objects creation doesn't matter because they aren't in the qt object tree. I am right or not? Thanks@Jimmy-Loyola
Yes, that in itself is true, if you have stack variables with no parentage it won't matter about destruction order.But as per @kshegunov code, in order to use these objects your code may have to set up parentage (or call Qt functions which implicitly do) as it goes along, and then the order will matter.
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@JonB Right, the last one thing, why should I set a parent to a stacked QObject if I don't want that qt manages memory for it?
I mean, if I never set a parent for stacked qobject the order of the objects creation doesn't matter because they aren't in the qt object tree. I am right or not? Thanks -
@Jimmy-Loyola said in Pro and cons about creating QObjects components on the stack instead of creating them dynamically.:
I mean, if I never set a parent for stacked qobject the order of the objects creation doesn't matter because they aren't in the qt object tree. I am right or not? Thanks
If you were to really do that, then you are fine. However, there is a slightly non-obvious way of setting a parent: When you put a widget into a layout it is re-parented. (And so its lifetime is again managed by Qt.) Basically the best rule you can have is that in general widgets should live on the heap. There are very few exceptions for this: 1) The MainWindow created in
maincan live on the stack (it does not have a parent). 2) A modal dialog can usually be constructed on the stack.As mentioned before, all the other non-widget class, e.g.
QFile, can be managed without parents and thus are allowed to live on the stack. -
@Jimmy-Loyola said in Pro and cons about creating QObjects components on the stack instead of creating them dynamically.:
I mean, if I never set a parent for stacked qobject the order of the objects creation doesn't matter because they aren't in the qt object tree. I am right or not? Thanks
If you were to really do that, then you are fine. However, there is a slightly non-obvious way of setting a parent: When you put a widget into a layout it is re-parented. (And so its lifetime is again managed by Qt.) Basically the best rule you can have is that in general widgets should live on the heap. There are very few exceptions for this: 1) The MainWindow created in
maincan live on the stack (it does not have a parent). 2) A modal dialog can usually be constructed on the stack.As mentioned before, all the other non-widget class, e.g.
QFile, can be managed without parents and thus are allowed to live on the stack.@SimonSchroeder said in Pro and cons about creating QObjects components on the stack instead of creating them dynamically.:
Basically the best rule you can have is that in general widgets should live on the heap.
Why do you do this to me? I just spent 2 days and about 7 posts to show there's no such rule. If there's such a recommendation even, that is to avoid putting objects in the stack, in the Qt documentation please point me to it.
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@kshegunov said in Pro and cons about creating QObjects components on the stack instead of creating them dynamically.:
Why do you do this to me? I just spent 2 days and about 7 posts to show there's no such rule. If there's such a recommendation even, that is to avoid putting objects in the stack, in the Qt documentation please point me to it.
You are right that it is nowhere written in the Qt documentation. And this is only a rule of thumb. As you have showed it is possible to put them on the stack. However, in most cases you do not have widgets living inside the scope of a function. In many cases I construct widgets inside the constructor of another widget's class. (Right now, I don't even use member variables because the parent tree will take care of everything.) In this case I would need a bunch of member variables inside the class. These would need to be in the right order inside the class declaration already (because they are initialized in that order). I think it is hard to add just another widget in the right place in this list of members (and beware if you forget to think about it only once). BTW, I am not even entirely sure how the widget will be re-parented when added to a layout. Is it reparented to the layout or the widget that the layout belongs to?
In short: If you put your widgets on the heap and you are consistently using layouts, you don't have to think about anything at all. You don't even have to think about setting parents. If, on the other hand, you are putting your widgets on the stack you always have to take care of the order of variable declarations. You need to think about two things at the same time: The look of your UI and memory management/stack layout. Adding a new widget to the layout is harder because you need to find the right place to declare it. Moving a widget to another place in your layout could mean to move its declaration as well if you are using sublayouts.
To me, it is much harder to reason about widgets on the stack. And it certainly is not beginner-friendly. It might even become a maintenaince nightmare. I usually try to use approaches that have the least chance of accidental errors. That is why I usually put everything I can on the stack. Then I get free lifetime management. The Qt parent tree also manages lifetime. Having two competing lifetime managers is just too error prone. I don't want to fight Qt's lifetime manager, but rather be friends with it.
The only reason left to not put widgets on the heap would be speed.
newanddeleteare notoriously slow. Nevertheless, we are talking about the GUI here and thus only have to match human speed. I have rarely seen a performance problem with creating layouted widgets fully on the heap. So, as long as you don't have a performance problem don't optimize prematurely.Basically, what it comes down to why I am proposing allocation of widgets on the heap as a rule of thumb is 1) maintainability and 2) saving hours of debugging mysterious errors.
