Solved QListWidget::addItems() Data type question
-
Ah, now I think I got it!
I did not know the internal management of QListWidget. So, to repeat, a QListWidget always copies the list I passed to it and stores this copy into a new allocated RAM.
Now, to avoid another copy for the function itsself to work with, I pass the source list by reference. As this is a potential risk to my source list, the Qt's function guarantees not to harm my list, and this guarantee is done by declaring the function parameter as "const". But "const" in this sense does not mean to change my variable into a const (this is still not possible, I hope), but instead it is like a "function character" to tell the compiler to take a look at the function algorithm and to ensure that it will not manipulate the variable.I hope, I got this right. If not, tell me please :-)
"There are no hard rules, but there's something to keep in mind - stack allocations are a lot faster but there's limited amount of space on it (by default something around 1MB per thread on Windows). The heap on the other side is large (it's the whole RAM you have) but access to it is slow, because allocations on it go through the OS. So a rule of thumb is use stack as much as possible for small and local things - variables, parameters, small objects etc."
Ok, I will notice that and I think I'll have to study the internal management of containers. I didn't know that Qt internally allocated heap for my container items. So, what would happen if I create a general object list containing 5 QWidgets, for example in a QVector? The container itsself will be allocated on the stack, but the items inside are already allocated dynamically. Will Qt then internally re-allocate them dynamically after storing them into a list container?Greetings, Binary
-
I hope, I got this right. If not, tell me please :-)
Yeah, that's close enough. There's basically some data object somewhere in the RAM. When you refer to it via your original variable it is non-const i.e. you can write to it. When you pass it to the function via const reference it creates another "view" so to speak, of that same area in RAM, only this time this view (or variable, whatever you call it) is treated as read only (const).
Ok, I will notice that and I think I'll have to study the internal management of containers.
That's how most containers work. Basically the
sizeof(yourContainer)
does not change (it can't really, it's a static type size) and the storage for data it holds is dynamically allocated/reallocated/deleted as an implementation detail of fuctions likeappend
,push_back
,erase
,clear
etc.So, what would happen if I create a general object list containing 5 QWidgets, for example in a QVector?
You can't. To put an object into a QVector it needs to be copyable. QObjects are not copyable (they have deleted copy constructors and assignment operators). In most cases you store pointers to widgets in containers, not widgets.
The container itsself will be allocated on the stack, but the items inside are already allocated dynamically. Will Qt then internally re-allocate them dynamically after storing them into a list container?
Your container holds some memory on the stack. in that memory it has a size counter and a pointer to dynamically allocated area of memory. In that memory it stores whatever the type of your container is. When you put something in a container it reallocates the memory to fit the new data and then calls a copy constructor to put the new thing in. It does not modify the original in any way. In case of pointers to widgets it just copies those pointers, so you get two containers containing pointers to the same widget object. Kinda like:
QWidget* w1 = new QWidget(); QWidget* w2 = w1; //now they point to the same thing but there's still only one widget
Except you copy entire container of those pointers, not just one. In case of
QStringList
you copy strings like you copy pointers, by value, i.e. you get another container with a fresh copy of those string values. -
@Chris-Kawa Thank you really much for this detailed support. Now I think it is clear for me.
What do you think is the best way to manage multidimensional string-lists that have to be viewed in 3 seperate QListWidgets that depend on each other?
Example:
Main QListWidget (#1) has 3 items. When the user clicks on one of those items, the second QListWidget displays the associated items, let's say again 3 items. When the user clicks on one of those 3 "sub-items", again 3 more items should be displayed in the third QListWidget.Till now, I'm using complicated, multi-dimensional QLists like this:
A simple QStringList for level #1, a QList<QStringList> for level #2 and a QList<QList<QStringList>> for level #3.
I catch user selection using "currentRowChanged", start a complicated for-loop to walk through the levels and choose the associated QStringList, clear the QListWidget and add the new List.Is there an easier and more performant way to manage this? Would QVector be a better solution for that? Or is a completely different approach better?
-
@Binary91-0 To avoid linear search you can use a QMap<QString, QstringList> for second and third list widgets. Key is the string selected in the previous list widget, value is the list of strings asotiated with that key.
-
@jsulm is probably the quickest/simplest suggestion to suffice you. However, being purist, be aware that as it stands it "cuts a corner": if you can have the same string text at different levels of the tree with different children (i.e. the fact that they are the same string is a "coincidence"), it will not work without some further massaging.
You could just invent a proper data structure to hold you data correctly. Create a
class
/struct
, namedNode
, which holds aQString text
for its string plus aQList<Node>
for its children. That is what your data really is. Finding a node by text at a given level is a linear search through its parent's child list (only).I would start by looking at whatever data structure you currently have for your items and their sub-items and take it from there.
-
@jsulm This sounds great! I did not test it so far, but after reading the documentation part, it seems to exactly fit my intentions.
@JonB Good aspect. Fortunatelly, no literally identical values will appear in the lists. The node system is very interessting. I think, for this purpose with only displaying the next level list-widget, maybe the node system is not needed, but it sounds like a good way to automatically walk through complex lists.
In addition, I will maybe use a combination of both. What do you think about inheriting QString in a new class and adding a specifier like a pointer to the next level string-list to it. Then, on the one hand, I get my fast interaction by QMap and as a backup (e. g. for storage and loading procedures of the whole data) I will always have unique connections.How would you guys manage the local storage of such three-dimensional data structures? I know database management from PHP server-based, but I have never done such stuff with Qt in a local directory. Is it a possible and "easy to learn" method for this purpose or do you suggest any other storage method?
-
@Binary91-0 said in QListWidget::addItems() Data type question:
What do you think about inheriting QString in a new class and adding a specifier like a pointer to the next level string-list to it.
No! Your structure, with a string text plus children, is not itself a
QString
, and does not behave anything like one. So inheritance here would be wrong, use encapsulation (memberQString
plus children, as I wrote). -
Ok, I understand.
But if I am right, there is still no direct way to get a connection from a selected list item to the node, right? For example, I create a complex node hirarchy and pass the string-values of those nodes to the QListWidget, including duplicates. How is it possible now to directly associate a selected list item with the corresponding node? Do I simultaniously have to mirror the list structure in my nodes and catch the "currentRow" signal to walk through my node structure till I found the corresponding item?
If this was the way I need to go, I'm asking myself how to easily react to changes in the list widget, e.g. changing the sort-mode from ascending alphabetically to descending alphabetically. Do I have to start a complex sort mechanism that walks through my nodes and sorts them by a given identifier or stuff like that?I mean, the perfect way would be to directly inherit from a list widget item or from QString to "infiltrate" the list so I can directly react to user selection with the corresponding node.
Or am I missing the point?
-
One possible answer is that a list widget is not the best tool for this task. QListWidget is sort of a entry level widget if you don't need anything fancy. Usually when whatever customization needs to be done going for a lower level solution turns out to be far more suitable. You could implement an item model with a tree structure of nodes and then have 3 QListViews that point to different levels of the tree. On selection change you would simply get the selected row index and switch the views to point to the right tree node. I discourage you from using strings as indices. It's slow and gets complicated with non-unique strings. It's better to just deal with QModelIndexes. Handling sorting and filtering in this case could be done via QSortFilterProxyModel, which would handle the translation of indices from what's in the view to what's in the model. See Model/View Programming for details and specifically Proxy Models section for handling filters and sorting.
-
Ok, sorry for the belated reply, but it took me some hours to carefully read the whole topic to Model/View Programming. Thank you for that hint, I think I know understood the basic principle of this concept!
I'm trying to solve my issue with a tree model now, what gives me most flexibility. For this, I'm coding an example as described in the link you posted (Simple Tree Model Example).
Here, I have a problem understanding what we already discussed eralier: The syntax of a function call with a const reference of X within this function call:int TreeItem::row() const { if (m_parentItem) return m_parentItem->m_childItems.indexOf(const_cast<TreeItem*>(this)); return 0; }
What is that "const_cast" good for? My google research told me that, in a const function like the above, the "this"-pointer is a const pointer to a CONST object. Instead, in a non-const function, the "this"-pointer was a const pointer to a NON-CONST object. Am I right?
Why do I have to use this const_cast now to call a const function? I mean, the "indexOf"-function is const, so why shouldn't it be possible to pass a const object?Sorry, but I really try to understand that stuff..
-
@Binary91-0 said in QListWidget::addItems() Data type question:
I mean, the "indexOf"-function is const, so why shouldn't it be possible to pass a const object?
Because m_childItems contains TreeItem pointers, not const TreeItem pointers.
-
@Christian-Ehrlicher So, it is not possible to pass either TYPE &var or const TYPE &var to a function that declares its argument itsself as const? What is the sense of that?
-
@Binary91-0 said in QListWidget::addItems() Data type question:
that declares its argument itsself as const?
I don't understand - m_childItems is of type QList<TreeItem*> - so indexOf() expects a TreeItem pointer, not a const TreeItem pointer. Therefore you have to cast the const this pointer to a non-const pointer. An automatic conversion from a const to a non-const value is not allowed (otherwise const would be completely useless when you think about it)
-
@Christian-Ehrlicher Ok, I know that I am wrong, but I try to explain what I do not understand:
The syntax of indexOf is:indexOf(const T &value) const
In my case, "T" is a TreeItem*, so the argument would be a "const TreeItem* &" what means a reference to a pointer of a const TreeItem, or not?
EDIT:
Ah, no. "T" is a Pointer to a TreeItem, and the indexOf()-function requires a const T &, so that means it requires a reference to a const pointer to a TreeItem, BUT NOT a const pointer to a CONST TreeItem, right?
And what does the const ensure in this function? Does it ensure that the pointer will not point to another QTreeItem after the function call or does it ensure that the QTreeItem will not be modified in any way? Or both? -
@Binary91-0 said in QListWidget::addItems() Data type question:
Does it ensure that the pointer will not point to another QTreeItem
The const means, as already explained from others above, that the value is not modified inside the function.
-
@Christian-Ehrlicher And the "value" in this example is the const pointer "this", right? So the only thing, the function guarantees is, that this pointer can't be changed, right? Does that mean, that the object it points to can be changed?
-
@Binary91-0 said in QListWidget::addItems() Data type question:
that the object it points to can be changed?
No, not in this case since the pointer itself is not const.
-
@Christian-Ehrlicher Oh, I don't know what's wrong actually but I simply do not understand it.
Look at this example that I found on my google research. It exactly explains this situation.
There is written, that a "this"-pointer is a const pointer, hence T* const this.
Now, in a const function, the object, this const pointer points to will also get const, hence const T* const thisSo why do you say that in this case, the pointer itsself was not const?
-
Sorry for double posting, but this may exceed the threads topic now.
I created an example application with a test class to try all possible combinations of pointers and classic variables in const member functions and non-const member functions.
class myQTest { private: int iPriv = 1, *iPPriv = nullptr; void fPriv(); public: int iPub = 2, *iPPub = nullptr; myQTest(); ~myQTest(); void fPub1(int); void fPub2(int) const; void fPub3(int*); void fPub4(int*) const; void fPub5(int *const); void fPub6(int *const) const; void fPub7(const int); void fPub8(const int) const; void fPub9(const int*); void fPub10(const int*) const; void fPub11(const int *const); void fPub12(const int *const) const; };
The first thing I realized is, that a "this"-pointer isn't const. So the explanation in the external link topic I mentioned above is either wrong or I missunderstood it.
The second thing I do not understand is, that I can write the following code without compiler error:
void myQTest::fPub3(int *const i) // The declaration was int*, NOT int const*! { }
How is it possible to define this function with a const pointer to int although it was declared as a non-const pointer to int?
-
@Binary91-0 I think it would be good to take a step back and go through it step by step, going back to your example:
int TreeItem::row() const { if (m_parentItem) return m_parentItem->m_childItems.indexOf(const_cast<TreeItem*>(this)); return 0; }
We're inside a const function, so
this
is of typeconst TreeItem*
, so pointer to constant TreeItem.m_childItems
is of typeQVector<TreeItem*>
, so vector'sT
in this case isTreeItem*
- non-const pointer to non-const TreeItem.QVector has a
int QVector::indexOf(const T & value, int from) const
method, so substitutingT
it is:int QVector::indexOf(TreeItem * const & value, int from) const
meaning it takes a reference to constant pointer to non-const TreeItem. The
const
on the function means it won't change any member of QVector. It has nothing to do with the parameter. And it makes sense, because finding an index of an item has no business modifying the container.Btw.
const
position in C++ syntax is a bit crazy so be careful not to fall into the trap. If you just naively replaceT
like this:const T&
->const TreeItem* &
then you've done it wrong. This would give you a reference to non-const pointer to const TreeItem, which is not what is happening here. Look up the "east const vs west const" to have a peek at the war that is going on in the language lawyers world :)Ok, so we have a
this
pointer which is a non-const pointer to const TreeItem and we have a function that expects a reference to const pointer to non const TreeItem. We have two mismatches here - first is on the type of the pointer and the second is on the item. The first one is easy because, as we previously discussed ,const can be added implicitly. The second one needs to take const off the item, and, as we also discussed, this can't be done automatically, so aconst_cast
is required to take the const off.So to summarize:
this -> const TreeItem* const_cast<TreeItem*>(this) -> TreeItem* indexOf(const_cast<TreeItem*>(this)) -> TreeItem* const &
that const in last type evaluation says that the function promises not to change the value of TreeItem*, so the pointer. It could, if it wanted, change the value of TreeItem or call a non-const members on it, but this is a generic container and it won't even know what T is, so it doesn't know T is a pointer and so it won't try to dereference that pointer.