Qt widget stacking child layouts on top of each other - c++

Question

Qt has a layout system kind of separate from the widgets, which is causing me problems. I know that when I have a QSplitter, I can add stuff to it just by doing
new SomeCustomQWidget(splitter);
However, if I have a QWidget, shouldn't I be able to do something similar? The Qt web pages only explain how to add layoutItems to layouts, only connecting them to widgets by inheriting both in a subclass. So, right no I am doing something like this
item = new QWidget(parentsplitter); //this works
subitem = new customWidget(item); //this too
subitem2 = new QSlider(item); //this isn't laid out with subitem
However this results in the slider being on top of the subitem instead of being next to it (either right or below I'd expect). Calling
//either
new QVBoxLayout(item);
//or
item->setLayout(new QVBoxLayout());
//or
item->setLayout(new QVBoxLayout(item));
after initializing the item doesn't help. (My logic is that the item takes the layout as it's child and will add subsequent children to it).
I went around this problem earlier but now I'm facing the exact same problem when making a widget inside a QGraphicsView. I feel like it shouldn't be necessary to make a custom subclass of every item I could ever want to dynamically add to those widgets, like the Qt website seems to expect. (If I simply want to add some text and a couple buttons, do I really need to make 2 custom subclasses?) I am using a .ui file for basic layout but these items need to be created dynamically.
Solution
Forgot to call item->show(); on the widget used for layout.
Turns out, I had tested otherwise correct code, but since some of my subclasses called show() but others didn't, it just seemed broken to me.
This however isn't the issue with the graphicswidgets, as graphicslayout can only be used to add graphicslayoutitems, which don't include for example graphicstextitems.
Solution 2
Using the QGraphicsProxyWidget instead of QGraphicsWidget fixed the issue!

Answer 1

QWidget *parent = new QWidget();
QVBoxLayout *rootLayout = new QVBoxLayout(parent);
QHBoxLayout *subLayout1 = new QHBoxLayout();
QHBoxLayout *subLayout2 = new QHBoxLayout();
rootLayout->addLayout(subLayout1);
rootLayout->addLayout(subLayout2);
subLayout1->addWidget(new QLabel("Foo"));
subLayout1->addWidget(new QLabel("Bar"));
subLayout2->addWidget(new QPushButton("Foo button"));
subLayout2->addWidget(new QPushButton("Bar button"));
parent->show();
Note once layout is assigned to a widget by setLayout or when widger is passed to constructor fo layout, the are in 1 to 1 relationship which last until widget or layout is destroyed. See documentation:
If there already is a layout manager installed on this widget, QWidget won't let you install another. You must first delete the existing layout manager (returned by layout()) before you can call setLayout() with the new layout.
That is why subitem2 = new QSlider(item) didn't work for you.

Answer 2

I think you're confusing parent/child relationship with managing layout.
Lets start with layouts. When set on a widget the layout becomes a child of that widget. When constructed with a widget parent the widget becomes a parent and the layout is set on the widget. When adding widgets to layout they become children of the widget that layout is set on, not the layout itself. Setting a parent on a widget or constructing widget with a parent does not automatically add the widget to its parent layout. Deleting a layout does not delete widgets governed by it, because they are not its children.
To summarize - layouting and parenting are two distinct mechanisms. first is for governing position and size of a widget, the other is for object hierarchy and resource management (parents delete their children). Layouts have a convenience constructor though, that does both - sets a parent of the layout and sets that layout on the parent widget.
In code:
item = new QWidget()
layout = new QVBoxLayout(item);
is equivalent to:
item = new QWidget();
layout = new QVBoxLayout();
layout->setParent(item);
item->setLayout(layout);
This code adds a child to a layout and item becomes its parent:
item = new QWidget();
layout = new QVBoxLayout(item);
child = new QWidget();
layout->addWidget(child);
This adds a child to a parent item, but does not add child to a layout:
item = new QWidget();
layout = new QVBoxLayout(item);
child = new QWidget(item);
A QSplitter is special in that it is a widget and it does its own layout of children i.e. when you use a splitter as a parent of a widget that widget's geometry is governed by the splitter. Other widgets don't behave like that.
So to comment your code:
item = new QWidget(parentsplitter); //this works because splitter does layout
subitem = new customWidget(item); //item is not splitter so subitem geometry is not managed
subitem2 = new QSlider(item); //item is not splitter so subitem2 geometry is not managed
If you want to lay out subitems in the item then item either has to be a splitter too or have a layout that will manage children, so:
childsplitter = new QWidget(parentsplitter); //note that parentsplitter has only 1 item
subitem = new customWidget(childsplitter);
subitem2 = new customWidget(childsplitter);
or
widget = new QWidget(parentsplitter); //note that parentsplitter has only 1 item
lay = new QHBoxLayout(widget);
subItem = new customWidget();
subItem2 = new customWidget();
lay->addWidget(subItem);
lay->addWidget(subItem2);
If you want parent splitter to have two items then you do:
item = new customWidget(parentsplitter);
item2 = new customWidget(parentsplitter);