[SOLVED]setupUi and using parent of an object

[qtlover]

Hi everybody;
Im rather new to Qt.I have 2 problems and 2 related questions.Here are they :

1. I created a Qt gui project,which includes a form. (I may remove form but i want to use it). I hand-code some controls,which include layout.Since I have a MainWindow (derived from QMainWindow) i have to use another widget (right?) on top of MainWindow for layout purposes.Up to here no problem.Now if I try this code

@ MainWindow::MainWindow(QWidget *parent) : QMainWindow(parent),
ui(new Ui::MainWindow)
{
QWidget *central=new QWidget;
this->setCentralWidget(central);
ui->setupUi(central);
.... @

I have this error : mainwindow.cpp:15: error: invalid conversion from 'QWidget*' to 'QMainWindow*'

Or, if I use this code

@ MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
QWidget *central=new QWidget;
this->setCentralWidget(central);
ui->setupUi(this);
....... @

program compiles and crashes at run-time.
My question is : How to solve this problem?Isnt it possible to use qtdesigner in cooepration with hand-coded controls and layout?

2)I have a MainWindow class derived from QMainWindow.I create one of this in main() function and use it as the main window of the application.
And i have a MyLabel class derived from QLabel which I use in MainWindow.And I pass MainWindow instance as parent to MyLabel constructor.
Now,in a member function of MyLabel I try to reach statusBar of MainWindow,which is now the parent.If Im not making a stupid error , I suppose that a code like this will work:

@ parent->statusBar()->showMessage("hello qt"); @

Errors : invalid use of member (did you forget the '&' ?)
error: base operand of '->' is not a pointer
What is wrongI thought I could emit a signal and consume it by mainwindow to update the statusbar.But is it possible to do this without emitting a signal,and using the parent?By the way, any other approach is appreciated.

Thanks

[Jake007]

Hi!

For your first problem.
First setupUi. Then add widgets and modify everything related to GUI.

@ MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)

{
QWidget *central=new QWidget; // It's OK to consturct and initialize other variables before setting up the ui
ui->setupUi(this); // Before setting central widget
this->setCentralWidget(central);
@

And for your second problem, if I remember correctly, unless it's in a constructor, parent is a method, so you need to do:
@ parent()->statusBar()->showMessage("hello qt"); // Notice: "parent()"->@

And if you want to use it as a QMainWindow, you just cast it:
@ ((QMainWindow *)parent())->statusBar()->showMessage("hello qt");@

Regards,
Jake

[qtlover]

Hi Jake,thanks for the quick reply,

I have further questions upon your clarifications,

For Q1 ) According to your code,we setup the ui "upon" this (which is a MainWindow instance) right?So what is the relation of central with ui ? Shall I create new widgets with parent as "this" or "central" ? For instance,if I add layout (assume that i didnt add layout in designer and want to add in code) shall I create it with parent = this or parent = central?Plus,whose setLayout method shall I call?

For Q2 ) Im not home right now,I will try both methods you proposed.

[Jake007]

Central is on of the widgets shown in main window. If you create new widgets and you'll add them to main window, pass main window ( this in your case).

And for setLayout(), it depends what you want. You probably want to call it on main window ( this). If you want widgets added to layout to be shown only in particular widget, you then call it on that widget, or you can use nested layouts.

Note: setCentralWidget() will automatically set given widget to center and resize it when window size changes.

Note: If you plan to use central widget as basis for adding widgets, don't do it. Because QMainWindow already has one for it's base :) .

I hope my explanation makes any sense and answers your question.

Regards,
Jake

[qtlover]

Hi Jake,
Thanks for your last reply.Now its clear that I have to setup ui first.

I still have confusion.

First,you said :

• And for setLayout(), it depends what you want. You probably want to call it on main window ( this). If you want widgets added to layout to be shown only in particular widget, you then call it on that widget, or you can use nested layouts.
  I dont use that central widget for that purpose only.Because as I know (correct me if I m wrong) QMAinWindow already has a layout of itself so we can not setLayout() directly on it.We have to employ an extra widget for that task.*

And you said :

• Note: If you plan to use central widget as basis for adding widgets, don’t do it. Because QMainWindow already has one for it’s base :) *
  I didnt understand this.QMainWindow already has what?

Also I wonder if ui->setupUi() sets up a layout or plays a role of laying out everything else,although I do NOT explicitly add a layout by code or designer?Or, setupUi() is just a connection bridge between the code and designer form?

[Jake007]

Hii!

Yes it has a layout only if your window has from( yes, ui->setupUi() sets up a form). In that case layout will automatically be added.
So use it on main window if you don't have a form file, otherwise, I'd recommend building as much GUI as possible from designer.

And for second:
I meant QMainWindow already has it's own QWidget, as it derives from it ( which actually makes it a one).

Also if you'll look in you build directory, you'll notice ui_[classname].h files. Those are automatically built based on you ui forms and inserted into your code.