# Qt problems

**URL:** <https://www.ruby-forum.com/t/qt-problems/170162>\
**Category:** Ruby\
**Created:** [June 13, 2009, 2:02am UTC](https://www.ruby-forum.com/t/qt-problems/170162 "2009-06-13T02:02:17Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![ther](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/977dab/32.png) [@ther](https://www.ruby-forum.com/u/ther)\
**Post date:** [June 13, 2009, 2:02am UTC](https://www.ruby-forum.com/t/qt-problems/170162/1 "2009-06-13T02:02:17Z")

</div>

require ‘Qt4’  
class MainWindow \< Qt::MainWindow  
def initialize  
super  
self.window\_title = ‘Hello QtRuby v1.0’  
resize(200, 100)

```
button = Qt::PushButton.new('Quit') do
  connect(SIGNAL :clicked) { Qt::Application.instance.quit }
end

label = Qt::Label.new(tr '<big>Hello Qt in the Ruby way!</big>')

layout1 = Qt::VBoxLayout.new do
  add_widget(label, 0, Qt::AlignCenter)
  add_widget(button, 0, Qt::AlignRight)
end
central_widget = layout1

#create_menubar
@statusbar = Qt::StatusBar.new(self)
@statusbar.setObjectName('statusbar')
set_status_bar(@statusbar)
@statusbar.show_message tr("Some text");

```

end  
end  
if $0 == **FILE**  
a = Qt::Application.new(ARGV)  
w = MainWindow.new  
w.show  
a.exec  
end

I am learning qt4 with ruby and have got to a problem that is driving me  
crazy. Acording to what I was able to find on Internet

```
central_widget = layout1

```

should paint a label and a box in the window. But it doesn’t. It doesn’t  
show anything.

Original c++ documentation points to setCentralWidget( layout1 ) method  
but it doesn’t exist in ruby Qt4.

It looks like that Qt with ruby is not used a lot. Can anybody confirm  
should this work at all or is it a bug.

by  
TheR

---

<div class="post-metadata">

**Author:** ![ther](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/977dab/32.png) [@ther](https://www.ruby-forum.com/u/ther)\
**Post date:** [June 13, 2009, 9:43am UTC](https://www.ruby-forum.com/t/qt-problems/170162/2 "2009-06-13T09:43:59Z")

</div>

On Saturday 13 June 2009, Damjan R. wrote:

> |  
> | @statusbar.setObjectName(‘statusbar’)  
> |  
> |I am learning qt4 with ruby and have got to a problem that is driving me  
> |crazy. Acording to what I was able to find on Internet  
> |  
> | central\_widget = layout1  
> |  
> |should paint a label and a box in the window. But it doesn’t. It doesn’t  
> |show anything.

I don’t know where you found this, but it’s clearly incorrect. The C++  
method  
setCentralWidget (which in ruby can be called with the three names  
setCentralWidget, set\_central\_widget and central\_widget=) takes a widget  
as  
argument. layout1 is a Qt::Layout, not a Qt::Widget, so you can’t pass  
it to  
setCentralWidget.

> |  
> |Original c++ documentation points to setCentralWidget( layout1 ) method  
> |but it doesn’t exist in ruby Qt4.

As I explained above, the setCentralWidget does exist, but it requires a  
Qt::Widget as argument. Here you’ve come across one of the peculiarities  
of  
QtRuby: most ruby libraries either don’t check methods arguments types  
or  
raise ArgumentError or TypeError if one argument is of the wrong type.  
QtRuby,  
instead, checks the arguments, but raises NoMethodError if the argument  
is of  
the wrong type. Since you’re calling setCentralWidget with an argument  
of the  
wrong type (Qt::Layout instead of Qt::Widget) you get a NoMethodError.

The reason why you don’t get the error when calling central\_widget= is  
that  
here you’re not actually calling the method, but creating a local  
variable  
called central\_widget. This isn’t a qtruby issue, but a quirk of the  
ruby  
syntax: whenever ruby sees something which can be a local variable  
followed by  
an equal sign, it interprets it as a local variable assignement. This  
means  
that to have ruby understand you’re calling the central\_widget= method  
of the  
main window, you have to call it using the dot notation:

self.central\_widget = widget

(this is an issue only when calling a method ending in = on self: if  
you’re  
calling it on another object, you naturally do it using the dot  
notation, so  
there can be no ambiguity).

Here’s a working version of your code:

require ‘Qt4’  
class MainWindow \< Qt::MainWindow  
def initialize  
super  
self.window\_title = ‘Hello QtRuby v1.0’  
resize(200, 100)  
cw = Qt::Widget.new self  
self.central\_widget = cw

```
button = Qt::PushButton.new('Quit', cw) do
  connect(SIGNAL :clicked) { Qt::Application.instance.quit }
end

label = Qt::Label.new(Qt::Object.tr('<big>Hello Qt in the Ruby

```

way!’), cw)

```
cw.layout = Qt::VBoxLayout.new cw do
  add_widget(label, 0, Qt::AlignCenter)
  add_widget(button, 0, Qt::AlignRight)
end

status_bar.show_message Qt::Object.tr('Some text')

```

end  
end  
if $0 == **FILE**  
a = Qt::Application.new(ARGV)  
w = MainWindow.new  
w.show  
a.exec  
end

I made several changes:

- replaced calls to tr with calls to Qt::Object.tr. In C++, QObject::tr  
is a  
static method, which means it becomes a class method in ruby. But class  
methods can’t be called directly from instances of the class, so you  
need to  
explicitly add the receiver (Qt::Object)
- removed most of the code related to status bar initialization: calling  
Qt::MainWindow#status\_bar when no status bar has been created  
automatically  
creats one, so there’s no need to do it manually (of course, if you  
wanted  
your status bar to be of a custom class, you’d need to create it  
manually)
- changed the code related to the central widget. Here, you’ll need to  
understand how Qt::MainWindow exactly works. It already has a layout (if  
you  
tried to change its layout using self.layout=Qt::VBoxLayout.new you’d  
get a  
warning telling you this). This layout makes one widget, called the  
central  
widget, fill all the window (except for the space reserved for the menu  
bar,  
the status bar and the toolbars). All the widgets you want to appear in  
the  
main window should actually go in this central widget. You could create  
a  
separate class for it or you can simply use a simple Qt::Widget, as I  
did.  
After creating the central widget (I assigned it to the local variable  
cw only  
to avoid typing central\_widget every time), we tell the main window to  
use it  
as central widget, using the central\_widget= method (note that we have  
to call  
it using self.central\_widget=, for the reason I explained above). After  
that,  
we create the button and the label (note that they’re children of the  
central  
widget now, not of the main window itself). Finally, we create the  
layout, set  
it as layout for the central widget and add the button and the label to  
it.

> |It looks like that Qt with ruby is not used a lot.

I’m not so sure about this.

I hope this helps

Stefano

---

<div class="post-metadata">

**Author:** ![ther](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/977dab/32.png) [@ther](https://www.ruby-forum.com/u/ther)\
**Post date:** [June 13, 2009, 9:03pm UTC](https://www.ruby-forum.com/t/qt-problems/170162/3 "2009-06-13T21:03:45Z")

</div>

Thanks man. You are a saviour. I was following what could be found here  
[http://techbase.kde.org/Development/Languages/Ruby](http://techbase.kde.org/Development/Languages/Ruby). And it seems a  
little rusty.

Can you tell any good Ruby Qt tutorial on the web?

by  
TheR

---

<div class="post-metadata">

**Author:** ![ther](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/977dab/32.png) [@ther](https://www.ruby-forum.com/u/ther)\
**Post date:** [June 14, 2009, 9:26am UTC](https://www.ruby-forum.com/t/qt-problems/170162/4 "2009-06-14T09:26:21Z")

</div>

On Saturday 13 June 2009, Damjan R. wrote:

> |Thanks man. You are a saviour. I was following what could be found here  
> |http://techbase.kde.org/Development/Languages/Ruby. And it seems a  
> |little rusty.  
> |  
> |Can you tell any good Ruby Qt tutorial on the web?  
> |  
> |  
> |by  
> |TheR  
> Have you looked at the tutorial which comes with qtruby itself? It’s  
> located  
> in the ruby/qtruby/examples/tutorial directory of the source  
> distribution and  
> corresponds to a very detailed tutorial which was shipped with qt until  
> version 4.5.

Stefano