My first program in QT



  • Hi!
    I am using QT Creator 2.4.1 based on QT 4.7.4 on Ubuntu 10.04
    [1] I created a C++ file with a form.
    [2] I drew a button and a text label on the form.
    [3] I right-clicked the button and chose "Go to Slot".
    [4] At the "Select Signal" Dialogue I chose "clicked()" and "OK"
    [5] I the inserted the code snippet :
    void MainWindow::on_pushButton_clicked()
    {
    lineEdit->setText("Hello");
    }

    [6] I then chose "Build Project 'first' "
    [7] I received this error:
    /home/owner/qt/first-build-desktop-Desktop_Qt_4_8_1_for_GCC__Qt_SDK__Release/../first/mainwindow.cpp:-1: In member function 'void MainWindow::on_pushButton_clicked()':
    /home/owner/qt/first-build-desktop-Desktop_Qt_4_8_1_for_GCC__Qt_SDK__Release/../first/mainwindow.cpp:18: error: 'lineEdit' was not declared in this scope.

    [1] Shouldn't QT have taken care of the declaration itself?
    [2] If not, where would i have to declare lineEdit and would I also have to declare the pushbutton also?

    I'm used to C++ builder and Delphi where all that is taken care of automatically for such a simple program.

    QT is obviously a quality program and I know I will enjoy working with it. I am not a proficient programmer in C++ and this is my first foray into QT. The signals and slots are confusing. I don't know why the developer chose to go the confusing java route instead of continuing with properties methos, and functions as in C++ :( Proprietary maybe?

    Please help. Thanks,
    Paul
    I should add that I have tried googling for simple tuts also I tried to follow an example in the help but they were to complicated for a first forms program and had to much extraneous widgets and such.



  • Try replace
    @lineEdit->setText(“Hello”);@

    with
    @ui->lineEdit->setText(“Hello”);@



  • Whew! I got a bunch of errors that time! Here they are:
    @/home/owner/qt/first-build-desktop-Desktop_Qt_4_8_1_for_GCC__Qt_SDK__Release/../first/mainwindow.cpp:18: error: stray '\342' in program

    /home/owner/qt/first-build-desktop-Desktop_Qt_4_8_1_for_GCC__Qt_SDK__Release/../first/mainwindow.cpp:18: error: stray '\200' in program

    /home/owner/qt/first-build-desktop-Desktop_Qt_4_8_1_for_GCC__Qt_SDK__Release/../first/mainwindow.cpp:18: error: stray '\234' in program

    /home/owner/qt/first-build-desktop-Desktop_Qt_4_8_1_for_GCC__Qt_SDK__Release/../first/mainwindow.cpp:18: error: stray '\342' in program

    /home/owner/qt/first-build-desktop-Desktop_Qt_4_8_1_for_GCC__Qt_SDK__Release/../first/mainwindow.cpp:18: error: stray '\200' in program

    /home/owner/qt/first-build-desktop-Desktop_Qt_4_8_1_for_GCC__Qt_SDK__Release/../first/mainwindow.cpp:18: error: stray '\235' in program

    And last but not least:

    /home/owner/qt/first-build-desktop-Desktop_Qt_4_8_1_for_GCC__Qt_SDK__Release/../first/mainwindow.cpp:-1: In member function 'void MainWindow::on_pushButton_clicked()':
    /home/owner/qt/first-build-desktop-Desktop_Qt_4_8_1_for_GCC__Qt_SDK__Release/../first/mainwindow.cpp:18: error: 'Hello' was not declared in this scope@

    I think I know why you added the ui: to reference the form?



  • [quote author="p3aul" date="1337358650"]

    [1] Shouldn't QT have taken care of the declaration itself?
    [2] If not, where would i have to declare lineEdit and would I also have to declare the pushbutton also?
    [/quote]

    You can always try to understand the code using your C++ knowledge.


    For a Normal C++ application:

    Source files:

    • *.cpp
    • *.h
      can be compiled by your C++ compiler, then linked by your linker.

    While a Normal Qt application:

    Source files:

    • x.cpp

    • x.h

    • x.h [Contains Q_OBJECT] ==> moc_x.cpp (generated by moc)

    • x.ui ==> ui_x.h (generated by uic)

    • x.rcc ==> qrc_x.cpp (generated by rcc)

    Then all you get is normal *.cpp and .h files now.

    All the *.cpp and *.h files can be compiled by your C++ compiler, then linked by your linker.



  • If you are used to C++ builder and Delphi, you can easily solve such issues, do you?

    Perhaps you copid some code from network which contains invalid C/C++ characters.
    [quote author="p3aul" date="1337361431"]Whew! I got a bunch of errors that time! Here they are:
    @/home/owner/qt/first-build-desktop-Desktop_Qt_4_8_1_for_GCC__Qt_SDK__Release/../first/mainwindow.cpp:18: error: stray '\342' in program

    /home/owner/qt/first-build-desktop-Desktop_Qt_4_8_1_for_GCC__Qt_SDK__Release/../first/mainwindow.cpp:18: error: stray '\200' in program

    /home/owner/qt/first-build-desktop-Desktop_Qt_4_8_1_for_GCC__Qt_SDK__Release/../first/mainwindow.cpp:18: error: stray '\234' in program

    /home/owner/qt/first-build-desktop-Desktop_Qt_4_8_1_for_GCC__Qt_SDK__Release/../first/mainwindow.cpp:18: error: stray '\342' in program

    /home/owner/qt/first-build-desktop-Desktop_Qt_4_8_1_for_GCC__Qt_SDK__Release/../first/mainwindow.cpp:18: error: stray '\200' in program

    /home/owner/qt/first-build-desktop-Desktop_Qt_4_8_1_for_GCC__Qt_SDK__Release/../first/mainwindow.cpp:18: error: stray '\235' in program

    And last but not least:

    /home/owner/qt/first-build-desktop-Desktop_Qt_4_8_1_for_GCC__Qt_SDK__Release/../first/mainwindow.cpp:-1: In member function 'void MainWindow::on_pushButton_clicked()':
    /home/owner/qt/first-build-desktop-Desktop_Qt_4_8_1_for_GCC__Qt_SDK__Release/../first/mainwindow.cpp:18: error: 'Hello' was not declared in this scope@

    I think I know why you added the ui: to reference the form?[/quote]



  • I just don't understand all this. My C++ builder coding was years ago. but I could do a simple app like this in under a minute. All I had to do was double-click the button, write that code snippet between the {} symbols and choose build project. compile and the choose run. It was that simple.
    This is as simple as it gets. one label one pushbutton. why can't I set the property with lineEdit->setText("Hello")? is this even the right syntax?



  • [quote author="p3aul" date="1337363837"]
    why can't I set the property with lineEdit->setText("Hello")? is this even the right syntax?
    [/quote]

    As I said before, you can always try to understand the code using your C++ knowledge as this is normal C++ code and there is no magic.

    Whether you can use "lineEdit" depends in which way you defined the variable.

    • Is a global variable
    • Is a class member variable
    • Is a member variable inherited from the parent class.
    • Is a local variable
    • ...

    If you find it's hard to understand, maybe a C++ book will be useful for you.

    Debao



  • From what you said, I guess you know the answer, but for your own reasons don't wish to reveal it. I see by observing parts of code in more complicated examples that my syntax is correct. Unfortunately, QT is not standard C++. If it was there wouldn't be this foolishness of Slots and Signals instead they would use Methods and Events as in standard C++. . I'm either going to have to get some answers either by waiting and hoping someone else will pitch in and help me or by creating another thread. I asked a simple question. Why can't I get a simple, complete answer. Is knowledge of QT hoarded and only given out bit by bit? I have a book by the way. It's just not a book on QT
    Thanks,
    Paul



  • Hello.
    The first, Qt instead of QT.
    The second, I guess you should read something about Qt. It's not the same as C++ builder.
    About your last problems: could you, please, post all code if mainwindow.cpp?



  • [quote author="p3aul" date="1337383959"]From what you said, I guess you know the answer, but for your own reasons don't wish to reveal it. I see by observing parts of code in more complicated examples that my syntax is correct.
    Paul[/quote]
    Sorry.

    And Sorry for my poor English.

    As C++ builder is a IDE of C++, and you said that you used to C++ builder, so I wrongly thought that you will be very familiar with C++, but seems you not.

    Qt is a library of C++, it isn't a good idea to deep into the Qt world before familiar with C++. Perhaps “C++ Primer” is a good book to start with.

    bq. Unfortunately, QT is not standard C++. If it was there wouldn’t be this foolishness of Slots and Signals instead they would use Methods and Events as in standard C++.

    It depends.

    All the “non-standard” sources will be pre-processed by moc, uic, rcc, and then all you get are C++ “standard” files.

    No matter what happened, your Qt application will be compiled by a “standard” C++ compiler such as GCC, MSVC, Intel-C++, Clang, ..., even C++ builder.



  • Main Window .cpp
    @#include "mainwindow.h"
    #include "ui_mainwindow.h"

    MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
    {
    ui->setupUi(this);
    }

    MainWindow::~MainWindow()
    {
    delete ui;
    }

    void MainWindow::on_pushButton_clicked()
    {
    lineEdit->setText("Hello");
    }
    @

    Errors:
    /home/owner/qt/newattempt-build-desktop-Desktop_Qt_4_8_1_for_GCC__Qt_SDK__Release/../newattempt/mainwindow.cpp:-1: In member function 'void MainWindow::on_pushButton_clicked()':
    /home/owner/qt/newattempt-build-desktop-Desktop_Qt_4_8_1_for_GCC__Qt_SDK__Release/../newattempt/mainwindow.cpp:18: error: 'lineEdit' was not declared in this scope

    This error about lineEdit seems to indicate that I need to declare it somewhere in mainwindow.cpp I don't know if this just the tip of the iceberg though. Once that is corrected others may show up. :(



  • Well, the simpliest solution is to declare a signal like sendText (QString) and connecting it with slot of your QLineEdit like setText, and then emit it in your on_pushButton_clicked () slot.



  • Robot Herder: I have an excellent book that came with C++ builder version 3. The syntax is correct. In Builder I didn't need to add declarations of Components(Widgets) because Builder added them automatically.



  • [quote author="Wilk" date="1337390529"]Well, the simpliest solution is to declare a signal like sendText (QString) and connecting it with slot of your QLineEdit like setText, and then emit it in your on_pushButton_clicked () slot.[/quote]
    It's an answer, but not the simpliest ;-) .

    The right answer have been given by VanDerSam. And I try to given reasons, but failed ;-(



  • [quote author="p3aul" date="1337390576"]Robot Herder: I have an excellent book that came with C++ builder version 3. The syntax is correct. In Builder I didn't need to add declarations of Components(Widgets) because Builder added them automatically.[/quote]

    If I am right, you never try to see the code behind the drag and drop operation when using C++ builder.



  • Wilk:
    @Well, the simpliest solution is to declare a signal like sendText (QString) and connecting it with slot of your QLineEdit like setText, and then emit it in your on_pushButton_clicked () slot.@
    That's a perfect example of what I'm talking about. What you wrote is NOT C++ it's Qt! I need a book on Qt, not C++!

    If that's the simplest solution than I need to go back to windows and CodeGear or Visual C++



  • The fact is that VanDerSam's solution makes your program dependent on UI and actual names of it's components, while you can change UI itself and/or it elements and/or names of UI elements. While my solution makes your application dependent only on interface of class.



  • Robot Hearder:
    @If I am right, you never try to see the code behind the drag and drop operation when using C++ builder.@

    Well yes, except for the code snippets you write your self. Let's face it, I'm no purist! I just want to get the job done, and Qt, like C++ builder is a RAD tool. Otherwise you would all just use a text editor and code it all for the fun of it.



  • [quote author="p3aul" date="1337391158"]Wilk:
    @Well, the simpliest solution is to declare a signal like sendText (QString) and connecting it with slot of your QLineEdit like setText, and then emit it in your on_pushButton_clicked () slot.@
    That's a perfect example of what I'm talking about. What you wrote is NOT C++ it's Qt! I need a book on Qt, not C++!

    If that's the simplest solution than I need to go back to windows and CodeGear or Visual C++ [/quote]

    You're free do do what you want, but Qt is better at least because it's cross platform. If you don't want to learn something new it totally your problem.
    P.S. Use "quote" function instead of code blocks highlighting for quotes.



  • ;-) Seems you don't know which problem encountered and complained by p3aul.

    What he need is a C++ book, no matter which GUI library he want to use.

    • Qt
    • MFC
    • wxWidgets
    • ...

    And which C++ IDE he want to selected

    • Visual Studio
    • C++ builder
    • Ellipse
    • Qt Creator
    • ...

    And which C++ compiler he want to choose

    • cl.exe (MSVC)
    • g++.exe
    • clang.exe
    • ....

    [quote author="Wilk" date="1337391395"]The fact is that VanDerSam's solution makes your program dependent on UI and actual names of it's components, while you can change UI itself and/or it elements and/or names of UI elements. While my solution makes your application dependent only on interface of class.[/quote]



  • I'm using the code format rather than the quote format because that's the only that I can figure out hon this forum. I guess I need a book on Qt forums also? Every one insists that i don't know C++ while I have proven that my syntax is correct. It's just a dead horse you're trying to beat. I know enough to know that I'll will never get an answer here because you are all die-hard loyalists to Qt who can't even accept its deficiencies. Well i guess this post will get me thrown off the forum, but it's just as well. Qt being what it is, i guess ya'll just love java!

    Robot Hearder:
    @If I am right, you never try to see the code behind the drag and drop operation when using C++ builder.@

    Well yes, except for the code snippets you write your self. Let's face it, I'm no purist! I just want to get the job done, and Qt, like C++ builder is a RAD tool. Otherwise you would all just use a text editor and code it all for the fun of it.

    @The right answer have been given by VanDerSam. And I try to given reasons, but failed ;-(@

    VanDerSam suggested that I use:

    ui->lineEdit->setText(“Hello”); and I tried it and came up with about 13 errors. I know I copied and pasted it to replace the code I had written so I know that I used his syntax. Now what is it about that line that makes it the best solution?
    Thanks,
    Paul



  • @1+1=2
    If p3aul needs some book about C++ why than he started a thread here?
    @p3aul
    You've mentioned that events are from standard C++... AFAIK they are not. Furthermore, if you think that signals and slots are foolishness, then you just don't understand the idea of them.
    And I didn't tell that you don't know C++.



  • Wilk:
    Mia culpa! you are right Events are a MS windows thing. C++ in the windows environment just makes use of them. Methods though are C++ (and Pascal/delphi which calls them functions) I guess I can sorta see why Qt would choose Slots and Signals I just wish they had chosen something a little more descriptive and a little less arcane! ;)

    Well it's not like I'm trying for a career as a professional programmer. I'm 70 years old and nobody would hire me anyway! But I used to Program! I learned on an NCR with 64K of memory. A lot back then!
    I guess I'll stick with web programming Since the scripts are easier and I can even use them on my Linux machine, I guess I'll stick with that.



  • In fact there is no arcane in signal/slots. They are used to make dynamic connections between objects. Also events are still used for processing of keyboard and mouse.



  • Then why did I have to use Signals and slots to interact with the mouse and push the button to get text into a textbox?



  • AFAIK you use signals not to interact with mouse, but to interact with button. Button has it's own method for processing mouse events. When you push button, it processes this event and emits signal.
    You have to use signals to get text into textbox because it's a more flexible way. As I've mentioned, you may totally change your UI, for example use text edit area instead of line edit. You may also use UI, created with desigber separate from your app, and then you will still be able to work with it just thanks to signal/slots.
    Furthermore, in you example you may also try
    @
    #include "mainwindow.h"
    #include "ui_mainwindow.h"

    MainWindow::MainWindow(QWidget *parent) :
        QMainWindow(parent),
        ui(new Ui::MainWindow)
    {
        ui->setupUi(this);
    }
     
    MainWindow::~MainWindow()
    {
        delete ui;
    }
     
    void MainWindow::on_pushButton_clicked()
    {
        QLineEdit *lineEdit = findChild <QLineEdit *> ();
        if (lineEdit) {
          lineEdit->setText("Hello");
        }
    }
    

    @
    In this code you ask, if main window has line edit element, and if it does, you set it's text.



  • It worked! A little more code than the program put in there, but it worked. Now, I ask; If that was the code needed to connect the button to the lineedit, why didn't Qt just put it there in the first place? Just to make me learn more about Qt and C++?



  • [quote author="p3aul" date="1337397384"]Now, I ask; If that was the code needed to connect the button to the lineedit, why didn't Qt just put it there in the first place? Just to make me learn more about Qt and C++?[/quote]

    It works, but it's really not a good way to do so.



  • I think it's so because Qt is a platform for building applications, it gives you some small independent blocks which you may use to build your own application. I have had the same problem as you have now. But then I've understood, that it's better not to use framework (Qt) classes directly, but create derived classes with behaviour you need.



  • [quote author="1+1=2" date="1337398002"][quote author="p3aul" date="1337397384"]Now, I ask; If that was the code needed to connect the button to the lineedit, why didn't Qt just put it there in the first place? Just to make me learn more about Qt and C++?[/quote]

    It works, but it's really not a good way to do so.
    [/quote]

    I guess better way is to find child by name. Even better is to find child and make connections in some method, and then call this method in constructor of main window. Am I wrong?



  • I don't know the reason behind finding the child anyway, but I like the idea of using a method anyway. Sounds more like I'm used to. I'm not going to write another "OpenOffice" type app. I write for my own pleasure and the pleasure I get from writing them. They are mostly simple, but in Delphi I have written front ends for databases.

    What IS the reason for doing the code the way you did?(with searching for the child of lineEdit? and why didn't simply using lineEdit->setText("Hello"); work? Aren't you calling a method called setText() to change the property "text" of the object lineEdit? And you used a pointer in there too. It seems like an awful lot of trouble just to change a property value.



  • [quote author="p3aul" date="1337401988"]I don't know the reason behind finding the child anyway, but I like the idea of using a method anyway. Sounds more like I'm used to. I'm not going to write another "OpenOffice" type app. I write for my own pleasure and the pleasure I get from writing them. They are mostly simple, but in Delphi I have written front ends for databases.

    What IS the reason for doing the code the way you did?(with searching for the child of lineEdit? and why didn't simply using lineEdit->setText("Hello"); work? Aren't you calling a method called setText() to change the property "text" of the object lineEdit? And you used a pointer in there too. It seems like an awful lot of trouble just to change a property value. [/quote]

    Yes, your are right, it really seems not good. and in more than 99% cases, Qt user need not to use this.

    http://qt-project.org/doc/qt-4.8/designer-using-a-ui-file.html



  • bq. Yes, your are right, it really seems not good. and in more than 99% cases, Qt user need not to use this.

    You said that Qt user would not use this. How would you code that simple example?
    thanks,
    Paul



  • It is difficult for me to explain this , if you still not familiar with some basic C++ knowledge, such as Class, Friendship, Inheritance, Polymorphism, Templates, ... ;-). C++ is very very complex, much more that java, C#, etc, but Qt doesn't require user is a C++ expert.

    [quote author="p3aul" date="1337443786"]
    You said that Qt user would not use this. How would you code that simple example?
    thanks,
    Paul
    [/quote]

    • First, you design a GUI Window through Qt Creator or Qt Designer, which called mainwindow.ui And of course, C++ compiler can not deal with such a "non-standard" file

    • Then one of the Qt Tools uic was called.
      @
      uic mainwindow.ui -o ui_mainwindow.h
      @
      which convert your your designed GUI Element to a standard C++ file. All the elements are in a new Class called Ui::MainWindow.

    • Then you need to use such a new Class in your code. There are more than 3 ways to do so and all of them are introduced in http://qt-project.org/doc/qt-4.8/designer-using-a-ui-file.html


    BTW, Let try to see what you are doing now.

    • You are using The Single Inheritance Approach(Using a Pointer Member Variable) mentioned in above link. Which called Aggregation as a pointer member in Qt Creator.(See Menus Tools/Options/Designer ..., and you can selected a different one)

    • So you Create a class called MainWindow, which has a class member call ui that is a pointer to instance of Ui::MainWindow.

    • Then all of the Elements you designed can be access through the ui member in your class MainWindow.
      @
      ui->
      @

    Note that, all Of them are standard C++ code, you can deep into the ui_mainwindow.h file if you have interested.


    OK, let's see what you are try to do.

    • You have a class MainWindow more or less like this

    @
    class MainWindow : public QMainWindow
    {
    Q_OBJECT
    public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();
    private:
    Ui::MainWindow *ui;
    };
    @

    • Then you want to use a variable call lineedit in one of its member functions. Of course it will fail.
      As it is not:
    • a global variable (doesn't defined outside all the functions and class)
    • a class member variable (doesn't defined in your Mainwindow class)
    • a member variable inherited from the parent class.(Not exists in QMainWindow)
    • a local variable (doesn't defined in you member function)

    ...

    Hope this will be helpful for you.

    Regards,
    Debao



  • @1+1=2
    As far as I understand, p3aul knows and understands C++. Furthermore, while Qt doesn't require user to be a C++ expert, you'd better be one.


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