[Solved] - Qt - GUI - Problem with Reference / Pointers

[yerbaguy]

Hallo,

I am learning C++ and trying to re-implement / add some code to the application that I gathered from the base of examples of Qt. It's about the Qt C++ GUI application. I have got a problem with sending by a reference a variable to a function ( actually to two functions - to which functions I created the buttons ). In the constructor I defined a variable named 'click' of the int type. Names of these functions are:

         forward();
            and 
          backwards();

I sent the click variable to these functions by reference, like:

    forward(&click);
    backwards(&click);

and the functions are:

   void MainWindow::forward()
  {
        (*click_forward)++;
  }
  void MainWindow::backwards()
  {
        (*click_backwards)--;
  }

In the header file, I defined these two functions as:

        forward();
        backwards();

I also defined the connections SIGNAL-SLOT, as:

     QObject::connect(nextbtn, &QPushButton::clicked, this, &MainWindow::forward());

     QObject::connect(beforebtn, &QPushButton::clicked, this, &MainWindow::backwards());

But when I compile the program, I receive an error like:

     no matching function for call to 'MainWindow::forward(int*);

     no matching function for call to 'MainWindow::backwards(int*);

When I put the parameters into the above functions:

         MainWindow::forward(int *click_forward)
         {

         }
       
        MainWindow::backwards(int *click_backwards)
        {

        }

   I received an error, like:

   /opt/Qt/5.4/gcc_64/include/QtCore/qglobal.h:694: błąd: invalid application of 'sizeof' to incomplete type 'QStaticAssertFailure<false>'
 enum {Q_STATIC_ASSERT_PRIVATE_JOIN(q_static_assert_result, __COUNTER__) = sizeof(QStaticAssertFailure<!!(Condition)>)}

     Could I please ask you to direct me, what I am doing wrong?

         Thank you,
                                                                                 ^

[mcosta]

Hi and welcome to devnet,

Qt is a C++ framework so the first requirement to learn how to use it is to have a good knowledge of the language.

The first concept is the declaration/definition of a method: in both case the signature of the function MUST be the same.

In you case you don't neet to pass arguments to your slots

// MainWindow.h
class MainWindow: public QWidget
{
Q_OBJECT // you need it to use signals/slots
public:
    MainWIndow(QWidget *parent=0);

public slots:
    void forward();
    void backward();

private:
    int _click;
};

// MainWindow.cpp

MainWindow::MainWindow(QWidget* parent): QWidget(parent)
{
.....
    connect(nextbtn, &QPushButton::clicked, this, &MainWindow::forward);
    connect(beforebtn, &QPushButton::clicked, this, &MainWindow::backward);
}

void MainWindow::backward()
{
    _click--;
}

void MainWindow::forward()
{
    _click++;
}

BTW, I suggest to learn a little bit more the language before to go forward with Qt

[koahnig]

Hi and welcome to devnet

You are doing a couple of things not correct.

My personal recommendation would be to start with C++ tutorial (e.g. this one, but there are many others) first. The GUI stuff and especially signals and slots make it more difficult. It will be a very frustrating experience when you continue to plow along those lines as you do at the moment.

void foo(int &ref)  // here we have a reference 
{
     ++ref;
}

void foo1(int *ptr) // here we have a ponter to an int
{
     *ptr += 1;   // increments by one 
}

void main ( int argc, char **argv) 
{
     int i;
     foo(i);  // calls by reference 

    foo1(&i);  // here the '&' does get the address/pointer 
}

Those are really the basics in a very tiny nutshell. Both functions do basically the same incrementing by one and also passing back the value. You should go through a tutorial and look especially for those differences in detail.

[yerbaguy]

@mcosta Thank you very much for your answer, I know that I have to learn C++ much more, but the thing is, that I would like to implement the GUI application - where users would have the manual ability to interact with the program. I understand that, frameworks themselves have got a lot of things which you have to deal with - I mean probably more that the language (C++) itself. May be you could suggest any other framework for C++ which I could use under Linux just to try to implement the code (GUI) and in the same time learn C++, or just to learn faster C++ and Qt? Once again, thank you.

[mcosta]

Hi,

in my opinion you should before learn more the language and after start with GUI frameworks (obviously I suggest Qt).

[yerbaguy]

@koahnig Thank you very much for your answer.