Baaah. The following is driving me mad for quite some time now. First off, I'm not an experienced Qt/C++ programmer, so please forgive me any stupidity or the overlooking blatant obviousness.

Basically I'm building a Qt application which uses a mainwindow designed in Qt Designer. I have a lot of functions belonging to my MainWindow class which makes the class bloated and many of those functions could be reused by other classes. What I wanted is some sort of separate base class (called DataModel) which provides several methods for manipulating GUI elements (e.g. QTreeViewWidgets which need to be filled with data from a database) to be called by different classes.

The problem is I can't seem to find out how to access those GUI elements from a separate class. So this might mean that it's either bad practice and should be avoided or I haven't found the holy grail yet. Any ideas? [size=x-small]Or am I too vague...[/size]

This should be possible. As long as the other classes have access to the object, they should be able to do it, by passing that to the DataModel class. ( or make the DataModel class handle the Window, avoiding that )

Hope that helped you, I can probably explain it better if you post the sources.

I know that in theory you should pass the object, in practice the only that happens is that my application segfaults.

Below are my mainwindow.h, mainwindow.cpp, datamodel.h and datamodel.cpp. I've stripped out most of the unnecessary functions, declarations, etc. for the sake of clarity.

mainwindow.h

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#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include
#include "ui_mainwindow.h"

class MainWindow : public QMainWindow, public Ui::MainWindow
{
  Q_OBJECT

  public:
    MainWindow(QWidget* parent=0);
 
  protected:
    void getData(const QString& queryString, const QString& dataType);

  public:
    Ui::MainWindow ui;
};

#endif

mainwindow.cpp

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#include
#include
#include
#include
#include "mainwindow.h"
#include "datamodel.h"
#include "client.h"
#include "project.h"
#include "document.h"
#include "work.h"
#include "ratestaxes.h"
#include "statusbar.h"

MainWindow::MainWindow(QWidget *parent) : QMainWindow(parent)
{
  setupUi(this);

  QSqlDatabase database = QSqlDatabase::addDatabase("QMYSQL");
  database.setHostName("localhost");
  database.setDatabaseName("***");
  database.setUserName("***");
  database.setPassword("***");

  if (database.open()) {
    //Initialise all data for all QTreeWidgets
    getData("SELECT * FROM clients","clients");
  }
}

datamodel.h

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#ifndef DATAMODEL_H
#define DATAMODEL_H

#include
#include
#include
#include

class dataModel : public QObject
{
  Q_OBJECT

  public:
    dataModel(QObject* parent);
};
#endif

datamodel.cpp

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#include "datamodel.h"
#include "mainwindow.h"
#include
#include
#include
#include

dataModel::dataModel(QObject* parent) : QObject(parent)
{
}

Looks fine to me, although I wouldn't make MainWindow a subclass of Ui::MainWindow. You should be able to pass around a pointer to MainWindow.ui without any problems ( unless it somehow is destroyed, which would require destroying MainWindow )

Without seeing how it is actually crashing, I would speculate that either the pointer became invalid, or you were passing around the Ui::MainWindow instance directly ( copying it ) instead of a pointer.

I've modified my code a bit and try to access a test function belonging to the MainWindow class, to see if the pointer is valid:

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MainWindow *mainWindow = MainWindow::test();

When building I get this error:

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error: cannot call member function 'void MainWindow::test()' without object

As the compiler tells you, you try to call a member function (a.k.a method) without having an object to call on.

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mainWindow->test();


Cheers,
_

I think that's the root of my problem; not having the right object passed. The declaration above (MainWindow *mainWindow = MainWindow::test();) doesn't seem to be correct.

The following code gives me this error (error: expected primary-expression before ';' token), which seems logical as well:

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MainWindow *mainWindow = MainWindow;
mainWindow->test();


This should work:

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MainWindow *mainWindow = new MainWindow();
mainWindow->test();

Then pass the mainWindow pointer to anything that needs it, as part of the function call that uses MainWindow functions.

It builds that way, but the resulting build won't start. It doesn't crash, but rather seems to stay looping forever.

You wouldn't be trying to create the MainWindow pointer inside the MainWindow constructor would you?

If so, try this instead:

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MainWindow *mainWindow = this;
mainWindow->test();

I'm afraid not. I'm calling it inside my dataModel class:

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dataModel::dataModel(QObject* parent) : QObject(parent)
{
  MainWindow *mainWindow = new MainWindow();
  mainWindow->test();
}

Whoops! I know what the culprit was and removed it. I had an old call to my dataModel class still in the MainWindow class. So in a sense I was calling it in the MainWindow constructor. All is well know, on to the next obstacle.

Thanks!

Is there a way to call dataModel in my MainWindow class, without triggering the eternal looping?

mensch wrote:Is there a way to call dataModel in my MainWindow class, without triggering the eternal looping?

You can just pass its instance to the MainWindow constructor or the other way around

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class MainWindow
{
public:
    MainWindow(dataModel* model);

private:
    dataModel* m_model;
};


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dataModel::dataModel()
{
    MainWindow* mainWindow = new MainWindow(this);
};


Cheers,
_

That seem to work.

However, if I try to call m_model->test() from my mainwindow class the application crashes due to a EXC_BAD_ACCESS error. The test function in dataModel should manipulate a QTreeWidget belonging to MainWindow (at the moment it should just printing the widget's name).

mainwindow.h

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#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include
#include "ui_mainwindow.h"

class dataModel;

class MainWindow : public QMainWindow, public Ui::MainWindow
{
  Q_OBJECT

  public:
    MainWindow(QWidget* parent=0);

  private:
    dataModel* m_model;
};
#endif

mainwindow.cpp

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MainWindow::MainWindow(QWidget *parent) : QMainWindow(parent)
{
  m_model->test();
}

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#ifndef DATAMODEL_H
#define DATAMODEL_H

#include
#include
#include
#include

class MainWindow;

class dataModel : public QObject
{
  Q_OBJECT

  public:
    dataModel(QObject* parent);
    void test();

    private:
      MainWindow *mainWindow;
};
#endif

datamodel.cpp

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dataModel::dataModel(QObject* parent) : QObject(parent)
{
  mainWindow = new MainWindow();
}

void dataModel::test() {
  qDebug() clientList->objectName();
}

This is because you never initialised m_model.
Change the MainWindow constructor as follows:

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MainWindow(QWidget* parent = 0, DataModel* parentModel);

Then change the following

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MainWindow::MainWindow(QWidget *parent) : QMainWindow(parent)

Into this

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MainWindow::MainWindow(QWidget *parent, DatModel* parentModel) : QMainWindow(parent)

Finally add the following before calling m_model->test();

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m_model = parentModel;

And change this:

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mainWindow = new MainWindow();

To this:

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mainWindow = new MainWindow(this, this);

Hope this helps.