I think I've tried a similar approach earlier, because I'm getting the same errors now as I got then. I didn't post my main.cpp code earlier, but this is where the errors start now. Understandably, because MainWindow should be called with two arguments this way, only I'm not sure what to pass in main.cpp:

main.cpp:

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

int main(int argc, char *argv[])
{
  QApplication myApp(argc, argv);

  MainWindow mainWindow;
  mainWindow.show();

  return myApp.exec();
}

This is the error:

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error: no matching function for call to 'MainWindow::MainWindow()

Other errors crop up in mainwindow.cpp:

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error: default argument missing for parameter 2 of 'MainWindow::MainWindow(QWidget*, dataModel*)'

And datamodel.cpp:

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error: no matching function for call to 'MainWindow::MainWindow(dataModel* const, dataModel* const)'

You have two choices really:
1: Make main.cpp create DataModel, which will then create the MainWindow.
2: Make main.cpp create MainWindow, which will create the DataModel.

Regarding error in datamodel.cpp, oops my mistake. It should have been this:

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mainWindow = new MainWindow(0, this);

Also, try this for a constructor:

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

I think option two is the best one. Create the mainwindow first and then populate it seems more logical.

When I put dataModel* parentModel = dataModel(this) in the constructor I get the error that you can't use "this" in this context.

There are other errors, but we're down to 9 instead of 21. One is the "error: default argument missing for parameter 2 of 'MainWindow::MainWindow(QWidget*, dataModel*)'" and the others have to do with a failure to make MainWindow privately or publicly available in datamodel.h for other functions (error: ISO C++ forbids declaration of 'MainWindow' with no type).

Given your choice.....
The MainWindow constructor no longer requires DataModel in it:

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MainWindow(QWidget* parent = 0)

The DataModel constructor needs to mention MainWindow now:

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dataModel::dataModel(MainWindow* parent)

You can fix the forbids declaration.... error by adding the following line around the top of datamodel.h:

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#include "mainwindow.h"

I think I'm still doing something wrong. Sorry. Hope I'm not testing your patience too much. :-$

Have I missed something in the declarations below?

I get the following errors:

datamodel.cpp:31: error: prototype for 'dataModel::dataModel(MainWindow*)' does not match any in class 'dataModel'
error: dataModel::dataModel()
error: candidates are: dataModel::dataModel(const dataModel& )
datamodel.h:37: error: expected `)' before '*' token


mainwindow.cpp:

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

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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);
};

#endif

datamodel.cpp:

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

dataModel::dataModel(MainWindow* parent)
{
  qDebug() clientList->objectName();
}

datamodel.h:

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#ifndef DATAMODEL_H
#define DATAMODEL_H

#include
#include
#include
#include

class dataModel
{
  public:
    dataModel(MainWindow* parent);
};
#endif

You need to add this to datamodel.h:

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class MainWindow;

Thanks again! Everything seems to work perfectly now. No errors and I'm able to access the things I want.

Thanks bcooksley and anda_skoa for all the help.

Back again with a related question. Is it possible to use slots defined in the datamodel class in the mainwindow class or is it considered bad practice?

It is completely acceptable to use a childs signals / slots to do something, I tend to avoid using a parents signals / slots however.

I'm sure I'm pointing to the wrong object again, because I get these warnings when declaring the slot like this. "model" is the publicly available dataModel class declared in mainwindow.h:

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connect(clientList, SIGNAL(itemClicked(QTreeWidgetItem*, int)), model, SLOT(updateProjectData(QTreeWidgetItem*)));

Warning:

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Object::connect: No such slot QObject::updateProjectData(QTreeWidgetItem*)

Data model is declared like this:

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dataModel::dataModel(QObject* parent, MainWindow* mainWindow) : QObject(parent)

You need to connect to it like so, since the mainWindow owns the object you are connecting to.

connect(clientList, SIGNAL(itemClicked(QTreeWidgetItem*, int)), mainWindow->model, SLOT(updateProjectData(QTreeWidgetItem*)));

mainWindow is only declared in datamodel.h, so I get an error on scope declaration. I'm calling the connect in mainwindow.cpp, while the slot function is in datamodel.cpp.

Oops. I misread the post. I should have put:
connect(clientList, SIGNAL(itemClicked(QTreeWidgetItem*, int)), dataModel->model, SLOT(updateProjectData(QTreeWidgetItem*)));

Tried that one as well. But I get the error:

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error: expected primary-expression before '->' token

It's being declared like this in mainwindow.h:

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

  public:
    MainWindow(QWidget* parent = 0);

  protected:
    dataModel* model;
};

And called in mainwindow.cpp:

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MainWindow::MainWindow(QWidget *parent) : QMainWindow(parent)
{
  setupUi(this);
  model = new dataModel(0, this);
}

Maybe try this instead:
connect(clientList, SIGNAL(itemClicked(QTreeWidgetItem*, int)), model->(**name of item as specified in ui file**), SLOT(updateProjectData(QTreeWidgetItem*)));

Make sure to replace the entire (**name of item as specified in ui file**)
with what is supposed to be there