Writing a function in C++ with Qt

Question

I know how to format functions in C++ for the most part, except for this scenario which I've never seen before.

I currently have this:

void MainWindow::on_viewEmployees_clicked()
{
QDebug debugTxt(QtDebugMsg);
DatabaseControl myDBControl; //this is the header file that I want to use
QSqlQuery qry;
bool ok;
int rows = myDBControl.getNumEmployees();
ui->table->setRowCount(rows);
int count = 0;
int currentCol = 0;

while(count < rows){
    qry.prepare("SELECT * FROM employees ORDER BY lastname LIMIT :f1, :f2");
    qry.bindValue(":f1", count);
    qry.bindValue(":f2", count+1);
    qry.exec();
    qry.next();
    currentCol = 0;
    while(currentCol < ui->table->columnCount()){
       QTableWidgetItem *setdes = new QTableWidgetItem;
       setdes->setText(qry.value(currentCol).toString());
       ui->table->setItem(count, currentCol-1, setdes);
       currentCol++;
    }
    count++;

}
debugTxt << "Table successfully populated";
ui->lblResult->setText("[+] Table successfully populated");
}

which compiles okay and everything, of course. It does exactly what it's supposed to do - scan the contents of a SQLite database and output it through a QTableWidget, ordered by employee last name, whenever you click the "viewEmployees" button. No issues.

However I want to convert this into a function...say, DatabaseControl.refreshTableView().

I copy/pasted this into the DatabaseControl class, and I know to change all of the DatabaseControl references to 'this,' however I don't know what to do with the 'ui->' bits, as ui-> is something in my mainwindow.cpp:

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

As a result, I can't alter any of the UI elements in MainWindow (as far as I know). What I want to do is be able to edit those UI elements through the DatabaseControl class. More specifically, I want to edit the contents of that QTableWidget through DatabaseControl.

Like I said in the post. When I reference DatabaseControl outside of DatabaseControl, I write DatabaseControl. When I reference DatabaseControl within DatabaseControl, I write 'this' instead. I have to reference 'ui' from MainWindow in DatabaseControl so I can edit the contents of the table in 'ui' from a function in the DatabaseControl class, but I don't know how. — ozzymado, Jan 02 '14 at 17:40

score 2 · Accepted Answer · edited Oct 27 '16 at 15:46

You could write public functions for the MainWindow class that can be accessed from your DataControl class that would modify the ui elements.
Edit: Or you could write a funciton in the MainWindow that would return a pointer to the ui variable and modify the the user interface from there.
like this:

MainWindow::getUI(){ return ui} // if your ui variable is already stored as a pointer

and now you can call the MainWindow::getUI() function from the DataControler class and manipulate your Ui

score 2 · Answer 2 · answered Jan 02 '14 at 17:44

Well, to answer your question directly, one way would be to pass the ui->table object into your refreshTableView() method. Or somehow give access to the MainWindow to your DatabaseControl object (e.g., like redshark suggests).

However, I suspect that might not be a very good way to structure the code. Many applications try to separate the database from the presentation of information in the database. One reason being that if you want to change how your data is represented, it should be independent of the manner in which information is retrieved from a database. Similarly, you don't want to break your presentation code if you have to switch to a new database with a different API, etc.

There's a whole lot more that could be said about this, but hopefully this is enough to get you thinking along a vein that has been well traversed and proven.

Writing a function in C++ with Qt

2 Answers2