QDataStream: Reading an object from disk, please advise.

**kramed** · 27th August 2007, 03:03

Hello,

I am attempting to include binary file save/open to a simple program that I am working on. So far, it appears to be writing the data I need correctly (well, at least the file is being written and populated). The wall I am running into is loading the data back into the program. I need to save what LineEdits contain, which boxes are checked, etc. I have manually entered each element/widget using a for loop with pointers. Is there an easier way, ie. write the object data to the disk? I am not sure how to do that with Qt. If I could write as an object, hopefully I could read it as an object. Anyways, you guys know better than I. Any help or pointers to a tutorial or helpful example would be greatly appreciated.

Forgive the formatting, the paste didnt take properly.

Qt Code:

Switch view

void MainWindow::save()	
{
    QFile file("grades.dat");
 
    if (!file.open(QIODevice::WriteOnly))
    {
        std::cout   << "Cannot open file for writing: "
                    << qPrintable(file.errorString()) << endl;
        return;
    }
 
    QDataStream output(&file);
    output.setVersion(QDataStream::Qt_4_3);
 
    output << quint32(0x39421872);
 
    for ( int i = 1 ; i <= 8 ; i++ )
    {   
        output << ass_label_LineEdit_[i]->text();
	output << ass_grade_SpinBox_[i]->value();
	output << ass_weight_SpinBox_[i]->value();
	output << ass_outof_SpinBox_[i]->value();
        output << ass_check_[i]->isChecked();
 
        output << test_label_LineEdit_[i]->text();
	output << test_grade_SpinBox_[i]->value();
	output << test_weight_SpinBox_[i]->value();
	output << test_outof_SpinBox_[i]->value();
        output << test_check_[i]->isChecked();
 
        output << other_label_LineEdit_[i]->text();
	output << other_grade_SpinBox_[i]->value();
	output << other_weight_SpinBox_[i]->value();
	output << other_outof_SpinBox_[i]->value();
        output << other_check_[i]->isChecked();
    }
}

void MainWindow::save()	
{
    QFile file("grades.dat");
 
    if (!file.open(QIODevice::WriteOnly))
    {
        std::cout   << "Cannot open file for writing: "
                    << qPrintable(file.errorString()) << endl;
        return;
    }
         
    QDataStream output(&file);
    output.setVersion(QDataStream::Qt_4_3);
    
    output << quint32(0x39421872);
    
    for ( int i = 1 ; i <= 8 ; i++ )
    {   
        output << ass_label_LineEdit_[i]->text();
	output << ass_grade_SpinBox_[i]->value();
	output << ass_weight_SpinBox_[i]->value();
	output << ass_outof_SpinBox_[i]->value();
        output << ass_check_[i]->isChecked();
               
        output << test_label_LineEdit_[i]->text();
	output << test_grade_SpinBox_[i]->value();
	output << test_weight_SpinBox_[i]->value();
	output << test_outof_SpinBox_[i]->value();
        output << test_check_[i]->isChecked();
                
        output << other_label_LineEdit_[i]->text();
	output << other_grade_SpinBox_[i]->value();
	output << other_weight_SpinBox_[i]->value();
	output << other_outof_SpinBox_[i]->value();
        output << other_check_[i]->isChecked();
    }
}

To copy to clipboard, switch view to plain text mode

**marcel** · 27th August 2007, 05:29

QDataStream does not have an << operator for QString, only for const char*.
You can either write a const char instead of the edit's text or use QSettings.

Regards

**kramed** · 27th August 2007, 05:52

Ok, thanks for the info. I will look into using QSetting.

**kramed** · 27th August 2007, 05:59

Apparently QSettings will not be useful to me in this case as I wish for the string to be stored in the file with the other elements.

Is it possible to cast the QString to a const char and then store it in the file?

**marcel** · 27th August 2007, 06:03

Yes, with QString::toAscii().constData().

**kramed** · 27th August 2007, 06:04

Thank you kindly for the info.

**kramed** · 27th August 2007, 06:07

Sorry, I forgot to ask. After I get the casting syntax properly, is it possible for me to use a similar loop to read the data back into the program? Do I have to initialize copies of the same variables and then read them in ?