config.xml

<?xml version="1.0" encoding="UTF-8"?>
<epandage>
  <parametres_parcelle>
    <nom>TEST</nom>
    <longueur>455</longueur>
    <largeur>22</largeur>
    <volume_pulverise>27</volume_pulverise>
    <volume_hectare>18</volume_hectare>
  </parametres_parcelle>
</epandage>

xml.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"

#include <QXmlStreamReader>
#include <QXmlStreamWriter>

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

MainWindow::~MainWindow()
{
    delete ui;
    delete xmlReader;
    delete xmlWriter;
}

void MainWindow::on_pushButton_clicked()
{
    QFile file(QCoreApplication::applicationDirPath() + "/config.log");

    if( file.open( QIODevice::ReadWrite ) )
    {
        xmlWriter->setDevice( &file );
        xmlWriter->setAutoFormatting( true );
        xmlWriter->setAutoFormattingIndent( 2 );

        generateXMLstructure(xmlWriter);

        file.close();
        
        QMessageBox::information(this, "INFO", "Le fichier de confifguration a bien été créé.");
    }
    else
    {
        QMessageBox::warning(this, "ERREUR", "Une erreur est survenue lors de la création du fichier de configuration.");
    }
}

void MainWindow::generateXMLstructure(QXmlStreamWriter* writer)
{
    writer->writeStartDocument();
    writer->writeStartElement("epandage");

    writer->writeStartElement("parametres_parcelle");
    writer->writeTextElement("nom", ui->nomParcelle->text());
    writer->writeTextElement("longueur", QString::number(ui->longueurParcelle->value()));
    writer->writeTextElement("largeur", QString::number(ui->largeurParcelle->value()));
    writer->writeTextElement("volume_pulverise", QString::number(ui->volumeParcelle->value()));
    writer->writeTextElement("volume_hectare", QString::number(ui->volumeHectare->value()));
    writer->writeEndElement();

    writer->writeEndElement();
    writer->writeEndDocument();
}

xml.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QFile>
#include <QMessageBox>

namespace Ui {
    class MainWindow;
}

class QXmlStreamReader;
class QXmlStreamWriter;

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget* parent = 0);
    ~MainWindow();

private slots:
    void on_pushButton_clicked();

private:
    Ui::MainWindow* ui;

    QXmlStreamWriter* xmlWriter;
    QXmlStreamReader* xmlReader;

    void generateXMLstructure(QXmlStreamWriter* writer);

};

#endif // MAINWINDOW_H