#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QString>
#include <QFont>
#include <QFontDialog>
#include <QDebug>

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

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


void MainWindow::on_pushButton_clicked()
{
    QString fontString("Nimbus Roman,36,-1,5,700,0,0,0,0,0,0,0,0,0,0,1");
    QFont initialFont;
    if(initialFont.fromString(fontString))
        qDebug() << "From string is GOOD";
    else
        qDebug() << "From string is BAD";
    QFontDialog dlg;
    dlg.setCurrentFont(initialFont);
    dlg.exec();
    QString altFont = dlg.currentFont().toString();
    qDebug() << altFont;
}


Resulting output in "Application Output" (having selected "Nimbus Roman Regular" in the dialog)

From string is GOOD
QFont::setPointSizeF: Point size <= 0 (0.000000), must be greater than 0
"Nimbus Roman,36,-1,5,400,0,0,0,0,0,0,0,0,0,0,1"