Qt Puzzle (question part)

There are 2 simple Qt programs.

pattern 1

#include <QtGui>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    int source_number = 100;
    QString message = "log10("
                      + QString::number(source_number)
                      + ") = "
                      + QString::number(log10(source_number));
    QLabel *label = new QLabel(message);
    label->show();
    return a.exec();
}

pattern 2

#include <QApplication>
#include <QLabel>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    int source_number = 100;
    QString message = "log10("
                      + QString::number(source_number)
                      + ") = "
                      + QString::number(log10(source_number));
    QLabel *label = new QLabel(message);
    label->show();
    return a.exec();
}

2 programs are the same mostly, and these seem to run correctly. But, only pattern 1 runs correctly, and pattern 2 doesn't. Why? and How? to be continued...