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QObject::connect(ui->pushButton, SIGNAL(clicked()), ui->lineEdit, SLOT(setText("123")));

MainWindow::MainWindow(...) {
 ...
 QObject::connect(ui->pushButton, SIGNAL(clicked()), this, SLOT(insertText()));
 ...
}

MainWindow::insertText() {
    ui->lineEdit->setText("123");
}

    cUpdateProgram *UpdateProgram;
    UpdateProgram = new cUpdateProgram;

    UpdateProgram = new cUpdateProgram;

#ifndef CUPDATEPROGRAM_H
#define CUPDATEPROGRAM_H

#include <QObject>

class cUpdateProgram : public QObject
{
    Q_OBJECT
public:
    explicit cUpdateProgram(QObject *parent = 0);

signals:

public slots:

};

#endif // CUPDATEPROGRAM_H

#include "cupdateprogram.h"

cUpdateProgram::cUpdateProgram(QObject *parent) :
    QObject(parent)
{
}