QMainWindow setLayout无效

提出问题：

1. mainWin.h

#ifndef MAINWIN_H
#define MAINWIN_H
#include <QMainWindow>
class QSignalMapper;
class MainWin : public QMainWindow
{
    Q_OBJECT

public:
    MainWin(QWidget *parent = 0);
    ~MainWin();
private slots:
    void doClicked(const QString & btnname);
private:
    QSignalMapper *signalMapper;
private:
};

#endif // MAINWIN_H

1. mainWin.cpp

#include "mainwin.h"
#include <QSignalMapper>
#include <QMessageBox>
#include <QGridLayout>
#include <QPushButton>