一个关于继承QDialog类的简单问题 [复制链接]

头文件内容为：

#ifndef _TMP_H
#define _TMP_H

#include <QtGui/QtGui>

class MyDialog : public QDialog
{
        Q_OBJECT

        public:
                MyDialog();
};

#endif

CPP文件内容为：

#include <QtGui/QtGui>

#include "tmp.h"

MyDialog::MyDialog()
{
        this->show();
}

int main(int argc, char **argv)
{
        QApplication app(argc, argv);

        MyDialog *pdlg = new MyDialog;

        return app.exec();
}
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请问为什么编译的时候出现：
g++ -c -pipe -O2 -Wall -W -D_REENTRANT -DQT_NO_DEBUG -DQT_GUI_LIB -DQT_CORE_LIB -DQT_SHARED -I/usr/local/Trolltech/Qt-4.4.3/mkspecs/linux-g++ -I. -I/usr/local/Trolltech/Qt-4.4.3/include/QtCore -I/usr/local/Trolltech/Qt-4.4.3/include/QtCore -I/usr/local/Trolltech/Qt-4.4.3/include/QtGui -I/usr/local/Trolltech/Qt-4.4.3/include/QtGui -I/usr/local/Trolltech/Qt-4.4.3/include -I. -I. -I. -o tmp.o tmp.cpp
tmp.cpp: In function ‘int main(int, char**)’:
tmp.cpp:14: warning: unused variable ‘pdlg’
g++ -Wl,-rpath,/usr/local/Trolltech/Qt-4.4.3/lib -o tmp tmp.o    -L/usr/local/Trolltech/Qt-4.4.3/lib -lQtGui -L/usr/local/Trolltech/Qt-4.4.3/lib -L/usr/X11R6/lib -pthread -lpng -lSM -lICE -pthread -pthread -lXi -lXrender -lXrandr -lfreetype -lfontconfig -lXext -lX11 -lQtCore -lz -lm -pthread -lgthread-2.0 -lrt -lglib-2.0 -ldl -lpthread
tmp.o: In function `MyDialog::MyDialog()':
tmp.cpp:(.text+0x2f): undefined reference to `vtable for MyDialog'
tmp.cpp:(.text+0x36): undefined reference to `vtable for MyDialog'
tmp.cpp:(.text+0x47): undefined reference to `vtable for MyDialog'
tmp.o: In function `MyDialog::MyDialog()':
tmp.cpp:(.text+0x9f): undefined reference to `vtable for MyDialog'
tmp.cpp:(.text+0xa6): undefined reference to `vtable for MyDialog'
tmp.o:tmp.cpp:(.text+0xb7): more undefined references to `vtable for MyDialog' follow
collect2: ld returned 1 exit status
make: *** [tmp] Error 1

为什么会出现上面的错误阿       

class MyDialog : public QDialog
{
        Q_OBJECT

        public:
                MyDialog( QWidget *parent = NULL);
};

MyDialog::MyDialog(QWidget *parent)
    : QDialog(parent)
{
        this->show();
}

楼上有理，构造函数不合