关于Qt动态加载dll的问题

[lateraware]

请高手为我解答下为什么我的加载是不成功的？
#include <QtGui/QApplication>
#include <QtGui/QWidget>
#include <QLibrary>
#include <Qt/qstring.h>
#include "PracDll.h"

typedef void (*my_SetupUi)(QWidget *);

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

#ifdef Q_OS_WIN
typedef void (*my_SetupUi)(QWidget *);
my_SetupUi test;
QString str = "PracDll.dll";
QLibrary lib(str);
test = (my_SetupUi)lib.resolve("setupUi");
QWidget *q = new QWidget();
test(q);
q->show();
lib.unload();
#endif

return app.exec();
}

[Red_angelX]

QLibrary lib(str);
test = (my_SetupUi)lib.resolve("setupUi");
每一步测试一下执行结果看哪里没成功