// mytree.h

 #ifndef MYTREE_H

 #define MYTREE_H

 #include <QTreeWidget>

 class MyTree : public QTreeWidget

 {

 public:

     MyTree(QWidget *parent=);

     void test();

 };

 #endif // MYTREE_H

// mytree.cpp

#include "mytree.h"

#include <QMessageBox>

MyTree::MyTree(QWidget *parent):

    QTreeWidget(parent)

{

}

void MyTree::test(){

    QMessageBox::warning(, "test", "test");

}

#include "mainwindow.h"

#include "ui_mainwindow.h"

MainWindow::MainWindow(QWidget *parent) :

    QMainWindow(parent),

    ui(new Ui::MainWindow)

{

    ui->setupUi(this);

    ui->treeWidget->test();

}

MainWindow::~MainWindow()

{

    delete ui;

}

// ui_mainwindow.h

treeWidget = new MyTree(centralWidget);