undefined reference to MainWindow::server

[rz]

I declare this in MainWindow.h

class MainWindow : public QMainWindow
{
Q_OBJECT

public:
static serversocket *server;

and in MainWindow.cpp

MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);


// serversocket *server = new serversocket();
bool success = server->listen(QHostAddress::Any, 4200);

compiler show the follow error
mainwindow.cpp:14: error: undefined reference to `MainWindow::server'

i don't wana declare server as public variable