Why does the cannot initialize object parameter for "some widgets" with an expression type 'MainWindow' occur?

#include "mainwindow.h"
# include <QApplication>

int main(intargc, char*argv[])
{
    QApplication a(argc,argv);
    MainWindow;
    w.show(); 
    return a.exec();
}

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

# include <QMainWindow>

    class MainWindow:public QMainWindow
    {
        Q_OBJECT

    public:
        MainWindow (QWidget* parent=nullptr);
        ~MainWindow();
    };

    #endif//MAINWINDOW_H

#-------------------------------------------------------
    #
    # Project created by Qt Creator 2018-09-14T 16:52:20
    #
    #-------------------------------------------------

    QT+=core gui widgets

    TARGET=MainWindow
    TEMPLATE=app

    # The following define makes your compiler permit warnings if you use
    # any feature of Qt which has been marked as deprecated (the exact warnings)
    # depend on your compiler).Please consult the documentation of the
    # Deprecated API in order to know how to port your code away from it.
    DEFINES+=QT_DEPRECATED_WARNINGS

    # You can make your code fail to compile if you use decremented APIs.
    # In order to do so, comment the following line.
    # You can select to disable decremented APIs only up to a certificate version of Qt.
    # DEFINES+=QT_DISABLE_DEPRECATED_BEFORE=0x060000#disables all the APIs detailed before Qt 6.0.0

    CONFIG+=c++11

    SOURCES+=\
            main.cpp\
            mainwindow.cpp

    HEADERS+=\
            mainwindow.h

# Default rules for deployment.
qnx —target.path=/tmp/$${TARGET}/bin
else:unix: !android:target.path=/opt/$${TARGET}/bin
!isEmpty(target.path)—INSTALLS+=target