[提问]提示variable 'QApplication a' has initializer but .. [复制链接]

我按照书上编写的代码，为什么总提示variable 'QApplication a' has initializer but incomplete type？

mainwindow.app中的内容

#include "mainwindow.h"
#include "ui_mainwindow.h"

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

MainWindow::~MainWindow()
{
    delete ui;
}


MainWindow::~MainWindow()
{
    QWidget *w=new QWidget;
    w->setWindowTitle("Enter Your Age");
    QSpinBox *spinBox=new QSpinBox;
    QSlider *slider=new QSlider(Qt::Horizontal);
    spinBox->setRange(0,130);
    slider->setRange(0,130);
    QObject::connect(spinBox,SIGNAL(valueChanged(int)),
                     slider,SLOT(setValue(int)));
    QObject::connect(slider,SIGNAL(valueChanged(int)),
                     spinBox,SLOT(setValue(int)));
    spinBox->setValue(35);
    QHBoxLayout *layout=new QHBoxLayout;
    layout->addWidget(spinBox);
    layout->addWidget(slider);
    w->setLayout(Layout);


}

main.cpp中的内容

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

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

    return a.exec();
}

mainwindow.h中的内容


#include <QMainWindow>
#include <QApplication>
#include <QHBoxLayout>
#include <QSlider>
#include <QSpinBox>

namespace Ui {
    class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();

private:
    Ui::MainWindow *ui;
};

#endif // MAINWINDOW_H

我是新手，什么也不懂，谢谢了！！

没有加头文件声明！！