我的Qt程序显示带有标题的空白窗口

Question

I am new to Qt and I have this code that is supposed to display a slider bar and number box in Qt main window. But all I am getting is the main window itself and nothing in it. I did use show() function but nothing happend

#include "mainwindow.h"
#include <QApplication>
#include <QSpinBox>
#include <QSlider>
#include <QHBoxLayout>
#include <QtGui/QApplicationStateChangeEvent>



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

    program.setWindowTitle("Title of window");


    QSpinBox *spinboxx = new QSpinBox();
    QSlider *slider = new QSlider(Qt::Horizontal);
    spinboxx->setRange(1,40);
    slider->setRange(1,40);

QObject::connect(spinboxx, SIGNAL(valueChanged(int)), slider, SLOT(setValue(int)));
QObject::connect(slider, SIGNAL(valueChanged(int)), spinboxx, SLOT(setValue(int)));
QHBoxLayout *layout = new QHBoxLayout;

layout->addWidget(slider);
layout->addWidget(spinboxx);
program.setLayout(layout);
    program.show();

    return app.exec();
}

Answer 1

There is an important warning when compiling your code:

QWidget::setLayout: Attempting to set QLayout "" on QMainWindow "", which already has a layout

In fact, you cannot set a layout for QMainWindow , since it has its own layout. From the Documentation of Qt5 :

A main window provides a framework for building an application's user interface. Qt has QMainWindow and its related classes for main window management. QMainWindow has its own layout to which you can add QToolBars, QDockWidgets, a QMenuBar, and a QStatusBar. The layout has a center area that can be occupied by any kind of widget.

You should assign a widget to the QMainWindow program instead like so:

QWidget *window = new QWidget;
QSpinBox *spinboxx = new QSpinBox();
QSlider *slider = new QSlider(Qt::Horizontal);

QHBoxLayout *layout = new QHBoxLayout;
layout->addWidget(slider);
layout->addWidget(spinboxx);

window->setLayout(layout);


QMainWindow program ;
program.setWindowTitle("Title of window");
program.setCentralWidget(window);
program.show();

PS: I kept the name convention you chose to make the changes clearer. I would rather use widget than window and window than program