Cannot add QImage to QGridLayout directly

Question

I am trying to write a trafficlight GUI program but I get the error message:

C:\Qt\Qt5.3.0\Tools\QtCreator\bin\A2Q2\trafficlight.cpp:29: error: no matching function for call to 'QGridLayout::addWidget(QImage*&, int, int)' layout->addWidget(image, 2, 1);

I can't even get one image to show and there must be three.

trafficlight.h

#ifndef TRAFFICLIGHT_H
#define TRAFFICLIGHT_H
#include "trafficpic.h"

#include <QWidget>
#include <QTimer>
#include <QLabel>
#include <QLineEdit>
#include <QPushButton>

class TrafficLight: public QWidget{
    Q_OBJECT
public:
    //constructor
    TrafficLight();

private slots:
    //set timer
    void setTimer();
    //display image
    void displayImage();
    //cancel timer
    void cancelTimer();

private:
    //widget data members
    QLabel* timerLabel;
    QLineEdit* timerEdit;
    QPushButton* okButton;
    QPushButton* cancelButton;
    QImage* image;
    //timer
    QTimer* timer;
    //instance of image
    TrafficPic tf;
    //sets up the GUI and connects signals and slots
    void setUpGUI();
};

#endif // TRAFFICLIGHT_H

trafficlight.cpp

#include "trafficlight.h"
#include <QGridLayout>
#include <QMessageBox>
#include <stdlib.h>
#include <time.h>

//constructor
TrafficLight::TrafficLight(){
    //initializes the timer
    timer = new QTimer(this);
    //sets up the GUI
    setUpGUI();
}

//sets up the GUI and connects signals and slots
void TrafficLight::setUpGUI(){
    //initializes all the widgets
    timerLabel = new QLabel("Set Timer [Range: 2000 - 10000 ms] ");
    timerEdit = new QLineEdit();
    okButton = new QPushButton("Set Timer");
    image = new QImage();
    cancelButton = new QPushButton("Cancel");

    //creates a layout and places widgets on the layout
    QGridLayout* layout = new QGridLayout();
    layout->addWidget(timerLabel, 0, 0);
    layout->addWidget(timerEdit, 0, 1);
    layout->addWidget(okButton, 1, 1);
    layout->addWidget(image, 2, 1);
    layout->addWidget(cancelButton, 3, 1);
    //layout is set to the root widget
    this->setLayout(layout);
    //title of the root widget is set
    this->setWindowTitle("Traffic Light");

    //signals and slots are connected
    connect(okButton,SIGNAL(clicked()), this, SLOT(setTimer()));
    connect(cancelButton, SIGNAL(clicked()), this, SLOT(cancelTimer()));
    connect(timer, SIGNAL(timeout()),this, SLOT(displayImage()));
}

//checks if the user input is valid and starts the timer
void TrafficLight::setTimer(){
     bool ok;
     QString timerText = timerEdit->text();
     int timerValue = timerText.toInt(&ok, 10);

     if (ok && (timerValue >= 2000) && (timerValue <= 10000)){
         timer->setInterval(timerValue);
         timer->start();
         timerEdit->setReadOnly(true);
         okButton->setDown(true);
     }
     else{
         QMessageBox:: information(this, "Invalid timer value", "Invalid or out of range timer value.");
     }
}

//display the image
void TrafficLight::displayImage(){
    tf.getImage();
}

//stops the timer and allows user to reset the timer
void TrafficLight::cancelTimer(){
    timer->stop();
    okButton->setDown(false);
    timerEdit->clear();
    timerEdit->setReadOnly(false);
}

trafficpic.h

#ifndef TRAFFICPIC_H
#define TRAFFICPIC_H

#include <QImage>

class TrafficPic{
public:
    //no-arg constructor
    TrafficPic();
    //returns an image
    QImage getImage();
private:
    //to store image
    QImage image;
};

#endif // TRAFFICPIC_H

trafficpic.cpp

#include "trafficpic.h"
//#include "cstdlib"
//#include "ctime"
#include <QLabel>

//constructor initializing data member with image
TrafficPic::TrafficPic(){
    image.load("C:/Qt/images/Green.jpg");
    QLabel myLabel;
    myLabel.setPixmap(QPixmap::fromImage(image));
    myLabel.show();
}

//returns an image
QImage TrafficPic::getImage(){
    return image;
}

main.cpp

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

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

Answer 1

You are trying to pass a QImage* as a QWidget*. QImage does not derive from QWidget. If a parameter takes a QWidget*, you need to pass it either a QWidget*, or a pointer to an instantiation of a class that is derived from QWidget.

You can put the image into a QLabel, as shown in this example