Use QTimer with its timeout event in another class of the project - c++

Question

I'm using Qt 5.7 trying to use a timer with timeout event in a class named MyClass. I created a public variable of this class in mainwindow.h. I put the timer's slot connect in the constructor of MyClass, and have a function in MyClass to start the timer. But when I execute the function, the timer slot doesn't work.
I have no error in building, but I get the comment below on the application window at run time:
QObject::connect: No such slot QObject::on_timeout()
Here is my code:
The mainwindow.h
#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include <QMainWindow>
#include "myclass.h"
namespace Ui {
class MainWindow;
}
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
MyClass mc;
explicit MainWindow(QWidget *parent = 0);
~MainWindow();
private slots:
void on_pushButton_clicked();
private:
Ui::MainWindow *ui;
};
#endif // MAINWINDOW_H
The mainwindow.cpp
#include "mainwindow.h"
#include "ui_mainwindow.h"
#include "myclass.h"
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
}
MainWindow::~MainWindow()
{
delete ui;
}
void MainWindow::on_pushButton_clicked()
{
mc.TimerStart();
}
myclass.h
#ifndef MYCLASS_H
#define MYCLASS_H
#include <QTimer>
class MyClass : public QObject
{
public:
MyClass();
TimerStart();
QTimer *timer;
public slots:
on_timeout();
};
#endif // MYCLASS_H
myclass.cpp
#include "myclass.h"
#include <QMessageBox>
#include <QTimer>
MyClass::MyClass()
{
timer = new QTimer(this);
}
MyClass::TimerStart()
{
timer->setInterval(1000);
connect(timer, SIGNAL(timeout()), this, SLOT(on_timeout()));
timer->start(1000);
}
MyClass::on_timeout()
{
QMessageBox msg;
msg.setText("updated");
msg.exec();
}

Answer 1

You forgot to add the Q_OBJECT to MyClass,
You're reentering the event loop in on_timeout.
You're setting the timer's interval multiple times.
You're connecting to the timer's signal multiple times.
You're making an implementation detail (the on_timeout slot) public.
You're pessimizing your code by manually managing the memory for QTimer.
You're including <QTimer> twice.
It's 2016 and you're not using C++11. Yes, you can use it even with Qt 4.
Here's how I'd write it:
Qt 5
#ifndef MYCLASS_H
#define MYCLASS_H
#include <QTimer>
#include <QMessageBox>
class MyClass : public QObject
{
Q_OBJECT
QMessageBox msg;
QTimer timer{this};
public:
explicit MyClass(QObject * parent = nullptr);
void timerStart();
};
#endif // MYCLASS_H
#include "myclass.h"
MyClass::MyClass(QObject * parent) : QObject{parent}
{
msg.setText(QStringLiteral("updated"));
connect(&timer, &QTimer::timeout, &msg, &QMessageBox::show);
}
void MyClass::timerStart()
{
timer.start(1000);
}
Qt 4
Here I demonstrate that implementation details should be private. You could of course do as above and set the msg's text in the constructor, and connect directly to the msg's SLOT(show()).
#ifndef MYCLASS_H
#define MYCLASS_H
#include <QTimer>
#include <QMessageBox>
class MyClass : public QObject
{
Q_OBJECT
QMessageBox msg;
QTimer timer{this};
Q_SLOT void on_timeout();
public:
explicit MyClass(QObject * parent = nullptr);
void timerStart();
};
#endif // MYCLASS_H
#include "myclass.h"
MyClass::MyClass(QObject * parent) : QObject{parent}
{
connect(&timer, SIGNAL(timeout()), SLOT(on_timeout()));
}
void MyClass::timerStart()
{
timer.start(1000);
}
void MyClass::on_timeout()
{
msg.setText("updated");
msg.show();
}

Answer 2

You forgot to add Q_OBJECT in your class declaration:
class MyClass : public QObject
{
Q_OBJECT
// ...
If you're using Qt 5.7, you should use the Qt5 syntax for connect:
connect(timer, &QTimer::timeout, this, &MyClass::on_timeout);
By doing so, the connection will be evaluated at compile time instead of runtime. Moreover, you can connect a signal to functions even if they are not declared as slots (so you won't need the Q_OBJECT identifier in the class declaration).
More info here.