How some objects are created before main event loop?

Question

While debugging, I came up with strange for me thing. In the main function, I commented out creation of window as follows:

#include <QApplication>
#include <QMetaType>
#include <QDebug>

//#include "mainwindow.h"

int main(int argc, char *argv[])
{
    qDebug() << "Creating QApplication";
    QApplication a(argc, argv);
    qDebug() << "QAplication has been created successfully";

    qDebug() << "Creating application window";
    // MainWindow w;
    qDebug() << "Displaying application window";
    // w.show();
    qDebug() << "Application window has been displayed successfully";

    return a.exec();
}

I thought I am just creating an event loop and lunching it. But output surprised me:

"17:28:32.793" ButtonControl: contructor.
"17:28:32.807" GestureControl: contructor
Creating QApplication
QAplication has been created successfully
Creating application window
Displaying application window
Application window has been displayed successfully

I have ButtonControl and GestureControl classes and first 2 lines of output are from their constructors. I create their objects inside of other class, which I use in MainWindow class. The strange thing for me is they are created before/without MainWindow and event loop. Their messages are printed even if I don't create QApplication object and call its exec() method. I tried cleaning and running qmake, and rebuilding, but no success. What's going on here? What is wrong with ButtonControl and GestureControl classes?

Env: win7, Qt 5.10, MSVC 2015.

Edit

Here is my Control class:

class Control : public QObject
{
    Q_OBJECT
public:
    explicit Control(QObject *parent = nullptr);
    static ButtonControl *getButtonController() {
        return &buttonController;
    }
    static GestureControl *getGestureController() {
        return &gestureController;
    }

private:
    static ButtonControl buttonController;
    static GestureControl gestureController;
};

I call this class inside my MainWindow. (As I understand from comments this code snippet is enough)

Without seeing the code for your classes, how would we know? Please post a [mcve]. — nvoigt, Apr 12 '18 at 09:03
Yes, they are static classes inside of `Control` class. BUT I don't create `Control` objects either. — Bobur, Apr 12 '18 at 09:08
Static class instances are automatically created during dynamic initialization stage which typically happens before `main` starts. — user7860670, Apr 12 '18 at 09:10
Just set a break point inside `ButtonControl: contructor`, inspect the stack trace. — xiaobing, Apr 12 '18 at 09:12

score 1 · Accepted Answer · edited Jun 20 '20 at 09:12

1

Following comments, I did a small research and found this answer:

Static members are initialized before main(), and they are destroyed in reverse order of creation after the return in main().

Static members are statically allocated, and their lifetime begins and ends with the program.

Thank you everyone who commented out.

edited Jun 20 '20 at 09:12

Community

1
1

answered Apr 13 '18 at 03:54

Bobur

545
1
6
21

How some objects are created before main event loop?

Edit

1 Answers1