Member Avatar for el33t

Please consider the below code.

1 #include <QApplication>
2 #include <QLabel>
3 int main(int argc, char *argv[])
4 {
5 QApplication app(argc, argv);
6 QLabel *label = new QLabel("Hello Qt!");
7 label->show();
8 return app.exec();
9 }


Now, I'm abit doubtful about line 5. According to the book(from which I extracted this code), it is said that QApplication in line 5 is a constructor. Now my questions are:

1. Every constructor should have a class (to which it belongs). So where is the class of this constructor?
2. Assuming that there IS a class for this constructor, then why the name of the constructor doesn't matches with the class to which it belongs?? I think that this is the rule for every constructor that the it should share the same name with the class to which it belongs. Here, we can see that the constructor is named QApplication app(argc, argv). [Assuming that the class name is QApplication.

Please clarify my doubts. I have quite basic knowledge of C++. So if I lack any basic points about constructor, I apologize and therefore request you to help me understand those missing points.

Thank You.

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Short, practical clarification that builds on 's helpful simulation: QApplication in the example is the class/type; the identifier that follows (the app name you saw) is simply the variable — an instance of that class constructed with the given arguments. The constructor itself is the QApplication function defined inside the class; the code reads as "create a QApplication object named app, passing argc, argv to its constructor."

Why you usually see that instance named and created early: the application object initializes the Qt runtime and GUI/platform integration and provides the event loop you start later. It should be created before any widgets are made so those widgets can hook into the platform and the event system. Passing argc and argv lets Qt parse and remove any Qt-specific command-line options during construction.

A couple of common pitfalls worth calling out: writing QApplication app(); does not create an object — it declares a function (the "most vexing parse"). To get a default or parameterized instance you must actually construct one. Prefer stack allocation for QApplication (so its destructor runs automatically at scope exit); allocating it with new requires you to delete it yourself. Also choose the right application class for your program: QCoreApplication for non-GUI, QGuiApplication for Qt GUI without widgets, and QApplication when using the QWidget-based UI.

For authoritative details and exact constructor signatures, see the official Qt documentation for QApplication (Qt 5/6).

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Member Avatar for Dave Sinkula

If I rolled a quick "simulation" of these classes as follows, would it help clarify things?

#include <iostream>
#include <string>

class QApplication // real definition probably in <QApplication>
{
public:
    QApplication(int argc, char *argv[])
    {
        std::cout << "QApplication ctor" << "\n";
    }
};

class QLabel // real definition probably in <Qlabel>
{
    std::string msg;
public:
    QLabel(const char *text) : msg(text)
    {
        std::cout << "QLabel ctor" << "\n";
    }
    void show()
    {
        std::cout << "QLabel show(): " << msg << "\n";
    }
};

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    QLabel *label = new QLabel("Hello Qt!");
    label->show();
    return 0;
}

/* my output
QApplication ctor
QLabel ctor
QLabel show(): Hello Qt!
*/

1. Every constructor should have a class (to which it belongs). So where is the class of this constructor?

Probably in #include <QApplication> .

2. Assuming that there IS a class for this constructor, then why the name of the constructor doesn't matches with the class to which it belongs??

What about QApplication app(argc, argv); doesn't match QApplication ?

Member Avatar for el33t

Allright, thanks, it helped a lot and my first doubt is cleared.

But you said that:

What about QApplication app(argc, argv); doesn't match QApplication

Here, why do we have that extra app. Isin't it supposed to be QApplication(argc, argv) and not QApplication app(argc, argv). What is the role of that app there?

Thanks

Member Avatar for el33t

Ohhh..... I'm sorry, all doubts cleard, even the 2nd one... I made some REAllY silly misunderstanding.

Anyways thanks.

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