What is "QApplication app(argc, argv)" trying to do? - c++

Question

#include <QtGui/QApplication>
#include <QtDeclarative>
#include "qmlapplicationviewer.h"
int main(int argc, char **argv) {
QApplication app(argc, argv);
QmlApplicationViewer viewer;
viewer.setMainQmlFile("app/native/assets/main.qml");
viewer.showFullScreen();
return app.exec();
}
My C++ is a bit rusty. Can someone please explain to me what is "QApplication app(argc, argv)" trying to do ?
Is it trying to declare a function which takes in 2 arguments (argc and argv) and return a variable of type QApplication ?

Answer 1

The line
QApplication app(argc, argv);
creates a new instance of type QApplication and invokes the constructor of this class. In your example, the variable app now stores this instance. It is somewhat (semantically) a shorthand of this:
QApplication app = QApplication(argc, argv);

Answer 2

Here's a quote from Qt Docs:
The QApplication class manages the GUI application's control flow and
main settings.
QApplication contains the main event loop, where all events from the
window system and other sources are processed and dispatched. It also
handles the application's initialization, finalization, and provides
session management. In addition, QApplication handles most of the
system-wide and application-wide settings.
For any GUI application using Qt, there is precisely one QApplication
object, no matter whether the application has 0, 1, 2 or more windows
at any given time. For non-GUI Qt applications, use QCoreApplication
instead, as it does not depend on the QtGui library.
The QApplication object is accessible through the instance() function
that returns a pointer equivalent to the global qApp pointer.
So, the line
QApplication app(argc, argv);
creates an instance of the QApplication class.

Answer 3

QApplication is a Qt class that contains the main event loop.
When you write QApplication app(argc, argv);
you are creating a object app of this class, by calling its constructor with argc and argv
When int main(int argc, char **argv) is called while running the program, int argc is intialized to contain the number of arguments passed while running the program. char **argv contains an array of arguments passed to the program when executing it.
char* argv[0] will contain (point to) the name of the program, while the subsequent elements will point to the other arguments passed.
argc and argv are in turn, passed to the constructor of QApplication, so that one can pass Qt specific arguments when running your program.
For an example of such arguments try running ./yourProgramName --help in a terminal window

Answer 4

app() is not a function, it is a constructor call.
If you come from C# or Java or something, imagine it as
QApplication app = new QApplication( argc, argv );
Just that app would be a pointer this way, while it actually is the object itself if it is created like in your example.
In short, Qt needs a QApplication instance to run so signals&slots are processed (if you are using them) and events like painting etc are handled