Learning Qt - Creating a QApplication

Question

I'm re-learning c++ (I have 10 years Java experience) and I'm also learning Qt in the process...

I'm used to creating objects (in Java) via:

MyObject o = new MyObject();

But when creating a QApplication in c++, the examples simply show:

QApplication app(argc, argv);
app.setOrganizationName("My Company");
app.setApplicationName("The App");

So suddenly, I have a reference to "app" and no obvious (to me) assignment to app...

Is this pattern a c++ thing or specific to Qt? What is this pattern called?

Thanks!

Answer 1

Not really a Qt question but,

//You have an assignment to app  
QApplication app(argc, argv);
// is just the same as
QApplication *app = new QApplication(argc, argv);

In C++ you have the choice of creating objects locally (on the stack) or with new (on the heap). Allocating it locally here makes more sense when the app object has a definite lifetime (the length of main) won't be deleted and recreated and only one exists.

One annoying feature of C++ (because of it's c heritage) is that accessing parts of the resulting object is different. If created directly you use "." app.name() but if allocated with new you need to use 'c' pointer notation app->name()

ps. There are a few Qt specific memory features. Qt does a lot of memory management for you, eg. copy-on-write, automatic delete. In Qt you rarely have to call delete for an object - for a C++ expert these are sometimes annoying but from Java it should look more natural.