Qt code questions perhaps pertaining to C++ in general

Question

I have recently began working with the Qt framework and realized there may be a bit about the syntax of C++ that I do not quite understand. Take for example this code that is given as a foundation when starting a Widget project in Qt.

MainWindow::MainWindow(QWidget *parent) :      
    QMainWindow(parent),                       
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
}

MainWindow::~MainWindow()
{
    delete ui;
}

Where the class MainWindow is defined as:

namespace Ui {
    class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();

private:
    Ui::MainWindow *ui;
};

---

Class declaration

• Stepping through this, I can see that a new namespace, Ui, is created. I imagine this to be an implementation effort to avoid naming conflicts with other user-implemented classes.

• A new class definition follows; it inherits QMainWindow, so we are essentially creating a custom window object.

• Q_OBJECT is apparently a macro that allows the user to use signals and slots in the class definitions, but I am not too worried about that code right now.

• We define the constructor to be explicit thereby disallowing any implicit conversions of this class type. There is also a destructor shown.

• In the private accesses, we look to the Ui namespace and create a MainWindow pointer.

Class definition

• We are using an initialization list I see, but I get lost here. I see that we have a default parameter of NULL ( or 0 ) for the parent of this widget, but if a class inherents another class why must you explicitly call the constructor for said inherited class? That is:

  class A {
  }
  
  class B : public A {
  }
  
  B testObject; 
  

Does this not automatically allocate memory on the stack sufficient memory to contain both classes A and B? Why does the Qt code shown above call the constructor for an inherited class within its own constructor?

• Continuing to the second parameter of the initialization list, I must say that I am confused on the syntax used here. Why do we have a pointer to a QMainWindow that we initialize by calling QMainWindow's constructor with a further call to new thereby creating an instance of itself? This initialization method seems circular.

• We then call a setupUi function and pass this object for initialization.

• And finally, in the destructor we simply deallocate the memory associated with the MainWindow.

As a final question, if an additional object is created, say a QPushButton, does the software automatically deallocate the memory assocated with these objects or must that be included in the destructor alongside delete ui? I have not seen a tutorial that actually does this, so I am not sure if the framework is designed such that it is handled for you.

I know there are few direct questions here, but is there any flaw in my logic for each statement?

tl;dr, skip to the first and second bullet under Class definition.

Answer 1

Please note that you should not ask so many sub-questions in a question, but I will try to answer your questions regardless ...

Does this not automatically allocate memory on the stack sufficient memory to contain both classes A and B?

It allocates memory for the base class, yes.

Why does the Qt code shown above call the constructor for an inherited class within its own constructor?

Because that is how C++ inheritance works. You need to initialize the base class explicitly in cases like this It could be implicit in an ideal with default constructors, but you are not even using that in here which is good.

Why do we have a pointer to a QMainWindow that we initialize by calling QMainWindow's constructor with a further call to new thereby creating an instance of itself? This initialization method seems circular.

As you explained in the beginning, you have two separate MainWindows, one generated by the Qt Designer, and one class implementation in your code, thus the namespaces you mentioned.

As a final question, if an additional object is created, say a QPushButton, does the software automatically deallocate the memory assocated with these objects or must that be included in the destructor alongside delete ui? I have not seen a tutorial that actually does this, so I am not sure if the framework is designed such that it is handled for you.

If you allocate it on the stack, it is not necessary since it will be destroyed automatically when running out of scope. If you allocate that on the heap, in the Qt world, you specify the parent as the argument for the constructor so that it gets deleted automatically when the parent gets deleted. This is done transparently for you by the Qt parent-child hierarchy and mechanism. That is why you do not see explicit delete usually in examples.