Why would a class allocate an instance of itself?

Guest · 10-04-2014

I'm currently learning the Qt framework and doing my first tutorial. Straight away I saw something that baffled me:

notepad.h

Code:

namespace Ui {
class Notepad;
}

class Notepad : public QMainWindow
{
    Q_OBJECT

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

private:
    Ui::Notepad *ui;
};

notepad.cpp

Code:

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

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

Note the ui pointer and the heap allocation in the class constructor; I can't wrap my head around why one would do this. What's going on here?

**Florian** · 10-04-2014

Ui::Notepad and ::Notepad are seperate classes.

I believe Ui::Notepad contains the generated UI code, but i am not that familiar with Qt.

Guest · 10-04-2014

Yes, that makes sense. I guess it's an unfortunate case of names overlapping, i.e. the two Notepads aren't directly related. Since UI::Notepad does not appear to be included in the header, it's probably a forward declaration. The source file does include a special header file that hasn't been created yet because I haven't built the unfinished project.

I was concerned this was some advanced C++ technique that I wasn't aware of. Thanks for the hint!

**Alpo** · 10-04-2014

Originally Posted by Guest

Yes, that makes sense. I guess it's an unfortunate case of names overlapping, i.e. the two Notepads aren't directly related. Since UI::Notepad does not appear to be included in the header, it's probably a forward declaration. The source file does include a special header file that hasn't been created yet because I haven't built the unfinished project.

I was concerned this was some advanced C++ technique that I wasn't aware of. Thanks for the hint!

I think the definition of Ui::Notepad is the auto generated class definition from a .ui file (created with the Designer). It should contain a C++ equivalent of the XML in the .ui file. The header generated might have a 'ui_' prefix, and it will contain a lot of other headers (so it's probably best to forward declare like this in another header, instead of including it)

Edit: One example of a class allocating itself though is called a singleton. You basically have a private constructor, and access the allocated instance with something like this:

Code:

    static EventHandler* Instance()
    {
        if(!s_pInstance)
        {
            s_pInstance = new EventHandler();
        }
        return s_pInstance;
    }

A singleton is basically a fancy global though. You can access it like this anywhere the header is included (the declaration of s_pInstance being in a source file for the class):

Code:

   EventHandler::Instance()->getMousePosition();

**Elysia** · 10-04-2014

Singleton is also considered an anti-pattern by many, though. Just so you know.

**nonpuz** · 10-05-2014

This is also an pretty example of "PIMPL" or "pointer to implementation" technique. Described best by the original author:
Pimpls - Beauty Marks You Can Depend On

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