Question

Hey, in a tutorial C++ code, I found this particular piece of confusion:

PlasmaTutorial1::PlasmaTutorial1(QObject *parent, const QVariantList &args)
    : Plasma::Applet(parent, args), // <- Okay, Plasma = namespace, Applet = class
    m_svg(this),                    // <- A member function of class "Applet"?
    m_icon("document")              // <- ditto?
{
    m_svg.setImagePath("widgets/background");
    // this will get us the standard applet background, for free!
    setBackgroundHints(DefaultBackground);
    resize(200, 200);
}

I’m not new to object oriented programming, so class derivation and super-classes are nothing complicated, but this syntax here got me confused.

The header file defines the class like this:

class PlasmaTutorial1 : public Plasma::Applet
{

Similar to above, namespace Plasma and class Applet. But what’s the public doing there?

I fear that I already know the concept but don’t grasp the C++ syntax/way of doing it.

In this question I picked up that these are called “superconstructors”, at least that’s what stuck in my memory, but I don’t get this to the full extend.

If we glance back at the first snippet, we see Constructor::Class(...) : NS::SuperClass(...), all fine ’till here. But what are m_svg(this), m_icon("document") doing there? Is this some kind of method to make these particular functions known to the derivated class?

Is this part of C++ basics or more immediate? While I’m not completly lost in C++, I feel much more at home in C 🙂

Most of the OOP I have done so far was done in D, Ruby or Python. For example in D I would just define class MyClass : MySuperClass, override what I needed to and call the super class’ constructor if I’d need to.

---

Okay, after reading some of the answers, is this…

PlasmaTutorial1::PlasmaTutorial1(QObject *parent, const QVariantList &args)
    : Plasma::Applet(parent, args), // <- Call the _parent_ constructor
    m_svg(this),         // <- set m_svg  (in the _derived_ class) to "this"
    m_icon("document")   // <- set m_icon (in the _derived_ class) to "document"
{
    ...
}

…assumption correct?

---

Test’s confirm my assumption so far. Thanks a lot! Picking a winning answer is a hard choice, tho…

Answer 1

What you are seeing is a initialization list. They are encouraged by the C++ FAQ Lite and speed up member initialization, because your members don’t need to take default values before entering the constructor.