Good question. I think it means how to write constructors correctly. Like:

My understanding of it is to just have constructors that are simple and only take a very small number of arguments (if any).

For example it is better to have code like:

as opposed to

as you just end up with a bunch of meaningless numbers in your code. With the former you can see the numbers in the context of the member function which gives a clue as to their meaning.

I was inclined to

explicit MyWidget(QWidget* parent, …etc): BaseWidget(parent) {}

Maybe Trolls themselves will answer? What is correct?

I was thinking of this article [doc.qt.nokia.com] when I wrote my reply.

In the Qt API, it is standard that the parent argument should be last, and should have a default value of 0. So it would be

The article ZapB links to is insightful if you want to learn about API design.

Looks like that Qt Essentials Curriculum Block is not perfect. I see that certified specialists don’t give an exact answer but explain only their own understanding of the term “clean constructor”.
Therefore I realize that there isn’t a documented definition of this term.

Would you mind giving it then? You seem in the know?

Note that this, IMHO, does not belong in the essentials curriculum at all. Perhaps in the C++ for Qt one though.

Unfortunately, I also don’t know the proper definition. I found this topic searching on the Internet to find it.
And as you can see at http://qt.nokia.com/developer/learning/certification/exams/preparation-prerequisites/qt-curriculum/qt-essentials clean constructors are in the Essentials block (See section “Writing your own widgets”)

Here are some more links that provide guidelines:

• http://doc.qt.nokia.com/qq/qq13-apis.html
• http://developer.qt.nokia.com/wiki/Coding_Conventions
• http://developer.qt.nokia.com/wiki/API_Design_Principles

Oh, great! These articles look to be very useful. Thanks a lot!