[SOLVED] Constructor difference

[A Former User]

Hey, could someone explain what is the difference between
@
explicit MainWindow(QWidget *parent = 0)
: QWidget(parent) { }@

and
@explicit MainWindow(QWidget *parent = 0);@

At the first example, what could I write inside of the { } ?

[Chris Kawa]

I understand you're a c++ novice? Maybe it would be better to grab a book or a tutorial on classes.

Anyway, both of these together define a constructor.

You have an error in the first part. there should be no explicit keyword here and no = 0 in the parameter list. These go only into the declaration in the header (the second snippet).

The first one is a definition. It goes into the .cpp file. It contains the actual code of the function. That's what you put in the {}. I can't tell you what exactly that is because I don't know what you're program is supposed to do ;) It can also initialize member variables (you heve none here) and call the base class constructor (here it's QWidget(parent)).

The second one is a declaration. You put it in a header and it describes what name and parameters a function has. It can also define default arguments values.

[A Former User]

Actually, both of these are from header files... When I create class in Qt it uses second example, but I've seen in some codes on the internet where people are using first example...

[Chris Kawa]

Oh, then someone is doing "the wrong thing"™ and is putting his code in the header, which you shouldn't do. That's usually good for examples though as it's shorter so you will find it on forums and such ;)

Basically you can do either this (which is what you should):
@
//class.h
class Foo {
public:
Foo();
};

//class.cpp
Foo::Foo() {}
@
Or this:
@
//class.h
class Foo {
public:
Foo() {}
};
@
The second one is shorter but will not compile if you happen to include that header in multiple places.

[A Former User]

I understand now, thank you...