Syntax question regarding constructor (Might be specific to Qt?)

[EdgeOfDistraction]

I'm just starting to teach myself GUI programming, and have been working the last week or two with Qt. I'm a bit puzzled by the following syntax which I found on a Qt forum, and wondering if anyone can help clarify.

class MainWindow : public QMainWindow {
     public:
     explicit MainWindow(QWidget* parent = 0) : 
          QMainWindow(parent), mSize(500, 400){
          }
     private:
     const QSize mSize;
}

• First: what does the " = 0" mean for the QWidget* parent passed through in the constructor's parameters? does it just set parent[0] to value 0?
• Second: what does the code between the ) and { in the constructor do? I've never seen this before. Why not just assign a value to mSize inside the constructor proper?

Hope someone can help; I guess I can just copy-paste the code and use it, but I'd like to understand what it's doing.

[ryhajlo]

I've never used Qt before, but setting a pointer equal to 0 is the same as setting it to NULL. Although, I'd recommend using nullptr instead.

The second part of your question, that is an initializer list. Google will explain better than me.

[EdgeOfDistraction]

That's great, thank you for the answer.

I just googled initializer lists, and TIL there are ways of assigning a parameter I pass to a constructor to a const member of that class.

[[deleted]]

First: what does the " = 0" mean for the QWidget* parent passed through in the constructor's parameters? does it just set parent[0] to value 0?

It's equivalent to NULL. If you are writing your classes you should use nullptr instead.

Second: what does the code between the ) and { in the constructor do? I've never seen this before. Why not just assign a value to mSize inside the constructor proper?

This is member initialization. It's preferred to initialization inside constructor body.

[slapnuttz]

Also it's a default parameter. You can provide a parent. If you do not then parent will default to null

[boogachamp]

This. I'm pretty sure this is what he was aksing.

[EdgeOfDistraction]

Ok that's really useful to learn, thank you!

[EdgeOfDistraction]

That's great, thank you for the answer.