构造函数MyClass（QWidget * parent = 0）中（QWidget * parent = 0）的含义;

Question

just a part of a usual structured class in Qt:

class MyClass  :  public QWidget           
{

Q_OBJECT

 public:

   MyClass(QWidget *parent = 0);
.
.
.
}

Looking at the constructor I don't understand the meaning of the parameter (QWidget *parent = 0) ? What does this mean?

greetings

Answer 1

MyClass(QWidget *parent = 0) defines a constructor that can take a QWidget* .

You might be confused at the = 0 part, which is C++ syntax for default arguments . Default arguments allow you to use the function without having to specify that particular argument. In the case of this, you could call this constructor like this:

mc = MyClass();

And this is equivalent to calling:

mc = MyClass(0); // or MyClass(NULL)

And that means that MyClass object will have no parent QWidget , because = 0 means the parent is a null pointer.

Answer 2

When you construct a new MyClass, you give it a pointer to a QWidget that you want to be the parent of the new one. The = 0 means that if you do not supply an argument, it will have no parent. Or, more strictly speaking, its parent pointer will be set to NULL.

Answer 3

First of all its good to use explicit keyword following constructor function like

explicit MyClass(QWidget *parent = 0 );

By using explicit ,it means you will not be able to do an implicit assignment.

For better understanding lets take an example

Suppose you create a class MyInteger that takes an integer in the constructor. Then you can write something like this:

MyInteger integer = 5;

For simple classes, this can work ok, but when you have complex classes, implicit assignments can result in errors. Therefor it is best to have explicit assignments:

MyInteger integer = MyInteger(5);

This way you are sure that everything is correct. It's a safeguard.

What about the statement "QWidget *parent = 0"!

What is that for?

You define a constructor with a default value. The default value in this case means that there is no parent. Assigning 0 to a pointer (in this case)

MyClass(QWidget *parent = 0)

means that you create a pointer to nothing. In any normal case, you want a pointer to point to a certain address in memory, like the address of a parent widget. But when you don't want to set a parent widget for example, you can point to nothing. Be aware though when using the pointer, always check that it actually points to something (and preferably something correct) or don't do anything with it.

Answer 4

That's a default parameter . If no parameter is supplied, the one provided is used.