Question

For example, I create a class named Circle and define it in Circle.h.

class Circle {
    public:
        Circle *parent;

        Circle();
};

And in a separate Circle.cpp file under the same directory as Circle.h, I define the constructor.

Circle::Circle() {
    // creates a root circle, parent is set to itself
    *parent = ????
}

What should I fill in the ???? part? In AS3, I know you can use the this keyword to represent the instance itself when defining functions of the class. What do you use in C++?

EDIT 2012/11/29 23:56

There is another constructor:

Circle(Circle*);

Circle::Circle(Circle *cParent) {
    *parent = *cParent;
}

And a function, when called, creates a new Circle instance and set the new instance’s *parent to caller.

void addChild();

void Circle::addChild() {
    Circle child(????);
}

???? is still that mystery part. According to resources I have read, this seems to be deprecated or has become of other meanings?

Answer 1

The keyword you’re looking for is this; it’s a pointer to the instance on which the function was invoked. In your example, you’d use it like this:

Circle::Circle()
{
  parent = this;
}

Note that it must be parent = this;, not *parent = this;. The latter would assign into the object parent is pointing to, which is not valid (parent has an indeterminate value). Fortunately, it wouldn’t compile, as you’d be assigning a pointer into an object.

For cases such as this (initialising a data member inside a constructor), C++ has the initialiser syntax, which is generally preferred to assigning the members in the constructor body. The code would then look like this:

Circle::Circle()
  : parent(this)
{}