C ++ ：. cpp文件中的实例本身的表示关键字定义了类的功能

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)
{}

Answer 2

Likewise, in C++ the this keyword is a pointer to the current object. Since it's a pointer, you should be assigning to the parent pointer directly instead of dereferencing it.

parent = this;

However, are you sure you want the parent of your circle to be itself? Often, the top-most object in a hierarchy has a null parent pointer:

parent = nullptr;

Or in a compiler that doesn't support nullptr , use NULL or 0 .

Since you didn't initialise your parent pointer to point at any valid Circle object, your code invokes undefined behaviour when you dereference it. You are then attempting to assign an object of type Circle* to an object of type Circle , which your compiler will complain about.

Answer 3

According to resources I have read, this seems to be deprecated or has become of other meanings?

You're either reading bad resources or misreading them. The this pointer is alive and well in C++(03|11).

Moving backwards, your mystery's solution is to use this :

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

This code is almost right:

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

But not quite right becuase *parent dereferences parent , which isn't what you want to do. What you want to do is assign the parent pointer, not make changes to the thing that parent points to:

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

Simalarly with the ctor:

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

However in this case, since a root node has no parent conceptually, the parent member of that node shouldn't point to itself, but to nothing:

Circle::Circle() {
    // creates a root circle, parent is set to itself
    parent = NULL; // or '0' or 'nullptr' with C++11
}