Static class members and constant initializing

[barliesque]

So, I'm learning to program classes in C++ and having some difficulty with the following...

class Rectangle {

	public:
		double x, y, width, height;

	public:
		int id;

	protected:
		static unsigned int count;

	//----------------------------

	public:  // CONSTRUCTOR
		Rectangle(double x, double y, double width, double height): x(x), y(y), width(width), height(height) {
			id = Rectangle::count++;
			cout << "Constructing Rectangle at position: (" << x << ", " << y << ")" << endl;
		}

	//----------------------------

	public:  // DESTRUCTOR
		~Rectangle() {
			cout << "Destructing Rectangle: " << id << endl;
		}
};

I get the following error:

obj\Debug\main.o||In function `Rectangle':|
[color=red]C:\Users\Barliesque\Documents\C++\First Project\main.cpp|33|undefined reference to `Rectangle::count'|
C:\Users\Barliesque\Documents\C++\First Project\main.cpp|33|undefined reference to `Rectangle::count'|[/color]
||=== Build finished: 2 errors, 0 warnings ===|

I just don't get it.  count is statically defined to the class, but I can't refer to it???

Another problem I'm having:  I wanted to set id to be a const so that it can't be changed after it's set in the constructor, but if it's a const I can't initialize it at all without getting compiler errors that I'm trying to modify a constant.

Any help is much appreciated!

[guestgulkan]

Well if you are learning - what does the reference material you are learning from have to say
about:

1. static member variables
2. constant member variables
?

[barliesque]

I've consulted several reference materials, and as far as I can tell, I'm doing exactly what needs to be done. I'm getting worn out reading and reading but not finding an answer to me specific questions, hence I've turned to the forums.

My understanding was that a variable defined using const could be assigned a value once and only once. But the errors that resulted suggest I can only assign a literal value when the const is declared. Is that the case?

Help me. I beg you.

[guestgulkan]

The static member has to be defined outside the class as weel as declared inide the class.
So somewhere in one of your cpp files - put
unsigned int Rectangle::count
or
unsigned int Rectangle::count = initial value



Constant member have to be initialised in the constructor initialisation list. (although static constant members that are integral types can be initialised in the class declaration).

[barliesque]

That's all I needed. Thank you for putting me out of my misery. :)

I don't know why it was so difficult to find out these simple rules from the reference materials. I've probably got some un-learning of other OOP languages to do. Having to declare statics outside the class just looks odd and redundant, but I'll get used to it!

[Disch]

Having to declare statics outside the class just looks odd and redundant,

I agree. This is one of C++'s warts, IMO.

But you get used to it.