Objective C static class variables

Question

I am new to Objective C and am reading a book called "Visual Quickstart Guide: Objective-C" by Steven Holzner, Peachpit Press

In Chapter 6: Object Oriented Programming, there is a section called Using Class Variables where he writes:

You can create class variables for use with your classes, but there’s a hitch: every object of that class shares the same variable, so if one object changes a class variable, that variable is changed for all objects. You create class variables with the static keyword. Class variables are often useful: for example, you can use a class variable to keep track of the number of objects of a particular class created in a program. You’ll do that in this task.

And says to enter the following code:

#import <stdio.h> #import <Foundation/NSObject.h> @interface TheClass: NSObject static int count; //error: cannot declare variable inside @interface or @protocol +(int) getCount; @end ... 

This code gives me an error in Xcode 4:

Cannot declare variable inside @interface or @protocol

Is the book wrong or am I doing something wrong?

Answer 1

You declare the static variable in the implementation file (.m file). This should work:

// TheClass.h @interface TheClass : NSObject + (int)count; @end  // TheClass.m static int theCount = 0;  @implementation TheClass + (int) count { return theCount; } @end 

It's not a class variable per se; Objective-C has no notion of a class variable. However, coupled with the class method for retrieving this variable, it functions similarly to a class variable. However, it's really just a C static variable that's accessible by the class's implementation.