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.