Question

Normally if I had Class 1 and I wanted to use it elsewhere, I would write Class1 *class1 = [[Class1 alloc] init]; to create a new instance. But what if I needed to reference variables in Class1 in another class and did not want to create a new instance, but use an existing one. How would I do that? If I call init, and create a new instance, then I will reset all variables back to 0. I understand this is very elementary, but maybe it is something I never really understood. Any help is appreciated.

Answer 1

Once you have an instance of a class, you can store it in a variable, pass it to other methods, and use it there:

Class1 *object1 = [[Class1 alloc] init];
[someOtherObject doSomethingWith:object1];

…

@implementation SomeOtherClass
- (void)doSomethingWith:(Class1 *)anObject {
    [anObject blah];
}
@end

Addendum

In other languages, you would want a “class variable”. Objective-C doesn’t have them, but you can simulate them using a static variable:

static NSString *foo;

@implementation ClassWithSharedFoo

+ (NSString *)sharedFoo {
    return foo;
}

+ (void)setSharedFoo:(NSString *)newFoo {
    if(newFoo != foo) {
        [foo release];
        foo = [newFoo retain];
    }
}

@end

Note that the methods defined here begin with a + instead of a -. This makes them class methods; that is, you send the message to the class itself, not any instance of it:

[ClassWithSharedFoo setSharedFoo:@"Foo"];
NSString *foo = [ClassWithSharedFoo sharedFoo];

In C, a static variable is like a global variable, but it is only visible within the file that it was declared in; so, it is shared by all instances of ClassWithSharedFoo, but can’t be accessed or modified elsewhere except by the provided class methods.