Question

I have found how to implement singleton in objective c (Non-ARC).

Matt Galloway’s singleton

// AppTools.h in my code

@interface AppTools : NSObject {

    NSString *className;
}

@property ( nonatomic, retain ) NSString *className;

+ ( id ) sharedInstance;

@end    // AppTools


// AppTools.m in my code

static AppTools *sharedAppToolsInstance = nil;

@implementation AppTools

@synthesize className;

- ( id ) init {

    self = [ super init ];
    if ( self ) {
        className = [ [ NSString alloc ] initWithString: @"AppTools" ];
    }
    return self;
}   // init

- ( void ) dealloc {

   // Should never be called, but just here for clarity really.
   [ className release ];
   [ super dealloc ];
}   // dealloc

+ ( id ) sharedInstance {

    @synchronized( self ) {
    if ( sharedAppToolsInstance == nil )
        sharedAppToolsInstance = [ [ super allocWithZone: NULL ] init ];
    }
    return sharedAppToolsInstance;
}   // sharedInstance

+ ( id ) allocWithZone: ( NSZone * )zone {

    return [ [ self sharedInstance ] retain ];
}   // allocWithZone:

- ( id ) copyWithZone: ( NSZone * )zone {

    return self;
}   // copyWithZone:

- ( id ) retain {

    return self;
}   // retain

- ( unsigned int ) retainCount {

    return UINT_MAX;    // denotes an object that cannot be released
}   // retainCount

- ( oneway void ) release {

    // never release
}   // release

- ( id ) autorelease {

    return self;
}   // autorelease

I’d like to know how to work allocWithZone: in sharedInstance method.
On this, the allocWithZone: method’s receiver is ‘super’ and ‘super’ is NSObject.
Though return value is NSObject instance, it is substituted to sharedInstance.

Where is className’s memory room then?
I don’t know how to work this part of the code.

Thank in advance.

Answer 1

You ask “Where is className’s memory room then?”

Most classes do not implement alloc or allocWithZone themselves but rely on the implementation inherited from NSObject. The NSObject implementation allocates an object of the original calling class.

So in your example AppTools does override allocWithZone, this implementation invokes NSObject‘s allocWithZone via a call to super, and NSObject‘s method performs the actual allocation and returns an object of type AppTools.

[Note: If you are wondering how NSObject‘s implementation knows what kind of object to allocate then this is simple – calling an inherited method does not alter the self argument to the method, alloc/allocWithZone are class methods, and the self argument of a class method references the class object (rather than an instance object of the class) itself.]