GCDSingleton Macro + @ArvinB changes.

GCDSingleton.h

/*!
 * @function Singleton GCD Macro
 */

#ifndef SINGLETON_GCD
#define SINGLETON_GCD(classname)                        \
                                                        \
+ (classname *)shared##classname {                      \
                                                        \
    static dispatch_once_t pred;                        \
    __strong static classname * shared##classname = nil;\
    dispatch_once( &pred, ^{                            \
        shared##classname = [[self alloc] init]; });    \
    return shared##classname;                           \
}                                                           
#endif

MyClass.h

@interface MyClass : NSObject
+ (MyClass *) sharedMyClass;
@end

MyClass.m

#import "MyClass.h"

@implementation MyClass

SINGLETON_GCD(MyClass);

- (id) init {
    if ( (self = [super init]) ) {
        // Initialization code here.
    }   
    return self;
}
@end