@interface MySingleton : NSObject {
    CTFontRef paintingFont;
}

@property (readonly) CTFontRef paintingFont;

@end


@implementation MySingleton

+ (MySingleton*)sharedInstance
{
    static dispatch_once_t once;
    static MySingleton *sharedInstance;
    dispatch_once(&once, ^{
        sharedInstance = [[self alloc] init];

        //NB
        sharedInstance->paintingFont =
           CTFontCreateWithName(CFSTR("Helvetica"), 80.0, nil);
    });
   return sharedInstance;
}

@end

sharedInstance->_paintingFont =
           CTFontCreateWithName(CFSTR("Helvetica"), 80.0, nil);

@interface MySingleton ()
@property (readwrite) CTFontRef paintingFont;
@end

@implementation MySingleton
@synthesize paintingFont;
....
@end