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#define SHARED_INSTANCE(...) ({\
    static dispatch_once_t pred;\
    static id sharedObject;\
    dispatch_once(&pred, ^{\
        sharedObject = (__VA_ARGS__);\
    });\
    sharedObject;\
})

// Single Line
+ (instancetype) sharedInstance {
    return SHARED_INSTANCE( [[self alloc] init] );
}

// Multi Line
+ (instancetype) sharedInstance {
    return SHARED_INSTANCE({
        NSLog(@"creating shared instance");
        CGFloat someValue = 84 / 2.0f;
        [[self alloc] initWithSomeValue:someValue]; // no return statement
    });
}