Why should I not separate alloc and init?

Question

The normal way to initialise and allocate in Objective-C is

NSObject *someObject = [[NSObject alloc] init];

Why is the following not practised

Answer 1

As per my understanding an allocated object makes no sense without it being initialized,

if you alloc an object first and then later plan to initialize it, there might be a case that you may forget to initialize the object and give a direct call to any of its instance method which would result in run time error.

Example:

    NSString *str = [NSString alloc];
// Override point for customization after application launch.
self.window = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];
NSLog(@"%ld",str.length);

When i run the above code i get this in my console

Did you forget to nest alloc and init?
 *** Terminating app due to uncaught exception 'NSInvalidArgumentException', 
reason: '*** -length only defined for abstract class.  Define -[NSPlaceholderString length]!'

if i would do the below I would still get the exception as str is not being initialized because whatever is being initialized is not being consumed or pointed by str

[str init];

Hence if you want to do it in two lines it should be like this

NSObject *someObject = [NSObject alloc];
someObject = [someObject init];

But it's always better to keep them nested

NSObject *someObject = [[NSObject alloc]init];

If you plan on doing it on single line then use the new keyword which servers the purpose of allocation and initialization on a single line.

Example: YourClass *object_ofClass = [YourClass new];