What is main difference between "auto" and "instancetype" type?

Question

What is difference between auto and clang type instancetype?
Where we have to use auto and where we have to user instancetype?

Answer 1

auto in Objective C is inherited from C and means auto keyword

Defines a local variable as having a local lifetime.

Keyword auto uses the following syntax:

[auto] data-definition; As the local lifetime is the default for local variables, auto keyword is extremely rarely used.

Note: GNU C extends auto keyword to allow forward declaration of nested functions.

If you are looking for equivalent for C++11's auto or C#'s var - in Objective C id is used.

id a = [NSString new];
id b = [NSNumber new];

But id is not resolved to concrete type at compile time like auto in C++11 does.

instancetype is a contextual keyword that can be used as a result type to signal that a method returns a related result type. For example:

@interface Person
+ (instancetype)personWithName:(NSString *)name;
@end

instancetype, unlike id, can only be used as the result type in a method declaration.

With instancetype, the compiler will correctly infer that the result of +personWithName: is an instance of a Person. And will generate an error if you will try to call

[[Person personWithName:@"Some Name"] methodThatNotExistsInPerson];

If you will use id compiler will not do so, you will not fix it and will receive runtime error!

Instancetype is used to add a little more "strong typing"to Objective C.