iOS Question about instancetype...

chrono1081 · May 28, 2014

Hi guys,

I'm catching up with modern Objective-C and found this document:

Adopting Modern Objective-C

and it is talking about using instance type but I'm a bit confused.

On one line it says:

Use the instancetype keyword as the return type of methods that return an instance of the class they are called on (or a subclass of that class). These methods include alloc, init, and class factory methods.

Then in another line it says:

In your code, replace occurrences of id as a return value with instancetype where appropriate. This is typically the case for init methods and class factory methods. Even though the compiler automatically converts methods that begin with “alloc,” “init,” or “new” and have a return type of id to return instancetype, it doesn’t convert other methods. Objective-C convention is to write instancetype explicitly for all methods.

So my question is is id never used anymore? Can anyone give a simple explanation of where to use it and where not to use it?

chown33 · May 28, 2014

Consider NSDictionary. It can hold objects of any type. Therefore, the return type for objectForKey: should be id, not instancetype.

The same principle applies to the key object; it can be any type, so the type of keys and objects in setObject:forKey: would also be id.

The principle also applies to other classes, such as everything in the Collections library. NSDictionary was just the first collection I thought of.

You can browse the class reference doc for NSDictionary and NSMutableDictionary and see where it uses instancetype vs. where it uses id.
https://developer.apple.com/library...ableDictionary_Class/Reference/Reference.html

PhoneyDeveloper · May 30, 2014

objectForKey can return an object of any type.

init can only return an object of the type of the class its called on or a subclass of that type.

chrono1081 · May 31, 2014

Thanks guys.

I'm still pretty confused but I'll work through it. A good amount has changed since I touched Objective-C last.

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iOS Question about instancetype...

chrono1081

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chrono1081

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