Would it be beneficial to begin using instancetype instead of id?

Question

Clang adds a keyword instancetype that, as far as I can see, replaces id as a return type in -alloc and init.

Is there a benefit to using instancetype instead of id?

Answer 1

Yes, there are benefits to using instancetype in all cases where it applies. I'll explain in more detail, but let me start with this bold statement: Use instancetype whenever it's appropriate, which is whenever a class returns an instance of that same class.

In fact, here's what Apple now says on the subject:

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.

• Emphasis mine. Source: Adopting Modern Objective-C

With that out of the way, let's move on and explain why it's a good idea.

First, some definitions:

 @interface Foo:NSObject
 - (id)initWithBar:(NSInteger)bar; // initializer
 + (id)fooWithBar:(NSInteger)bar;  // class factory
 @end

For a class factory, you should always use instancetype. The compiler does not automatically convert id to instancetype. That id is a generic object. But if you make it an instancetype the compiler knows what type of object the method returns.

This is not an academic problem. For instance, [[NSFileHandle fileHandleWithStandardOutput] writeData:formattedData] will generate an error on Mac OS X (only) Multiple methods named 'writeData:' found with mismatched result, parameter type or attributes. The reason is that both NSFileHandle and NSURLHandle provide a writeData:. Since [NSFileHandle fileHandleWithStandardOutput] returns an id, the compiler is not certain what class writeData: is being called on.

You need to work around this, using either:

[(NSFileHandle *)[NSFileHandle fileHandleWithStandardOutput] writeData:formattedData];

or:

NSFileHandle *fileHandle = [NSFileHandle fileHandleWithStandardOutput];
[fileHandle writeData:formattedData];

Of course, the better solution is to declare fileHandleWithStandardOutput as returning an instancetype. Then the cast or assignment isn't necessary.

(Note that on iOS, this example won't produce an error as only NSFileHandle provides a writeData: there. Other examples exist, such as length, which returns a CGFloat from UILayoutSupport but a NSUInteger from NSString.)

Note: Since I wrote this, the macOS headers have been modified to return a NSFileHandle instead of an id.