Would it be beneficial to begin using instancetype

问题:

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?

回答1:

There definitely is a benefit. When you use 'id', you get essentially no type checking at all. With instancetype, the compiler and IDE know what type of thing is being returned, and can check your code better and autocomplete better.

Only use it where it makes sense of course (i.e. a method that is returning an instance of that class); id is still useful.

回答2:

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.

For initializers, it's more complicated. When you type this:

- (id)initWithBar:(NSInteger)bar

…the compiler will pretend you typed this instead:

- (instancetype)initWithBar:(NSInteger)bar

This was necessary for ARC. This is described in Clang Language Extensions Related result types. This is why people will tell you it isn't necessary to use instancetype, though I contend you should. The rest of this answer deals with this.

There's three advantages:

1. Explicit. Your code is doing what it says, rather than something else.
2. Pattern. You're building good habits for times it does matter, which do exist.
3. Consistency. You've established some consistency to your code, which makes it more readable.

Explicit

It's true that there's no technical benefit to returning instancetype from an init. But this is because the compiler automatically converts the id to instancetype. You are relying on this quirk; while you're writing that the init returns an id, the compiler is interpreting it as if it returns an instancetype.

These are equivalent to the compiler:

- (id)initWithBar:(NSInteger)bar;
- (instancetype)initWithBar:(NSInteger)bar;

These are not equivalent to your eyes. At best, you will learn to ignore the difference and skim over it. This is not something you should learn to ignore.

Pattern

While there's no difference with init and other methods, there is a difference as soon as you define a class factory.

These two are not equivalent:

+ (id)fooWithBar:(NSInteger)bar;
+ (instancetype)fooWithBar:(NSInteger)bar;

You want the second form. If you are used to typing instancetype as the return type of a constructor, you'll get it right every time.

Consistency

Finally, imagine if you put it all together: you want an init function and also a class factory.

If you use id for init, you end up with code like this:

- (id)initWithBar:(NSInteger)bar;
+ (instancetype)fooWithBar:(NSInteger)bar;

But if you use instancetype, you get this:

- (instancetype)initWithBar:(NSInteger)bar;
+ (instancetype)fooWithBar:(NSInteger)bar;

It's more consistent and more readable. They return the same thing, and now that's obvious.

Conclusion

Unless you're intentionally writing code for old compilers, you should use instancetype when appropriate.

You should hesitate before writing a message that returns id. Ask yourself: Is this returning an instance of this class? If so, it's an instancetype.

There are certainly cases where you need to return id, but you'll probably use instancetype much more frequently.

回答3:

Above answers are more than enough to explain this question. I would just like to add an example for the readers to understand it in terms of coding.

ClassA

@interface ClassA : NSObject

- (id)methodA;
- (instancetype)methodB;

@end

---

Class B

@interface ClassB : NSObject

- (id)methodX;

@end

---

TestViewController.m

#import "ClassA.h"
#import "ClassB.h"

- (void)viewDidLoad {

    [[[[ClassA alloc] init] methodA] methodX]; //This will NOT generate a compiler warning or error because the return type for methodA is id. Eventually this will generate exception at runtime

    [[[[ClassA alloc] init] methodB] methodX]; //This will generate a compiler error saying "No visible @interface ClassA declares selector methodX" because the methodB returns instanceType i.e. the type of the receiver
}

回答4:

You also can get detail at The Designated Initializer

**

INSTANCETYPE

** This keyword can only be used for return type, that it matches with return type of receiver. init method always declared to return instancetype. Why not make the return type Party for party instance, for example? That would cause a problem if the Party class was ever subclassed. The subclass would inherit all of the methods from Party, including initializer and its return type. If an instance of the subclass was sent this initializer message, that would be return? Not a pointer to a Party instance, but a pointer to an instance of subclass. You might think that is No problem, I will override the initializer in the subclass to change the return type. But in Objective-C, you cannot have two methods with the same selector and different return types (or arguments). By specifying that an initialization method return "an instance of the receiving object," you would never have to worry what happens in this situation. **

ID

** Before the instancetype has been introduced in Objective-C, initializers return id (eye-dee). This type is defined as "a pointer to any object". (id is a lot like void * in C.) As of this writing, XCode class templates still use id as the return type of initializers added in boilerplate code. Unlike instancetype, id can be used as more than just a return type. You can declare variables or method parameters of type id when you are unsure what type of object the variable will end up pointing to. You can use id when using fast enumeration to iterate over an array of multiple or unknow types of objects. Note that because id is undefined as "a pointer to any object," you do not include an * when declaring a variable or object parameter of this type.