Why is instancetype used?

问题:

Can someone please explain to me (in simple terms) why an instancetype is used in Objective-C?

- (instancetype) init { 
    self = [super init];
    if (self) { 
        // Custom initialization
    } 
    return self; 
} 

回答1:

It's to increase type safety.

Back in the old days, initialisers just returned an object of type id (any object).

With normal initialisers (those that begin with "init", "alloc" or "new"), this wasn't usually a problem. The compiler would automatically infer the type that it returned and therefore restrict any method calls on the object to the instance methods of that class.

However, this was a problem with static convenience initialisers or "factory methods" that didn't necessarily follow the same naming convention - therefore it was unable to apply the same type safety.

This means that with a class like this:

@interface Foo : NSObject

+(id) aConvenienceInit;

@end

The compiler would accept code like this:

NSArray* subviews = [Foo aConvenienceInit].subviews;

Why? Because the returned object could be any object, so if you try and access a UIView property - there's no type safety to stop you.

However, now with instancetype, the result you get back is of type of your given instance. Now with this code:

@interface Foo : NSObject

+(instancetype) aConvenienceInit;

@end

...

NSArray* subviews = [Foo aConvenienceInit].subviews;

You'll get a compiler warning saying that the property subviews is not a member of Foo*:

Although it's worth noting that the compiler will automatically convert the return type from id to instancetype if your method begins with "alloc", "init" or "new" - but nonetheless using instancetype wherever you can is a good habit to get into.

---

See the Apple docs on instancetype for more info.

回答2:

Imagine two classes:

@interface A : NSObject
- (instancetype)init;
@end

@interface B : A
@end

The init method from A is inherited to B. However, in both classes the method has a different return type. In A the return type is A and in B the return type is B.

There is no other way to declare the return type for initializers correctly. Note that most programming languages with classes don't even have return types for constructors, therefore they completely avoid the issue.

This is the reason why Obj-C needs instancetype but of course it can be used outside initializers, too.