Objective-C中的instancetype和id区别

Related result types¶

According to Cocoa conventions, Objective-C methods with certain names (“init”, “alloc”, etc.) always return objects that are an instance of the receiving class’s type. Such methods are said to have a “related result type”, meaning that a message send to one of these methods will have the same static type as an instance of the receiver class. For example, given the following classes:

@interface NSObject+ (id)alloc;- (id)init;@end@interface NSArray : NSObject@end

and this common initialization pattern

NSArray *array = [[NSArray alloc] init];

the type of the expression [NSArray alloc] is NSArray* because alloc implicitly has a related result type. Similarly, the type of the expression [[NSArray alloc]init] is NSArray*, since init has a related result type and its receiver is known to have the type NSArray *. If neither alloc nor init had a related result type, the expressions would have had type id, as declared in the method signature.

A method with a related result type can be declared by using the type instancetype as its result type.  instancetype is a contextual keyword that is only permitted in the result type of an Objective-C method, e.g.

@interface A+ (instancetype)constructAnA;@end

The related result type can also be inferred for some methods. To determine whether a method has an inferred related result type, the first word in the camel-case selector (e.g., “init” in “initWithObjects”) is considered, and the method will have a related result type if its return type is compatible with the type of its class and if:

• the first word is “alloc” or “new”, and the method is a class method, or
• the first word is “autorelease”, “init”, “retain”, or “self”, and the method is an instance method.

If a method with a related result type is overridden by a subclass method, the subclass method must also return a type that is compatible with the subclass type. For example:

@interface NSString : NSObject- (NSUnrelated *)init; // incorrect usage: NSUnrelated is not NSString or a superclass of NSString@end

Related result types only affect the type of a message send or property access via the given method. In all other respects, a method with a related result type is treated the same way as method that returns id.

Use __has_feature(objc_instancetype) to determine whether the instancetype contextual keyword is available.

使用instancetype有三点好处：

1、明确性。代码只做你让它做的事，而不是其他。

2、程式化。你会养成好习惯，这些习惯在某些时候会很有用，而且肯定有用武之地。

3、一致性。让代码可读性更好。

明确性

用instancetype代替id作为返回值的确没有技术上的好处。但这是因为编译器自动将id转化成了instancetype。你以为init返回的值类型是id，其实编译器返回了instancetype。

这两行代码对于编译器来说是一样的：

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

但在你眼里，这两行代码却不同。你不该学着忽视它。

模式化

在使用init等方法时的确没有区别，但在定义简易构造函数时就有区别了。

这两行代码并不等价：

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

如果用instancetype作为函数的返回类型，就不会出错。

一致性：

最后，想象把所有东西放到一起时的情景：你想要一个init方法和一个简易构造函数。

如果你用id来作为init函数的返回类型，最终代码如下：

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

但如果你用instancetype，代码如下：

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

代码更加一致，可读性更强。它们返回相同的东西，这一点一目了然。

结论

除非你有意为旧编译器写代码，不然你在合适的时候都应该用instancetype。

在写一条返回id的消息前，问自己：这个类返回实例吗？如果返回，用instancetype。

肯定有需要返回id的时候，但你用instancetype的频率应该会更高

注意：

instancetype只可以作为返回值类型，不可以像id那样作为参数类型

参考：http://clang.llvm.org/docs/LanguageExtensions.html#objective-c-features