Adopting Modern Objective-C 译文

 1 @interface MyObject : NSObject
 2 + (instancetype)factoryMethodA;
 3 + (id)factoryMethodB;
 4 @end
 5  
 6 @implementation MyObject
 7 + (instancetype)factoryMethodA { return [[[self class] alloc] init]; }
 8 + (id)factoryMethodB { return [[[self class] alloc] init]; }
 9 @end
10  
11 void doSomething() {
12    NSUInteger x, y;
13  
14     x = [[MyObject factoryMethodA] count]; // Return type of +factoryMethodA is taken to be "MyObject *"
15     y = [[MyObject factoryMethodB] count]; // Return type of +factoryMethodB is "id"
16 }

1 @interface MyObjectSubclass : MyObjet
2 @end
3  
4 void doSomethingElse() {
5         NSString *aString = [MyObjectSubclass factoryMethodA];
6 }

1 main.m: Incompatible pointer types initializing ’NSString *’ with an expression of type ’MyObjectSubclass *’