iOS中instancetype和id的区别

1、instancetype

1. instancetype不可以跟id那样，作为参数进行传递，只可以作为方法的返回值
2. 当一个类返回相同类的实例时可以使用instancetype

2、id

1. 不在描述

3、代码反馈
    Person.h

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1. #import <Foundation/Foundation.h>
   
2. 
   
3. @interface Person : NSObject
   
4. 
   
5. + (instancetype)personInstancetype;
   
6. + (id)personID;
   
7. 
   
8. - (void)does;
   
9. @end

    Person.m

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1. #import "Person.h"
   
2. 
   
3. @implementation Person
   
4. 
   
5. + (instancetype)personInstancetype
   
6. {
   
7.     return [[self alloc] init];
   
8. }
   
9. + (id)personID
   
10. {
    
11.     return [[self alloc] init];
    
12. }
    
13. - (void)does
    
14. {
    
15.     NSLog(@"person does");
    
16. }
    
17. @end

    
    Man.h

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1. #import "Person.h"
   
2. 
   
3. @interface Man : Person
   
4. 
   
5. - (void)doIt;
   
6. @end

    Man.m

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1. #import "Man.h"
   
2. 
   
3. @implementation Man
   
4. 
   
5. - (void)doIt
   
6. {
   
7.     NSLog(@"man do it ");
   
8. }
   
9. @end

    调用执行过程如下：
    main.m

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1. #import <UIKit/UIKit.h>
   
2. 
   
3. #import "AppDelegate.h"
   
4. #import "Person.h"
   
5. #import "Man.h"
   
6. 
   
7. int main(int argc, char *argv[])
   
8. {
   
9.     @autoreleasepool {
   
10.         [[Person personID] does];
    
11.         [[Person personID] doIt];
    
12.         
    
13.         [[Person personInstancetype] doIt];
    
14.         [[Person personInstancetype] does];
    
15.         
    
16.         [[Man personInstancetype] doIt];
    
17.         [[Man personInstancetype] does];
    
18.         
    
19.         [[Man personID] does];
    
20.         [[Man personID] doIt];
    
21.         
    
22.         return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class]));
    
23.     }
    
24. }

      在13行会有编译器提示，如下：

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1. /Users/...../Demo/Demo/main.m:21:38: No visible @interface for 'Person' declares the selector 'doIt'