IOS爲什麼初始化失敗時使用_variable，但不使用self.variable替換NSNumber？

Question

我有以下類別：

---- .h文件中

#import <Foundation/Foundation.h> 
#import "MapperProtocoll.h" 

@interface ServiceRequest : NSObject { 
} 

@property (strong, nonatomic) NSString *url; 
@property (strong, nonatomic) NSString *postData; 
@property (strong, nonatomic) NSNumber *requestId; 
@property (strong, nonatomic) id<ServiceMapperDelegate> mapper; 

-(id)initWithUrl:(NSString *)url andWithPostdata:(NSString *)postData; 

@end 

------- .m文件

#import "ServiceRequest.h" 

@implementation ServiceRequest 

@synthesize url = _url, postData = _postData, requestId = _requestId, mapper = _mapper; 

-(id)initWithUrl:(NSString *)url andWithPostdata:(NSString *)postData { 
    if (self = [super init]) { 
     _url = url; 
     _postData = postData; 
     self.requestId = [NSNumber numberWithInt:-1]; // HERE IS THE PROBLEM 
    } 
    return self; 
} 

@end 

爲什麼 self.requestId = [NSNumber numberWithInt：-1]; 工作，但 _requestId = [NSNumber numberWithInt：-1]; 拋出運行時錯誤？

Answer 1

類方法[NSNumber numberWithInt:-1]正在返回一個自動釋放值。當您使用合成的setter方法時，值將被setter保留。當你繞開setter，沒有保留...所以一旦autorelease池耗盡，你有一個懸掛指針。