iphone - IOS 为什么使用 _variable 初始化失败而不是使用 NSNumber 的 self.variable？

Question

我有以下类(class):

----.h文件

#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

为什么self.requestId = [NSNumber numberWithInt:-1];工作但是_requestId = [NSNumber numberWithInt:-1];抛出运行时错误？

Answer 1

类方法 [NSNumber numberWithInt:-1] 正在返回一个自动释放的值。当您使用合成 setter 方法时，该值将由 setter 保留。当您绕过 setter 时，没有保留...因此，一旦自动释放池被耗尽，您就会有一个悬空指针。