NSDate isMemberOfClass：[NSDate class]返回false？ [英] NSDate isMemberOfClass: [NSDate class] returns false?

问题描述

这很奇怪。以下if语句失败。可能出错？

  NSDate * date = [NSDate date]; 
 
 if（[date isMemberOfClass：[NSDate class]]）{
 //未执行。 
} 
  

解决方案

code> NSArray ， NSDictionary ， NSString 和
NSData 是类集群。这个概念在
文档，这意味着您不会得到该类的直接实例
。

由于要处理的数据的多样性，类具有内部的
专用子类;当你创建一个实例时，将
确定哪些内部子类是最好的选项，
你的对象将是该子类的实例（不是 NSData
本身）。

在这种情况下，如果你需要检查，使用 isKindOfClass： code>这将
也为真子类。

  NSDate * date = [NSDate date] ; 
 
 if（[date isKindOfClass：[NSDate class]]）{
 / * ... * / 
} 
  pre> 
 
 
 
 
 
 编辑：
另一个例子，调用 NSStringFromClass（[obj class]） 
在这些对象中：
  NSData * data = [NSData data]; 
 NSData * str_data = [@stringdataUsingEncoding：NSUTF8StringEncoding]; 
 NSNumber * n_bool = [NSNumber numberWithBool：YES]; 
 NSNumber * n_int = [NSNumber numberWithInt：42]; 
 NSArray * array = [NSArray array]; 
  
结果：
  _NSZeroData 
 NSConcreteMutableData 
 __NSCFBoolean 
 __NSCFNumber 
 __NSArrayI 
  
 
this is bizarre. The following if statement is failing. What could be wrong?
 NSDate *date = [NSDate date];

 if ([date isMemberOfClass: [NSDate class]]) {
    // Not executed.
 }

解决方案
It happens  that classes like NSArray,  NSDictionary, NSString and
NSData are  class clusters.  This concept is  explained better  in the
documentation, and it means that you will not get a direct instance
for that class.

Due  to the  variety of  "data" to  be handled,  the class  has internal
specialised  subclasses; and  when you  create  an instance  it will  be
determined which  of these internal  subclasses is the best  option, and
your object will  then be an instance of that  subclass (not of NSData
itself).

In this case, if you need to check that, use isKindOfClass: which will
be true for subclasses as well.
NSDate *date = [NSDate date];

if ([date isKindOfClass: [NSDate class]]) {
    /* ... */
}




Edit:
Just as an additional  example, calling NSStringFromClass([obj class])
in these objects:
NSData   * data     = [NSData data];
NSData   * str_data = [@"string" dataUsingEncoding:NSUTF8StringEncoding];
NSNumber * n_bool   = [NSNumber numberWithBool:YES];
NSNumber * n_int    = [NSNumber numberWithInt:42];
NSArray  * array    = [NSArray array];
Results in:
_NSZeroData
NSConcreteMutableData
__NSCFBoolean
__NSCFNumber
__NSArrayI


                        
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