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OC中的经常使用nil, NSNUll, 你对这些理解正确吗?

题目: 以下各个表达式的结果是 YES or NO ?

NSNull *null = [NSNull null];
NSString *str = nil;

if (!null)
if (null == nil)
if ([null isKindOfClass:[NSNull class]])


if (!str)
if ([str isKindofClass:[NSNull class]])
if ([str isKindofClass:[NSString class]])

if(nil == 0)
if(null == 0)
if(nil == NULL)

答案:

 NilStudy[29401:12682571] ========================================
 NilStudy[29401:12682571] =                NSNull                =
 NilStudy[29401:12682571] |    NSNull *null = [NSNull null];    |
 NilStudy[29401:12682571] if (null) ==> YES        // null是对象
 NilStudy[29401:12682571] if (!null) ==> NO
 NilStudy[29401:12682571] if (null == nil) ==> NO
 NilStudy[29401:12682571] if (null != nil) ==> YES
 NilStudy[29401:12682571] if ([null isKindOfClass:[NSNull class]]) ==> YES
 NilStudy[29401:12682571] if ([null isEqual:[NSNull class]]) ==> NO    //class是类, null是对象
 NilStudy[29401:12682571] if ([null isEqual:[NSNull null]]) ==> YES
 NilStudy[29401:12682571] ========================================
 NilStudy[29401:12682571] ========================================
 NilStudy[29401:12682571] =                  nil                 =
 NilStudy[29401:12682571] |         NSString *str = nil;         |
 NilStudy[29401:12682571] if (str) ==> NO        
 NilStudy[29401:12682571] if (!str) ==> YES        // nil为0,则!0为YES
 NilStudy[29401:12682571] if (str == nil) ==> YES
 NilStudy[29401:12682571] if (str != nil) ==> NO
 NilStudy[29401:12682571] if ([str isKindOfClass:[NSNull class]]) ==> NO  //nil与NSNull没关系
 NilStudy[29401:12682571] if ([str isKindOfClass:[NSString class]]) ==> YES //类型不变
 NilStudy[29401:12682571] ========================================
 NilStudy[29401:12682571] ========================================
 NilStudy[29401:12682571] =                值类型                =
 NilStudy[29401:12682571] if (0) ==> NO
 NilStudy[29401:12682571] if (3) ==> YES    // 非0即真
 NilStudy[29401:12682571] if (nil == 0)  ==> YES
 NilStudy[29401:12682571] if (null == 0)   ==> NO    // NSNull为对象
 NilStudy[29401:12682571] if (nil == NULL) ==> YES   // NULL为C中的空指针

结论:

• NSNull是类;
• null是对象;
• nil是空指针;
• NULL是C中的空指针
• Nil 对应的Class类型的空标示

定义:

Nil:
#ifndef Nil
# if __has_feature(cxx_nullptr)
#   define Nil nullptr
# else
#   define Nil __DARWIN_NULL
# endif
#endif

nil:
#ifndef nil
# if __has_feature(cxx_nullptr)
#   define nil nullptr
# else
#   define nil __DARWIN_NULL
# endif
#endif

NSNull:
@interface NSNull : NSObject <NSCopying, NSSecureCoding>
+ (NSNull *)null;
@end

更多参考: http://www.cnblogs.com/baozou/p/3304096.html

DEMO点这