NSString *someString = nil; NSURL *someURL = nil; id someObject = nil; if (anotherObject == nil) // do something |
// objc.h #ifndef nil # if __has_feature(cxx_nullptr) # define nil nullptr # else # define nil __DARWIN_NULL # endif #endif // __DARWIN_NULL in _types.h #define __DARWIN_NULL ((void *)0) |
Class someClass = Nil; Class anotherClass = [NSString class]; |
// objc.h #ifndef Nil # if __has_feature(cxx_nullptr) # define Nil nullptr # else # define Nil __DARWIN_NULL # endif #endif |
int *pointerToInt = NULL; char *pointerToChar = NULL; struct TreeNode *rootNode = NULL; |
// in stddef.h #define NULL ((void*)0) |
// 因为 nil 被用来用为集合结束的标志,所以 nil 不能存储在 Foundation 集合里。 NSArray *array = [NSArray arrayWithObjects:@"one", @"two", nil]; // 错误的使用 NSMutableDictionary *dict = [NSMutableDictionary dictionary]; [dict setObject:nil forKey:@"someKey"]; // 正确的使用 NSMutableDictionary *dict = [NSMutableDictionary dictionary]; [dict setObject:[NSNull null] forKey:@"someKey"]; |
/* NSNull.h Copyright (c) 1994-2012, Apple Inc. All rights reserved. */ #import <Foundation/NSObject.h> @interface NSNull : NSObject <NSCopying, NSSecureCoding> + (NSNull *)null; @end |
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