Objective-c 中 nil, Nil, NULL和NSNull的区别
nil:?A null pointer to an Objective-C?object.
( #define nil ((id)0)? )
Nil: A null pointer to an Objective-C?class.
NULL: A null pointer to anything else, ?is?for?C-style memory pointers.
( #define NULL ((void *)0)? )
NSNull: A class defines a singleton object used to represent null values in collection objects (which don't allow nil values).
[NSNull null]: The singleton instance of NSNull.
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Technically they're all the same,,, but in practice they give someone reading your code some hints about what's going on; just like naming classes with a capital letter and instances with lowercase is recommended, but not required.
If someone sees you passing NULL, they know the receiver expects a C pointer. If they see nil, they know the receiver is expecting an object. If they see Nil, they know the receiver is expecting a class. Readability.
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if obj is nil , [obj message] will return NO, without NSException
if obj is NSNull , [obj message will throw a NSException
[NSApp beginSheet:sheet modalForWindow:mainWindow? modalDelegate:nil //pointing to an object didEndSelector:NULL //pointing to a non object/class contextInfo:NULL]; //pointing to a non object/class
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NSObject *obj1; if (obj1 != nil) { NSLog(@"object is not nil"); }else { NSLog(@"object is nil"); } testClass *c1; if (c1 != Nil) { NSLog(@"class is not Nil"); }else { NSLog(@"class is Nil"); } int *money; if (money != NULL) { NSLog(@"money is not NULL"); }else { NSLog(@"money is NULL"); }?
NSObject *obj1 = [[NSObject alloc] init]; NSObject *obj2 = [NSNull null]; NSObject *obj3 = [NSObject new]; NSObject *obj4; NSArray *arr1 = [NSArray arrayWithObjects:obj1, obj2, obj3, obj4, nil]; NSLog(@"arr1 count: %ld", [arr1 count]); //arr1 count: 3 NSObject *obj1; NSObject *obj2 = [[NSObject alloc] init]; NSObject *obj3 = [NSNull null]; NSObject *obj4 = [NSObject new]; NSArray *arr2 = [NSArray arrayWithObjects:obj1, obj2, obj3, obj4, nil]; NSLog(@"arr2 count: %ld", [arr2 count]); //arr2 count: 0
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//有异常！ NSObject *obj1 = [NSNull null]; NSArray *arr1 = [NSArray arrayWithObjects:@"One", @"TWO", obj1, @"three" ,nil]; for (NSString *str in arr1) { NSLog(@"array object: %@", [str lowercaseString]); } //修改 NSObject *obj1 = [NSNull null]; NSArray *arr1 = [NSArray arrayWithObjects:@"One", @"TWO", obj1, @"three" ,nil]; for (NSString *str in arr1) { if (![str isEqual:[NSNull null]]){ NSLog(@"array object: %@", [str lowercaseString]); } } ?