标签：
nil: A null pointer to an Objective-C object.
( #define nil ((id)0) )
nil表示一个Objective-C对象，这个对象的指针指向空
Nil: A null pointer to an Objective-C class.
首字母大写的Nil 和nil有一点不一样，Nil定义一个指向空的类，是class，而不是对象。
#define Nil 0
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.
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.
if obj is nil , [obj message] will return NO, without NSException
if obj is NSNull , [obj message will throw a NSException
Demo1:
[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
Demo2:
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");
}
Demo3:
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
Demo4:
//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]);
}
}
Objective-c 中 nil, Nil, NULL和NSNull的区别
标签：
原文地址：http://www.cnblogs.com/lee4519/p/4222235.html
