nil、Nil、NULL、NSNull、[NSNull null] in Objective-C
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很多初学者对nil,Nil,NULL,NSNull,[NSNull null]这五个概念很混淆!先借用网上的解释:
nil: A null pointer to an Objective-C object.
Nil: A null pointer to an Objective-C class.
NULL: A null pointer to anything else. (primitive type or absence of data)
NSNull: A class defines a singleton object used to represent null values in collection objects (which don't allow nil values. nil mean the end of the collection).
[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;
nil和NULL从字面意思来理解比较简单,nil是一个对象,而NULL是一个值,我的理解为nil是将对象设置为空,而NULL是将基本类型设置为空的。而且我们对于nil调用方法,不会产生crash或者抛出异常。
看一下用法
NSURL *url = nil;
Class class = Nil;
int *pointerInt = NULL;
nil是一个对象指针为空,Nil是一个类指针为空,NULL是基本数据类型为空。
这五个概念很容易混淆,在一些场合下他们具有同样的用法。首先解释一下“空指针”意思,空指针是指该指针未指向一块有意义的内存区。如int *p; int *p = NULL;
- 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]); //count: 3 因为obj=nil,在加入obj4时就结束添加对象
- 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]); //count: 0,因为obj1=nil,所以后面的对象没有添加进去
[NSNull null]通常作为占位符作用,如下:
- NSObject *obj1 = [NSNull null];
- NSArray *arr1 = [NSArray arrayWithObjects:@"One", @"TWO", obj1,@"three",nil];
- for (NSString *str in arr1) {
- NSLog(@"array object: %@", str);
- } //result:One、Two、<NULL>、three
- NSObject *obj1 = [NSNull null];
- NSArray *arr1 = [NSArray arrayWithObjects:@"One", @"TWO", obj1,@"three",nil];
- for (NSString *str in arr1) {
- if (str != [NSNull null]){
- NSLog(@"array object: %@", str);
- }
- }//result:One、Two、three
转:http://blog.csdn.net/lyrebing/article/details/8228663
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