zoukankan      html  css  js  c++  java
  • 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]);
                }
            } 
  • 相关阅读:
    kubernetes进阶(一) kubectl工具使用详解
    二进制安装kubernetes(七) 部署知识点总结
    1024程序员节:这就是国内开发者的现状?
    php 伪协议
    浅谈 PHP 与手机 APP 开发(API 接口开发)
    比RBAC更好的权限认证方式(Auth类认证)
    PHP获得毫秒数
    2020年PHP面试题附答案(实战经验)
    分享几套2019年各大公司最新的PHP面试题,几斤几两一试便知
    PHP面试题2019年百度工程师面试题及答案解析
  • 原文地址:https://www.cnblogs.com/lee4519/p/4222235.html
Copyright © 2011-2022 走看看