在Objective-c中,YES/NO,TRUE/FALSE和true/false之间是否有区别？

Question

简单的问题; 这些值之间是否存在差异(BOOL和bool之间是否存在差异)？一位同事提到他们在Objective-C中评估不同的东西,但是当我查看各自的.h文件中的typedef时,YES/TRUE/true都被定义为,1而NO/FALSE/false都被定义为0.真的有什么区别吗？

Answer 1

我相信有是之间的差异bool和BOOL,看看这个网页为什么的解释:
http://iosdevelopertips.com/objective-c/of-bool-and-yes.html

因为BOOL是一个unsigned char而不是一个原始类型,所以类型的变量BOOL可以包含除YES和之外的值NO.

考虑以下代码:

BOOL b = 42;

if (b) {
    printf("b is not NO!\n");
}

if (b != YES) {
    printf("b is not YES!\n");
}

输出是:

b不是NO!
b不是!

对于大多数人来说,这是一个不必要的问题,但如果你真的想要一个布尔值,最好使用一个bool.我应该补充一点:iOS SDK通常BOOL在其接口定义上使用,因此这是一个坚持的参数BOOL.

Answer 2

如果您使用BOOL变量作为布尔值,则没有实际差异.C根据它们是否计算为0来处理布尔表达式.所以:

if(someVar ) { ... }
if(!someVar) { ... }

意思是一样的

if(someVar!=0) { ... }
if(someVar==0) { ... }

这就是为什么你可以将任何原始类型或表达式评估为布尔测试(包括例如指针).请注意,你应该做前者,而不是后者.

请注意,如果将obtuse值分配给所谓的变量并测试特定值,则会有所不同BOOL,因此请始终将它们用作布尔值,并仅从其#define值中分配它们.

重要的是,永远不要使用字符比较来测试布尔值 - 这不仅有风险,因为someVar可以分配一个非零的非零值,但在我看来更重要的是,它无法正确表达意图:

if(someVar==YES) { ... } // don't do this!
if(someVar==NO ) { ... } // don't do this either!

换句话说,使用构造,因为它们的目的和记录使用,你将使自己免于C的伤害世界.

Answer 3

我对此做了详尽的测试.我的结果应该说明一切:

//These will all print "1"
NSLog(@"%d", true == true);
NSLog(@"%d", TRUE == true);
NSLog(@"%d", YES  == true);
NSLog(@"%d", true == TRUE);
NSLog(@"%d", TRUE == TRUE);
NSLog(@"%d", YES  == TRUE);
NSLog(@"%d", true == YES);
NSLog(@"%d", TRUE == YES);
NSLog(@"%d", YES  == YES);

NSLog(@"%d", false == false);
NSLog(@"%d", FALSE == false);
NSLog(@"%d", NO    == false);
NSLog(@"%d", false == FALSE);
NSLog(@"%d", FALSE == FALSE);
NSLog(@"%d", NO    == FALSE);
NSLog(@"%d", false == NO);
NSLog(@"%d", FALSE == NO);
NSLog(@"%d", NO    == NO);


//These will all print "0"
NSLog(@"%d", false == true);
NSLog(@"%d", FALSE == true);
NSLog(@"%d", NO    == true);
NSLog(@"%d", false == TRUE);
NSLog(@"%d", FALSE == TRUE);
NSLog(@"%d", NO    == TRUE);
NSLog(@"%d", false == YES);
NSLog(@"%d", FALSE == YES);
NSLog(@"%d", NO    == YES);

NSLog(@"%d", true == false);
NSLog(@"%d", TRUE == false);
NSLog(@"%d", YES  == false);
NSLog(@"%d", true == FALSE);
NSLog(@"%d", TRUE == FALSE);
NSLog(@"%d", YES  == FALSE);
NSLog(@"%d", true == NO);
NSLog(@"%d", TRUE == NO);
NSLog(@"%d", YES  == NO);

输出是:

2013-02-19 20:30:37.061 BooleanTests[27433:a0f] 1
2013-02-19 20:30:37.061 BooleanTests[27433:a0f] 1
2013-02-19 20:30:37.072 BooleanTests[27433:a0f] 1
2013-02-19 20:30:37.073 BooleanTests[27433:a0f] 1
2013-02-19 20:30:37.073 BooleanTests[27433:a0f] 1
2013-02-19 20:30:37.074 BooleanTests[27433:a0f] 1
2013-02-19 20:30:37.074 BooleanTests[27433:a0f] 1
2013-02-19 20:30:37.075 BooleanTests[27433:a0f] 1
2013-02-19 20:30:37.075 BooleanTests[27433:a0f] 1
2013-02-19 20:30:37.076 BooleanTests[27433:a0f] 1
2013-02-19 20:30:37.077 BooleanTests[27433:a0f] 1
2013-02-19 20:30:37.077 BooleanTests[27433:a0f] 1
2013-02-19 20:30:37.078 BooleanTests[27433:a0f] 1
2013-02-19 20:30:37.078 BooleanTests[27433:a0f] 1
2013-02-19 20:30:37.079 BooleanTests[27433:a0f] 1
2013-02-19 20:30:37.079 BooleanTests[27433:a0f] 1
2013-02-19 20:30:37.080 BooleanTests[27433:a0f] 1
2013-02-19 20:30:37.080 BooleanTests[27433:a0f] 1
2013-02-19 20:30:37.081 BooleanTests[27433:a0f] 0
2013-02-19 20:30:37.081 BooleanTests[27433:a0f] 0
2013-02-19 20:30:37.082 BooleanTests[27433:a0f] 0
2013-02-19 20:30:37.091 BooleanTests[27433:a0f] 0
2013-02-19 20:30:37.092 BooleanTests[27433:a0f] 0
2013-02-19 20:30:37.093 BooleanTests[27433:a0f] 0
2013-02-19 20:30:37.093 BooleanTests[27433:a0f] 0
2013-02-19 20:30:37.094 BooleanTests[27433:a0f] 0
2013-02-19 20:30:37.094 BooleanTests[27433:a0f] 0
2013-02-19 20:30:37.095 BooleanTests[27433:a0f] 0
2013-02-19 20:30:37.095 BooleanTests[27433:a0f] 0
2013-02-19 20:30:37.096 BooleanTests[27433:a0f] 0
2013-02-19 20:30:37.096 BooleanTests[27433:a0f] 0
2013-02-19 20:30:37.097 BooleanTests[27433:a0f] 0
2013-02-19 20:30:37.098 BooleanTests[27433:a0f] 0
2013-02-19 20:30:37.101 BooleanTests[27433:a0f] 0
2013-02-19 20:30:37.102 BooleanTests[27433:a0f] 0
2013-02-19 20:30:37.102 BooleanTests[27433:a0f] 0

Answer 4

您可能想要阅读此问题的答案.总之,在Objective-C中(来自objc.h中的定义):

typedef signed char        BOOL; 
// BOOL is explicitly signed so @encode(BOOL) == "c" rather than "C" 
// even if -funsigned-char is used.
#define OBJC_BOOL_DEFINED


#define YES             (BOOL)1
#define NO              (BOOL)0

Answer 5

主要的(危险!)之间的差异true,并YES在JSON序列化.

例如,我们有JSON类型的服务器请求,需要在json sence中发送true/false:

NSDictionary *r1 = @{@"bool" : @(true)};
NSDictionary *r2 = @{@"bool" : @(YES)};
NSDictionary *r3 = @{@"bool" : @((BOOL)true)};

然后我们在发送之前将其转换为JSON字符串

NSData *data = [NSJSONSerialization  dataWithJSONObject:requestParams options:0 error:nil];
NSString *jsonString = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];

结果是

jsonString1 // {"bool":1}
jsonString2 // {"bool":true}
jsonString3 // {"bool":true}

由于API逻辑jsonString1可能会导致错误.

因此,请注意Objective-C中的布尔值.

PS你可以使用

@{@"bool" : @"true"}; // in JSON {"bool":true}