Objective-C中的YES / NO，TRUE / FALSE和true / false

Question

确实是一个简单的问题；这些值之间是否有区别（BOOL和bool之间是否有区别）？一位同事提到他们在Objective-C中对不同的事物进行评估，但是当我查看各自.h文件中的typedef时，YES / TRUE / true都定义为，1而NO / FALSE / false都定义为0。真的有区别吗？

Answer 1

如果您将BOOL变量用作布尔值，则没有实际区别。C根据布尔表达式的计算结果是否为0或0来处理它们。

if(someVar ) { ... }

if(!someVar) { ... }

的意思与

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

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

这就是为什么您可以将任何原始类型或表达式作为布尔测试（包括指针）进行评估的原因。请注意，您应该做前者，而不是后者。

请注意，如果您将钝值分配给所谓的变量并测试特定值，则有所不同BOOL，因此请始终将其用作布尔值，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-这是冒险的，因为someVar可以为它分配一个非零的非YES值，但我认为更重要的是，它不能正确表达意图：

if(someVar==YES) { ... } // don't do this!

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

换句话说，按预期使用和记录使用构造，您将避免遭受C伤害。

Answer 2

我对此进行了详尽的测试。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);

输出为：

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