ObjectC中的BOOL类型

绝不要直接将BOOL值和YES比较

Objective-C中的BOOL实际上是一种带符号的的字符类型（signed char）的定义，它使用8位存储空间。YES定义为1，而NO定义为0.

#import <Foundation/Foundation.h>
BOOL areIntsDifferent(int things1, int things2)
{
    if(things1 == things2)
        return (NO);
    else
        return (YES);
}//areIntsDifferent

NSString *boolString(BOOL yesNo)
{
    if (yesNo == NO)
        return (@"NO");
    else {
        return (@"YES");
    }
}

int main(int argc,  const char *argv[])
{
    BOOL areTheyDifferent;
    areTheyDifferent = areIntsDifferent(5, 5);
    NSLog(@"are %d and %d different? %@", 5, 5, boolString(areTheyDifferent));
    areTheyDifferent = areIntsDifferent(23, 24);
    NSLog(@"are %d and %d different? %@", 23, 24, boolString(areTheyDifferent));
    return 0;
}

这是一段很简单的代码，很容易得到如下结果：

注意到上面我们说了YES其实是1 ，并不是非零，将代码改变如下

#import <Foundation/Foundation.h>
BOOL areIntsDifferent(int things1, int things2)
{
/*
    if(things1 == things2)
        return (NO);
    else
        return (YES);
*/
    int diff = things1 - things2;
    if (diff == YES)
        return (YES);
    else
        return (NO);
}//areIntsDifferent

NSString *boolString(BOOL yesNo)
{
    if (yesNo == NO)
        return (@"NO");
    else {
        return (@"YES");
    }
}

int main(int argc,  const char *argv[])
{
    BOOL areTheyDifferent;
    areTheyDifferent = areIntsDifferent(5, 5);
    NSLog(@"are %d and %d different? %@", 5, 5, boolString(areTheyDifferent));
    areTheyDifferent = areIntsDifferent(23, 24);
    NSLog(@"are %d and %d different? %@", 23, 24, boolString(areTheyDifferent));
    return 0;
}

在C语言中，默认非0就是yes，但是从上面的代码可以看出，23-24 == -1，此时应该返回YES的，但是运行结果如下：

主要原因是Objective-C中YES被define为1，并不是非0，在代码的编写过程中，这一点一定要注意。

来自为知笔记(Wiz)