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NSUInteger 的 “负值” 问题

说明:

今天遇到一个特别的Bug

NSArray *array;
// 定义 array 为:存有 100 个数的数组;
NSInteger index = -1
if (index >= array.count) {
NSLog(@"index >= 100");
} else {
NSLog(@"index < 100");
}
// 打印:index >= 100

显然,结果和我们期待的很不一样:难道不应该是-1 < 100吗?经过一番查证,终于找到了问题所在,特此以记之。

问题详述:
  • NSArray.count的类型为NSUInteger.
    // NSArray.h 中 count 属性如下:
    @property (readonly) NSUInteger count;
  • 若给NSUInteger型变量赋值为-1,它将被自动转换成NSUInteger的最大值,且自-1向下一一对应.
    NSUInteger a = 0;
    a = 2; NSLog(@" a=2 === %lu",(unsigned long)a);
    a = 1; NSLog(@" a=1 === %lu",(unsigned long)a);
    a = 0; NSLog(@" a=0 === %lu",(unsigned long)a);
    NSLog(@"");
    a = -1; NSLog(@"a=-1 === %lu",(unsigned long)a);
    a = -2; NSLog(@"a=-2 === %lu",(unsigned long)a);
    a = -3; NSLog(@"a=-3 === %lu",(unsigned long)a);
    a = -4; NSLog(@"a=-4 === %lu",(unsigned long)a);
    a = -5; NSLog(@"a=-5 === %lu",(unsigned long)a);
    NSLog(@"");
    a = -1;
    a = a - 18446744073709551615;
    //编译器警告18446744073709551615太大不能放进NSInteger,会自动转换为NSUInteger
    NSLog(@"a(= -1) - 18446744073709551615 = %lu",(unsigned long)a);

    // 打印结果如下:
    // a=2 === 2
    // a=1 === 1
    // a=0 === 0
    //
    // a=-1 === 18446744073709551615
    // a=-2 === 18446744073709551614
    // a=-3 === 18446744073709551613
    // a=-4 === 18446744073709551612
    // a=-5 === 18446744073709551611
    //
    // a(=-1) - 18446744073709551615 === 0
  • NSIntegerNSUInteger进行运算时,编译器会自动将NSInteger转换成对应的NSUInteger,这便是造成文章开头所提出的,产生与直觉相悖结果的原因.
    NSInteger b = -1;
    NSUInteger ub = 0;
    if (b > ub) {
    NSLog(@"-1 > 0");
    } else {
    NSLog(@"-1 < 0");
    }
    // 打印:-1 > 0
解决方法:
NSInteger b = -1;
NSUInteger ub = 0;
if (b > (NSInteger)ub) {
NSLog(@"-1 > 0");
} else {
NSLog(@"-1 < 0");
}
// 打印:-1 < 0
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