使用带有%u和%x的unsigned int时出现意外值

Question

今天我正在开发一个示例iOS应用程序,其中包含以下代码:

unsigned int uCount = 0;
int iJoke = -7;
uCount = uCount + iJoke;

但是当我打印它时:

????????????????????????????????????????????????????????
? Format Specifier ?   Print Statement    ?   Output   ?
????????????????????????????????????????????????????????
? %d               ? NSLog(@"%d",uCount); ? -7         ?
? %u               ? NSLog(@"%u",uCount); ? 4294967289 ?
? %x               ? NSLog(@"%x",uCount); ? fffffff9   ?
????????????????????????????????????????????????????????

我预计%u的输出为7.

然后我用过:

unsigned int i = 0;
int j = -7;
i = i + abs(j);

输出如下:

????????????????????????????????????????????????????
? Format Specifier ?   Print Statement    ? Output ?
????????????????????????????????????????????????????
? %d               ? NSLog(@"%d",uCount); ?      7 ?
? %u               ? NSLog(@"%u",uCount); ?      7 ?
? %x               ? NSLog(@"%x",uCount); ?      7 ?
????????????????????????????????????????????????????

虽然我的问题已得到解决abs(),但我很想知道为什么%u 4294967289在我的第一个案例中给出了结果.

请帮助,提前致谢.

Answer 1

此赋值将以模式表示-7(以2的补码)分配给unsigned int.这将是非常大的无符号值.

对于32位int,这将是 2^32 - 7 = 4294967289

标准说它如下所示

"If the destination type is unsigned, the resulting value is the least unsigned integer congruent to the source integer (modulo 2^n where n is the number of bits used to represent the unsigned type). [ Note: In a two’s complement representation, this conversion is conceptual and there is no change in the bit pattern (if there is no truncation). —end note ]