什么时候一个整数的符号性真的重要吗？ [英] When does the signedness of an integer really matter?

问题描述

由于转换和操作都在C中定义的方式，它似乎很少不管您是否使用符号或无符号变量：

Due to the way conversions and operations are defined in C, it seems to rarely matter whether you use a signed or an unsigned variable:

uint8_t u; int8_t i;

u = -3;    i = -3;
u *= 2;    i *= 2;
u += 15;   i += 15;
u >>= 2;   i >>= 2;

printf("%u",u); // -> 2
printf("%u",i); // -> 2

那么，有没有一套规则，告诉在什么条件下一个变量的符号性真的有差别？

So, is there a set of rules to tell under which conditions the signedness of a variable really makes a difference?

推荐答案

它的问题在这些背景：

• 除法和模数： -2/2 = 1 ， -2U / 2 = UINT_MAX / 2-1 ， -3％4 = -3 ， -3u％4 = 1


• 的变化。对于负符号值，＆GT的结果;＆GT; 和＆LT;＆LT; 是实现定义或定义，RESP 。对于无符号的价值，他们总是定义。


• relationals -2＆LT; 0 ， -2U＆GT; 0


• 溢出。 X + 1＆GT; X 可通过编译器假设总是正确的当且仅当的 X 签署类型。

• division and modulo: -2/2 = 1, -2u/2 = UINT_MAX/2-1, -3%4 = -3, -3u%4 = 1
• shifts. For negative signed values, the result of >> and << are implementation defined or undefined, resp. For unsigned values, they are always defined.
• relationals -2 < 0, -2u > 0
• overflows. x+1 > x may be assumed by the compiler to be always true iff x has signed type.

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