如果将负值赋给无符号变量会发生什么? [英] What happens if I assign a negative value to an unsigned variable?

问题描述

我很想知道如果我将一个负值赋给一个无符号变量会发生什么.

I was curious to know what would happen if I assign a negative value to an unsigned variable.

代码看起来有点像这样.

The code will look somewhat like this.

unsigned int nVal = 0;
nVal = -5;

它没有给我任何编译器错误.当我运行程序时，nVal 被分配了一个奇怪的值！会不会是一些 2 的补码值被分配给了 nVal?

It didn't give me any compiler error. When I ran the program the nVal was assigned a strange value! Could it be that some 2's complement value gets assigned to nVal?

推荐答案

官方回答 - 第 4.7 节 conv.integral

For the official answer - Section 4.7 conv.integral

"如果目标类型是无符号的，则结果值是与源整数一致的最小无符号整数(模 2ⁿ 其中 n 是使用的位数来表示无符号类型.[注意:在二进制补码表示中，这种转换是概念性的，位模式没有变化(如果没有截断).-end note]

"If the destination type is unsigned, the resulting value is the least unsigned integer congruent to the source integer (modulo 2ⁿ 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 ]

这实质上意味着，如果底层架构存储在不是二进制补码的方法中(如有符号幅度或一个补码)，则转换为无符号的行为必须表现得好像它是二进制补码一样.

This essentially means that if the underlying architecture stores in a method that is not Two's Complement (like Signed Magnitude, or One's Complement), that the conversion to unsigned must behave as if it was Two's Complement.

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