printf 类型提升和符号扩展 [英] printf type promotion and sign extension

问题描述

我对在 printf 和一般情况下如何进行类型提升感到困惑.我尝试了以下代码

I am confused about how type promotion happens in case of printf and in general. I tried the following code

unsigned char uc = 255
signed char sc = -128

printf("unsigned char value = %d \n", uc);
printf("signed char value = %d \n", sc);

这给出了以下输出:

unsigned char value = 255
signed char value = -128

这让我想知道促销实际上是如何进行的，以及是否会发生标志扩展.如果进行了符号扩展，则值 255 应打印为负值(-128 保持不变)，如果未进行符号扩展，则应将 -128 打印为正值(255 保持不变).请解释.

This has left me wondering about how promotion actually takes place and whether a sign extension happens or not. If a sign extension is done then the value 255 should be printed as negative value (-128 remaining the same) and if no sign extension is done then -128 should have been printed as a positive value (255 remaining the same). Please explain.

推荐答案

如果进行了符号扩展，则值 255 应打印为负值

If a sign extension is done then the value 255 should be printed as negative value

这就是你错的地方 - unsigned char 类型的所有值，包括 255，都可以用 int 表示，所以升级为 int 来自 unsigned char 只是发生了没有任何有趣的事情.

This is where you're wrong - all values of type unsigned char including 255, can be represented in a int, so the promotion to int from unsigned char just happens without any funny business.

出现问题的地方是当有符号值必须转换(这与提升不同，并且为操作数创建通用类型)为无符号值.如果该有符号类型具有负值，则转换为无符号类型将更改该值.

Where problems occur is when a signed value must be converted (which is a different thing than promotion, and occurs to create a common type for operands) to an unsigned value. If that signed type has a negative value, then the conversion to an unsigned type will change the value.

综上所述，整数提升保值(包括符号)，转换可以改变值.

In summary, integer promotion preserves value (including sign), conversion can change value.

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