是否使用 char 未定义行为访问数组元素? [英] Is accessing an array element using a char undefined behaviour?

问题描述

由于不清楚什么是未定义行为，什么不是 C 语言，我想知道使用 char 访问数组元素是否是未定义行为.例如:

Since it's not clear what's undefined behaviour and what's not in C, I'm wondering if accessing an array element using a char is or not undefined behaviour. For example:

char c = 'A';
int a[3000];
printf("%i\n", a[c]);

我知道实际上字符和整数在某种程度上是可以互换的，但我仍然不确定.

I know that actually chars and ints are somehow interchangeable, but still, I'm not sure.

推荐答案

是否使用 char 未定义行为访问数组元素?

Is accessing an array element using a char undefined behaviour?

这不是未定义的行为.它的工作方式类似于另一种整数类型.然而，char 的数值可能令人惊讶地为负数.

It is not undefined behavior. It works like another integer type. Yet the numeric value of a char may surprisingly be negative.

char 与 signed char 或 unsigned char 具有相同的范围.它是实现定义的.

A char has the same range as signed char or an unsigned char. It is implementation defined.

使用 c 作为索引是可以的，如果提升的索引加上指针会产生一个有效的内存地址.详细信息:char 将被提升为 int，或可能的 unsigned.

Using c as an index is fine, if the promoted index plus the pointer results in a valid memory address. Detail: A char will be promoted to int, or possible unsigned.

如果 c 为负值，则以下可能是一个问题.在 OP 的情况下，使用 ASCII 编码，'A' 的值为 65，因此 0 <= 65 <;3000.@Joachim Pileborg

The following is potentially a problem had c had a negative value. In OP's case, with ASCII encoding, 'A' has the value of 65, so it does not have a problem as 0 <= 65 < 3000. @Joachim Pileborg

char c = 'A';
int a[3000] = { 0 };
printf("%i\n", a[c]);  // OK other than a[] not initialize in OP's code.

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