“12345”+ 2在C是做什么的?

I've seen this done in C before:

我以前在C中见过这种情况:

#define MY_STRING "12345"
...
#define SOMETHING (MY_STRING + 2)

What does SOMETHING get expanded to, here? Is this even legal? Or do they mean this?:

什么东西会膨胀到这里?这是即使是合法吗?或者他们是这个意思?

#define SOMETHING (MY_STRING[2])

2 个解决方案

#1

String literals exist in the fixed data segment of the program, so they appear to the compiler as a type of pointer.

在程序的固定数据段中存在字符串常量，因此它们作为指针的类型出现在编译器中。

+-+-+-+-+-+--+
|1|2|3|4|5|\0|
+-+-+-+-+-+--+
 ^ MY_STRING
     ^ MY_STRING + 2

#2

When you have an array or pointer, p+x is equivalent to &p[x]. So MY_STRING + 2 is equivalent to &MY_STRING[2]: it yields the address of the third character in the string.

当你有一个数组或指针时，p+x等于&p[x]。MY_STRING + 2等价于&MY_STRING[2]:它生成字符串中第三个字符的地址。

Notice what happens when you add 0. MY_STRING + 0 is the same as &MY_STRING[0], both of which are the same as writing simply MY_STRING since a string reference is nothing more than a pointer to the first character in the string. Happily, then, the identity operation "add 0" is a no-op. Consider this a sort of mental unit test we can use to check that our idea about what + means is correct.

注意加0会发生什么。MY_STRING + 0与&MY_STRING[0]是相同的，两者都与只写MY_STRING相同，因为字符串引用仅仅是字符串中第一个字符的指针。令人高兴的是，身份操作“添加0”是一个禁止操作。考虑到这是一种心理单元测试，我们可以用它来检验“+”的意思是否正确。

#1