c - 如何理解在宏中有# 或## 的情况下重新扫描替换 token 序列以获得更多定义的标识符？

Question

来自 KRC 的 The C Programming Language，关于宏定义中的运算符 # 和 ##

Two special operators influence the replacement process. First, if an occurrence of a parameter in the replacement token sequence is immediately preceded by #, string quotes (") are placed around the corresponding parameter, and then both the # and the parameter identifier are replaced by the quoted argument. A \ character is inserted before each " or \ character that appears surrounding, or inside, a string literal or character constant in the argument.

Second, if the definition token sequence for either kind of macro contains a ## operator, then just after replacement of the parameters, each ## is deleted, together with any white space on either side, so as to concatenate the adjacent tokens and form a new token. The effect is undefined if invalid tokens are produced, or if the result depends on the order of processing of the ## operators. Also, ## may not appear at the beginning or end of a replacement token sequence.

In both kinds of macro, the replacement token sequence is repeatedly rescanned for more defined identifiers. However, once a given identifier has been replaced in a given expansion, it is not replaced if it turns up again during rescanning; instead it is left unchanged.

我无法理解最后一段，尤其是加粗的句子。

您能否改写一下，和/或举一些例子？谢谢。

Answer 1

考虑片段:

#define A   B + C
#define B   1
#define C   2

int k = A;

在这种情况下第一次通过

将替换A:

int k = B + C;

第二遍将替换 B 和 C

int k = 1 + 2;

现在考虑另一个片段:

#define A   B + C
#define B   A
#define C   A

int k = A;

现在第一遍将展开A一次，和以前一样:

int k = B + C;

第二个将像以前一样替换 B 和 C:

int k = A + A;

但是这里它会停止，因为 A 在第一遍之前已经展开了。