C++ macro: The ## operator

1. The `##` operator

The ## (double number sign) operator concatenates two tokens in a macro invocation (text and/or arguments) given in a macro definition.

The last token of the argument for x is concatenated with the first token of the argument for y.
x 的自变量的最后一个标记与 y 的自变量的第一个标记合并。

#define XY(x,y) x##y

Use the ## operator according to the following rules:

The ## operator cannot be the very first or very last item in the replacement list of a macro definition.
## 运算符不能是宏定义的替换列表中的第一个或最后一个项。
The last token of the item in front of the ## operator is concatenated with first token of the item following the ## operator.
## 运算符前面的项的最后一个标记与 ## 运算符后面的项的第一个标记合并。
Concatenation takes place before any macros in arguments are expanded.

在展开自变量中的任何宏之前进行合并。
If the result of a concatenation is a valid macro name, it is available for further replacement even if it appears in a context in which it would not normally be available.

如果合并的结果是有效的宏名称，那么即使它出现在通常不可用的上下文中，也可以进行进一步替换。
If more than one ## operator and/or # operator appears in the replacement list of a macro definition, the order of evaluation of the operators is not defined.

#define ArgArg(x, y) x##y
#define ArgText(x) x##TEXT
#define TextArg(x) TEXT##x
#define TextText TEXT##text
#define CHENG 1
#define YONGQIANG 2

ArgArg(123, 456)
ArgArg(yong, qiang)
ArgText(name)
TextArg(book)
TextText
ArgArg(YONG, QIANG)

Result of macro expansion:

123456
yongqiang
nameTEXT
TEXTbook
TEXTtext
2