It is often useful to merge two tokens into one while expanding macros.
進行宏展開的時候將兩個符號合併成爲一個符號是很有用的。
This is called token pasting or token concatenation.
這就叫做符號拼接。
The ‘##’ preprocessing operator performs token pasting.
##預處理操作符就用來執行符號拼接
When a macro is expanded, the two tokens on either side of each ‘##’ operator are combined into a single token,
進行宏展開的時候，在##兩端的符號被拼成一個符號
which then replaces the ‘##’ and the two original tokens in the macro expansion.
在展開後的結果中，新拼接的符號就取代了兩個舊符號和##
Usually both will be identifiers, or one will be an identifier and the other a preprocessing number.
When pasted, they make a longer identifier.
This isn't the only valid case.
It is also possible to concatenate two numbers (or a number and a name, such as 1.5 and e3) into a number.
Also, multi-character operators such as += can be formed by token pasting.
However, two tokens that don't together form a valid token cannot be pasted together. For example, you cannot concatenate x with + in either order. If you try, the preprocessor issues a warning and emits the two tokens. Whether it puts white space between
the tokens is undefined. It is common to find unnecessary uses of ‘##’ in complex macros. If you get this warning, it is likely that you can simply remove the ‘##’.
Both the tokens combined by ‘##’ could come from the macro body, but you could just as well write them as one token in the first place. Token pasting is most useful when one or both of the tokens comes from a macro argument. If either of the tokens next to
an ‘##’ is a parameter name, it is replaced by its actual argument before ‘##’ executes. As with stringification, the actual argument is not macro-expanded first. If the argument is empty, that ‘##’ has no effect.
Keep in mind that the C preprocessor converts comments to whitespace before macros are even considered. Therefore, you cannot create a comment by concatenating ‘/’ and ‘*’. You can put as much whitespace between ‘##’ and its operands as you like, including
comments, and you can put comments in arguments that will be concatenated. However, it is an error if ‘##’ appears at either end of a macro body.
Consider a C program that interprets named commands. There probably needs to be a table of commands, perhaps an array of structures declared as follows:
一個用來解析命名命令的C程序， 可能需要一張由結構體數組構成的命令列表。
struct command
{
char *name;
void (*function) (void);
};
struct command commands[] =
{
{ "quit", quit_command },
{ "help", help_command },
...
};
It would be cleaner not to have to give each command name twice, once in the string constant and once in the function name.
A macro which takes the name of a command as an argument can make this unnecessary.
The string constant can be created with stringification, and the function name by concatenating the argument with ‘_command’. Here is how it is done:
字符串常數可以用#來產生，函數名就可以採用##拼接產生。
#define COMMAND(NAME) { #NAME, NAME ## _command }
struct command commands[] =
{
COMMAND (quit),
COMMAND (help),
...
};
