与##和__LINE__（带定位宏令牌串联）创建C代码宏 [英] Creating C macro with ## and __LINE__ (token concatenation with positioning macro)

问题描述

我要创建一个基于一个名字创建了一个函数是C宏
上的行号。
我以为我可以像做（真正的功能将有括号内的语句）：

I want to create a C macro that creates a function with a name based on the line number. I thought I could do something like (the real function would have statements within the braces):

#define UNIQUE static void Unique_##__LINE__(void) {}

这点我希望这将扩大到是这样的：

Which I hoped would expand to something like:

static void Unique_23(void) {}

这是行不通的。随着令牌串联，定位宏
字面上处理，结束了扩大为：

That doesn't work. With token concatenation, the positioning macros are treated literally, ending up expanding to:

static void Unique___LINE__(void) {}

这是可能的吗？

（是的，有一个真正的原因，我想这样做再好也没用，这似乎）。

(Yes, there's a real reason I want to do this no matter how useless this seems).

推荐答案

的问题是，当你有一个宏替换，则preprocessor只会扩大宏递归如果既字符串化操作符 ＃也不令牌粘贴运算符 ## 被应用到它。所以，你必须使用间接一些额外的图层，可以使用令牌粘贴运营商递归扩展的参数：

The problem is that when you have a macro replacement, the preprocessor will only expand the macros recursively if neither the stringizing operator # nor the token-pasting operator ## are applied to it. So, you have to use some extra layers of indirection, you can use the token-pasting operator with a recursively expanded argument:

#define TOKENPASTE(x, y) x ## y
#define TOKENPASTE2(x, y) TOKENPASTE(x, y)
#define UNIQUE static void TOKENPASTE2(Unique_, __LINE__)(void) {}

然后， __ LINE __ UNIQUE （扩建过程中被扩展到行号，因为它不会参与任何＃或 ## ），然后标记粘贴的扩张过程中会发生 TOKENPASTE 。

Then, __LINE__ gets expanded to the line number during the expansion of UNIQUE (since it's not involved with either # or ##), and then the token pasting happens during the expansion of TOKENPASTE.

还应当指出的是，也 __计数器__ 宏，其中每个被评价时间扩展为一个新的整数，以便在需要时具有的多个实例在同一行的 UNIQUE 宏。注： __ __ COUNTER 由MS Visual Studio中，海湾合作委员会（自V4.3），并支持锵，而不是标准的C

It should also be noted that there is also the __COUNTER__ macro, which expands to a new integer each time it is evaluated, in case you need to have multiple instantiations of the UNIQUE macro on the same line. Note: __COUNTER__ is supported by MS Visual Studio, GCC (since V4.3), and Clang, but is not standard C.

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