使用 ## 和 __LINE__ 创建 C 宏(与定位宏的标记连接)

问题描述

我想创建一个 C 宏来创建一个基于名称的函数行号上.我想我可以做类似的事情(真正的函数会在大括号内声明):

I want to create a C macro that creates a function with a name basedon 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) {}

我希望可以扩展为:

static void Unique_23(void) {}

这行不通.使用令牌连接，定位宏被逐字处理，最终扩展为:

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

static void Unique___LINE__(void) {}

这可能吗?

推荐答案

问题是当你有一个宏替换时，预处理器只会递归地扩展宏，如果字符串化操作符 # 和标记粘贴运算符 ## 应用于它.因此，您必须使用一些额外的间接层，您可以使用带有递归扩展参数的标记粘贴运算符:

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__ 在 UNIQUE 的扩展过程中被扩展为行号(因为它不涉及 # 或 ##)，然后令牌粘贴发生在TOKENPASTE的扩展过程中.

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.

还应注意，还有 __COUNTER__ 宏，每次计算时都会扩展为一个新整数，以防您需要对 UNIQUE 宏在同一行.注意:MS Visual Studio、GCC(自 V4.3 起)和 Clang 支持 __COUNTER__，但不是标准 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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