C++ 在#define 宏中转义# 符号？
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/1135822/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
Escaping a # symbol in a #define macro?
提问by Rob
Without going into the gory details I want to use a #definemacro that will expand to a #includebut the '#' sign is confusing the preprocessor (as it thinks I want to quote an argument.)
无需深入细节，我想使用一个#define宏，该宏将扩展为 a#include但“#”符号使预处理器感到困惑（因为它认为我想引用一个参数。）
For example, I want to do something like this:
例如，我想做这样的事情：
#define MACRO(name) #include "name##foo"
And use it thus:
并使用它：
MACRO(Test)
Which will expand to:
这将扩展到：
#include "Testfoo"
The humble # sign is causing the preprocessor to barf. MinGW gives me the following error:
不起眼的# 符号导致预处理器呕吐。MinGW 给了我以下错误：
'#' is not followed by a macro parameter
'#' is not followed by a macro parameter
I guess I need to escape the # sign but I don't if this is even possible.
我想我需要转义 # 符号，但如果这可能的话，我不需要。
Yes, macros are indeed evil...
是的，宏确实是邪恶的......
采纳答案by EFraim
As far as I remember you cannot use another preprocessor directive in define.
据我所知，你不能在定义中使用另一个预处理器指令。
回答by ybungalobill
It ispossible to insert a hash token into the preprocessed token stream. You can do it as follows:
它是可以插入标记插入到预处理令牌流的哈希值。你可以这样做：
#define MACRO(hash, name) hash include name
MACRO(#,"hello")
—expands to:
——扩展为：
# include "hello"
However, the standard explicitly rules out any further analysis of such line for the existence of preprocessing directives [cpp.rescan]:
但是，该标准明确排除了对预处理指令 [cpp.rescan] 存在的此类行的任何进一步分析：
The resulting completely macro-replaced preprocessing token sequence is not processed as a preprocessing directive even if it resembles one.
生成的完全宏替换的预处理标记序列不会作为预处理指令处理，即使它类似于一个指令。
回答by MSalters
The problem isn't actually getting a # symbol in the output of your preprocessor.
问题实际上并不是在预处理器的输出中得到 # 符号。
Apparently you want the preprocessor to reparse your file, to deal with newly created #include directives as part of macro expansion. It doesn't work that way. If a line starts with #, it's an instruction for the preprocessor and interpreted. If a line doesn't start with #, it's only subject to preprocessor transformation including macro substitution. This is a once-per-line test.
显然，您希望预处理器重新解析您的文件，以处理新创建的 #include 指令作为宏扩展的一部分。它不会那样工作。如果一行以# 开头，则它是预处理器和解释器的指令。如果一行不以 # 开头，则它仅受预处理器转换的影响，包括宏替换。这是每行一次的测试。
MACRO(Test)
does not start with #. Therefore it is not interpreted as a preprocessor directive; instead it's subject to macro replacement rules.
不以#开头。因此它不被解释为预处理器指令；相反，它受宏替换规则的约束。
回答by Martin York
This is because the # has special meaning when used in a macro.
这是因为 # 在宏中使用时具有特殊含义。
# means quote the following token (which should be a macro parameter name)
## means concatenate the preceding and following tokens.
In your situation the # is not followed by a proper token. So in your situation we need to go through a level of indirection:
在您的情况下，# 后面没有正确的标记。所以在你的情况下，我们需要通过一个间接级别：
#define QUOTE(name) #name
#define TEST(name) QUOTE(name ## foo)
#include TEST(scot)
回答by John Bode
You can't do that. Preprocessor directives are recognized before macro expansion; if the macro expands into something that looks like a preprocessor directive, that directive will not be recognized. The best you can do is create a macro for the file name:
你不能那样做。在宏扩展之前识别预处理器指令；如果宏扩展为看起来像预处理器指令的内容，则将无法识别该指令。您能做的最好的事情是为文件名创建一个宏：
#define MACRO(name) "name##foo"
...
#include MACRO(Test)
回答by David F.
#define HASH_SIGN #
BOOST_PP_CAT(HASH_SIGN, include)
回答by Sam Harwell
This mightwork (it works for regular #definemacros with no parameters, but I haven't tested it with macros with parameters).
这可能有效（它适用于#define不带参数的常规宏，但我还没有用带参数的宏对其进行测试）。
#define MACRO(name) <name##foo>
#include MACRO(Test)
回答by user6858363
#define PARAM_NAME Param
#define GETNAME_(a) #a
#define GETNAME(a) GETNAME_(a)
int Param;
printf("%s = %i\n", GETNAME(PARAM_NAME), PARAM_NAME);
