如何在两个宏之间插入一个点，而不插入空格？ [英] How do I concatenate two macros with a dot between them without inserting spaces?

问题描述

我正在预处理InfoPlist文件以包括我的修订号。我的标头如下所示：

  #import svn.h 
 
 #define APP_VERSION 1.0 
 #define APP_BUILD APP_VERSION.SVN_REVISION 
  

当我从程序中检查构建版本时，它是 1.0。 123456 。但是，如果我尝试这样做：

  #import svn.h 
 
 #define APP_VERSION 1.0 
＃定义APP_BUILD APP_VERSION ## ## SVN_REVISION 
  

我知道

 错误：粘贴格式为'APP_VERSION。'，无效的预处理令牌
错误：粘贴格式为'.SVN_REVISION'，无效的预处理令牌
  

我见过，但实际上并没有给出答案； OP实际上并不需要连接令牌。我做。如何在两个宏之间用点连接而不插入空格？

解决方案

问题似乎是由< href = http://gcc.gnu.org/onlinedocs/cpp/Argument-Prescan.html#Argument-Prescan rel = noreferrer>预处理器的怪癖：串联运算符aren的参数首先不进行扩展（或者……不管怎样，规则都很复杂），所以当前预处理器不尝试连接 1.0 和。 / code>，它实际上是试图将单词 APP_VERSION 粘贴到输出令牌中。单词在C中没有点，因此这不是一个有效的标记，因此会出错。

通常可以通过几次操作来强制执行此操作。包装宏层，以便将连接操作隐藏在至少两个替换项之后，例如：

  #define APP_VERSION 1.0 
＃定义SVN_REVISION 123456 
 
＃定义M_CONC（A，B）M_CONC_（A，B）
＃定义M_CONC_（A，B）A ## B 
 
＃定义APP_BUILD M_CONC（APP_VERSION，M_CONC（。，SVN_REVISION））
 
 APP_BUILD //扩展为单个令牌1.0.123456 
  

您很幸运，因为C预处理程序编号
= noreferrer>可以根据需要添加任意数量的点。

I'm preprocessing my InfoPlist file to include my revision number. My header looks like this:

#import "svn.h"

#define APP_VERSION 1.0
#define APP_BUILD APP_VERSION.SVN_REVISION

When I check my build version from within the program, it's 1.0 . 123456. But if I try this:

#import "svn.h"

#define APP_VERSION 1.0
#define APP_BUILD APP_VERSION ## . ## SVN_REVISION

I get

error: pasting formed 'APP_VERSION.', an invalid preprocessing token
error: pasting formed '.SVN_REVISION', an invalid preprocessing token

I've seen this question but it doesn't actually give an answer; the OP didn't actually need to concatenate the tokens. I do. How do I concatenate two macros with a dot between them without inserting spaces?

解决方案

The problem looks like it is being caused by a quirk of the preprocessor: arguments to the concatenation operator aren't expanded first (or... whatever, the rules are complicated), so currently the preprocessor isn't trying to concatenate 1.0 and ., it's actually trying to paste the word APP_VERSION into the output token. Words don't have dots in them in C so this is not a single valid token, hence the error.

You can usually force the issue by going through a couple of layers of wrapper macros so that the concatenation operation is hidden behind at least two substitutions, like this:

#define APP_VERSION 1.0
#define SVN_REVISION 123456

#define M_CONC(A, B) M_CONC_(A, B)
#define M_CONC_(A, B) A##B

#define APP_BUILD M_CONC(APP_VERSION, M_CONC(.,SVN_REVISION))

APP_BUILD    // Expands to the single token 1.0.123456

You're in luck in that a C Preprocessor number is allowed to have as many dots as you like, even though a C float constant may only have the one.

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