C ++中的多语句宏 [英] Multi-statement Macros in C++

问题描述

在C ++ 中，是否可以像下面这样创建一个带有 nested if语句的多语句宏?我已经尝试了一段时间，第二个if语句无法看到' symbol '，我遇到了范围问题.也许我需要进一步了解宏.

In C++, is it possible to make a multi-statement macro with nested if statements inside of it like the one below? I've been attempting it for a while now and I'm getting a scope issue for the second if statement not being able to see 'symbol'. Maybe I need to understand macros further.

#define MATCH_SYMBOL( symbol, token)
     if(something == symbol){
          if( symbol == '-'){
          }else if (symbol != '-'){
          }
     other steps;
     }

推荐答案

对于多行宏，您需要在除最后一行以外的所有末尾添加\字符，以告诉宏处理器继续解析下一行中的宏，如下所示:

For a multi-line macro you need to add a \ character to the end of all but the last line to tell the macro processor to continue parsing the macro on the next line, like so:

#define MATCH_SYMBOL( symbol, token) \
     if(something == symbol){        \
          if( symbol == '-'){        \
          }else if (symbol != '-'){  \
          }                          \
     other steps;                    \
     }

现在，它试图将其解释为1行宏，然后在文件顶部显示一些实际代码，这不是您想要的:

Right now, it's trying to interpret it as a 1-line macro and then some actual code at the top of your file, which isn't what you want:

#define MATCH_SYMBOL( symbol, token)

// and then... wrongly thinking this is separate...

 if(something == symbol){ // symbol was never defined, because the macro was never used here!
      if( symbol == '-'){
      }else if (symbol != '-'){
      }
 other steps;
 }

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