C++中特殊的宏定义
常规用法不再介绍,做如下几点说明和介绍
1. 带参数的宏只完成简单字符替换,之前不做计算实参的工作,如下
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
结果应该是 s=a+b+b*c+b+c=10。
2. define中的特殊标识符
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
结果是 a=1234,c='a',c="1234";
可以看出 ## 是简单的连接符,#@用来给参数加单引号,#用来给参数加双引号即转成字符串。
更神奇的是
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
char *s = x(a b/**/ c);
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
结果是*s="a b c",*p="a//nb",#s果然很厉害
3.define的多行定义
define可以替代多行的代码
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
关键是要在每一个换行的时候加上一个"/"
由此联想到了C中的关于字符串的一些默认规则
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
其中只有s4会产生编译错误,s1="abcefg",s2="abcefg",s3="abc"
4. 宏调用顺序
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
结果是a=5;可见一个宏的参数是另一个宏,先替换外面的宏,后替换参数。即Conn(A,B)=>AB,后AB=>5
5.解决重复定义的问题
由于头文件包含可以嵌套,那么C文件就有可能包含多次同一个头文件,就可能出现重复定义的问题的。
通过条件编译开关来避免重复包含(重复定义)
例如
#ifndef __headerfileXXX__
#define __headerfileXXX__
…
文件内容
…
#endif