C++宏#号的用法

1. ＃:在宏展开的时候会将#后面的参数替换成字符串，如：
          ＃define p(exp) printf(#exp);
       调用p(hello)的时候会将#exp换成”hello”

The # operator should not be confused with the null directive.

Use the # operator in a function-like macro definition according to the following rules:

• A parameter following # operator in a function-like macro is converted into a character string literal containing the argument passed to the macro.
• White-space characters that appear before or after the argument passed to the macro are deleted.
• Multiple white-space characters imbedded within the argument passed to the macro are replaced by a single space character.
• If the argument passed to the macro contains a string literal and if a \ (backslash) character appears within the literal, a second \ character is inserted before the original \ when the macro is expanded.
• If the argument passed to the macro contains a " (double quotation mark) character, a \ character is inserted before the " when the macro is expanded.
• The conversion of an argument into a string literal occurs before macro expansion on that argument.
• If more than one ## operator or # operator appears in the replacement list of a macro definition, the order of evaluation of the operators is not defined.
• If the result of the macro expansion is not a valid character string literal, the behavior is undefined.

Example: # operator

The following examples demonstrate the use of the # operator:

   #define STR(x)        #x
   #define XSTR(x)       STR(x)
   #define ONE           1

Invocation	Result of Macro Expansion
STR(\n "\n" '\n')	"\n \"\\n\" '\\n'"
STR(ONE)	"ONE"
XSTR(ONE)	"1"
XSTR("hello")	"\"hello\""

2. ##:将前后两个的单词拼接在一起。例如《The C Programming Language》中的例子：
          #define cat(x,y) x##y
       调用cat(var, 123)展开后成为var123.
3. #@:将值序列变为一个字符
          #define ch(c)  #@c
       调用ch(a)展开后成为’a’.


ref:
http://publib.boulder.ibm.com/infocenter/iadthelp/v7r0/index.jsp?topic=/com.ibm.etools.iseries.langref.doc/ilcrefer17.htm
http://www.trueeyu.com/?p=505

作者：翁志艺