#,## in C + +, and "

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Want to apply macros flexibly, inseparable # from and ## .

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learning # and ## , first look at a " Example:

#include <stdio.h>#include <string.h>int Main(){    Const Char* P1 = "Hello," "world!";     //A space    Const Char* P2 = "Hello,"    "world!";  //Multiple spaces    Const Char* P3 = " Hello, " "world!.";      //No spaces    Const Char* P4 = "hello,world!";        //A whole string    Const Char* P5 = " Nihao, " "shijie!.";  //A different string    printf("P1 =%s, strlen (p1) =%d\ n", P1, strlen(P1));    printf("P2 =%s, strlen (p2) =%d\ n", P2, strlen(P2));    printf("p3 =%s, strlen (p3) =%d\ n", P3, strlen(P3));    printf("P4 =%s, strlen (p4) =%d\ n", P4, strlen(P4));    printf("P5 =%s, strlen (p5) =%d\ n", P5, strlen(P5));    return 0;}

The output is:

P1 = hello,world!, strlen (p1) = 12p2 = hello,world!, strlen (p2) = 12P3 = hello,world!, Strle N (p3) = 12P4 = hello,world!, strlen (p4) = 12P5 = nihao,shijie!, strlen (p5) =  

View the constant string segment of a PE file and discover that only one string exists after the compiler has been optimized Hello,World! .

That is to say, p1,p2,p3,p4 these four kinds of writing is equivalent, this is the premise that explains # usage afterwards.

String Operation (#)

When used as a string operation, # the primary role is to convert macro parameters to string constants without being extended.
Points:

• The space between the left and right sides of the macro definition parameter is ignored, and multiple spaces between tokens of the parameter are converted to a single space.
  
• The
• macro definition parameter contains the need for special meaning word such as " or \ , they are automatically preceded by an escape character \ .
  

or through the MSDN example to see easy to understand:

 #define F ABC   #define B def   #define FB (ARG) #arg  # Define FB1 (ARG) FB (ARG)  fb   ( f  b  )  fb1   ( f  b  )  

When I first saw that the two lines pre-compile the effect is the same, but see the use of GCC-E compiled code, it is understood that MSDN on the "no extension" has a deeper understanding of the actual pre-compiled code:

"F B";"abc def";

Presumably the transformation process should be as follows:

FB(F B) --> #F B -->"F B"  FB1(F B) --> FB1(abc def) --> FB(abc def) --> #abc def --> "abc def"

Tag Join operation (# #)

Connect multiple tokens to a token.
Points:

• It cannot be the first or last token in a macro definition.
  
• Before and after the space is optional.
  

To understand the examples on MSDN:

#include <stdio.h>#define Paster (n) printf_s ("token" #n "=%d", token# #n)int Token9 = 9;int Main(){   Paster(9);}

paster(9);The preprocessing steps should be as follows:

• paster(9);
  
• printf_s( "token" #9 " = %d", token##9 );
  
• printf_s( "token" "9" " = %d", token9 );
  
• printf_s( "token9 = %d", token9 );
  

This should be a good understanding.

Refer to MSDN

• Stringizing Operator (#): http://msdn.microsoft.com/EN-US/library/7e3a913x (V=vs.110,d=hv.2). aspx
  
• Token-pasting Operator (# #): Http://msdn.microsoft.com/EN-US/library/09dwwt6y (V=vs.110,d=hv.2). aspx

#,## in C + +, and "