Special method for connecting strings by # define [go]

In # define, the standard defines only # and # #两种操作. #用来把参数转换成字符串, # #则用来连接两个前后两个参数, turn them into a string.

#define CONN (x, y) x# #y//connection X, Y

#define TOCHAR (x) #@x//Plus Single quote

#define TOSTRING (x) #x//plus double quotation marks

Recently read the COM related information, see CCmdTarget Implement COM interface, read the definition of some macros, in the Afxdisp.h header file

#define Begin_interface_part (Localclass, baseclass) \
Class x# #localClass: Public baseclass \

Originally this macro definition is easy to understand, but there is a x## here, I really have not seen this usage, do not know what it is intended.
Later asked a few friends also do not know.

You know what?

Perhaps you do not know ~ hehe, at last I still found the relevant information, interpretation of this define, but also by the way to know the other two less commonly used define

#define CONN (x, y) x# #y
#define TOCHAR (x) #@x
#define TOSTRING (x) #x

x# #y表示什么? Represents x connection Y, for example:
int n = Conn (123,456); The result is n=123456;
char* str = Conn ("asdf", "ADF") result is str = "ASDFADF";
It's amazing, isn't it?

To see #@x, in fact, is to add single quotation marks to x, the result is a const char. For example,
char a = ToChar (1); The result is a= ' 1 ';
Do a cross-border test char a = TOCHAR (123); The result is a= ' 3 ';
But if your parameter exceeds four characters, the compiler will give you an error! Error C2015:too many characters in Constant:p

Finally look at #x, and you know, he's giving X double quotes.
char* str = ToString (123132); str= "123132";

Finally leave a few small experiments for everyone to test:
#define DEC (x, y) (X-y)
int n = Dec (A (123,1), 1230);
n = Conn (123, Conn (123,332));
char* str = A ("n", ToString (Dec (3,1));
What would be the result? Hey Hey hehe ~


Some of the other assorted things:
I defined a macro first.
#define TE Text
I want to define another macro that will change Te to a string ("text").
That is defined:
#define STR (str)??? can be replaced with "text" when using str (TE).
Excuse me??? How does the part come about? The use of #str is not up to the goal, with ' ' # #str # # ' is also wrong.

#不能做二次替換, you can define a medium macro again
#define TE Text
#define AD TE
#define STR (AD) #AD
Try

Define two macros directly!
#define TE Text
#define TE_S "Text"


#define STR ABC
Char arr[10] = "STR";
Why is str rather than abc,define not a simple substitution of characters in the arr array?

#define A "AAA"
#define B "BBB" A
I'm not familiar with macro this thing ... However this can succeed printf ("%s\n", B);


In # define, the standard defines only # and # #两种操作. #用来把参数转换成字符串, # #则用来连接两个前后两个参数, turn them into a string.
#include <stdio.h>
#define PASTER (N) printf ("token" #n "=%d\n", token# #n)
int main ()
{
int token9=10;
Paster (9);
return 0;
}
Output to
[Email protected] src]$./a.out
Token 9 = 10

To add: When defining with # define, you can use a slash ("\") to continue the line. With the underscore ("_") in VB.
Like what:
#define ADD1 (x, y) (x + y)
Can also be expressed as:
#define ADD1 (x, y) \
(x + y)

3 General Usage
1.
C: #define X----> C + +: const int x=100;

2.
C: #define MAX (a) > (b) ( A):(B))
C + +: inline int max (int a,int b) {return a>b?a:b;}

3.
#ifndef NULL
#define NULL ((void*) 0)
#endif

#ifndef _myheadfile_h
#define _myheadfile_h
#endif

Special method for connecting strings by # define [go]