You may not know # define usage: concatenate a string through # define

I recently read the com-related documents. When I saw that csf-target implemented the COM interface, I read some macro definitions in the afxdisp. h header file.

# Define begin_interface_part (localclass, baseclass )\
Class X # localclass: Public baseclass \

This macro definition is easy to understand, but here is an extra X #. I have never seen such a definition, and I don't know what it means.
I asked a few friends, but I didn't know either.

Do you know?

Maybe you don't know ~ Well, I finally found the relevant materials, explained the define, and also met the other two less commonly used define

# Define conn (x, y) x # Y
# Define tochar (x) # @ x
# Define tostring (x) # x

X ## what does y mean? X connects to Y, for example:
Int n = conn (123,456); the result is n = 123456;
Char * STR = conn ("ASDF", "ADF") returns STR = "asdfadf ";
How about it? It's amazing.

Let's take a look at # @ X. In fact, a single quotation mark is added to X, and the returned result is a const char. For example:
Char A = tochar (1); the result is a = '1 ';
Make an out-of-bounds test char a = tochar (123); the result is a = '3 ';
However, if your parameter contains more than four characters, the compiler reports an error! Error c2015: too failed
Characters in constant: P

Finally, let's take a look at # X. You may also understand that it adds double quotation marks to X.
Char * STR = tostring (123132); then STR = "123132 ";

Finally, let's leave a few small tests for everyone to test:
# Define Dec (x, y) (x-y)
Int n = Dec (A (1230 );
N = conn (123, Conn (123,332 ));
Char * STR = a ("12", tostring (Dec (3, 1 ));
What will happen? Hey hey ~

Other things:
I first defined a macro
# Define te text
I want to define another macro to convert te into a string ("text ").
Definition:
# Define STR (STR )??? // Replace STR (TE) with "text ".
Excuse me ??? How to implement this part? # STR cannot achieve the goal. '"' # STR # '"' is also incorrect.

# Secondary replacement is not allowed, and an intermediate macro can be defined.
# 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 in the ARR array? Isn't define a simple replacement of characters?

# Define a "AAA"
# Define B "BBB"
I'm not familiar with Macro ...... However, printf ("% s \ n", B) can be successful );

In # define, only the # And # operations are defined. # Used to convert parameters into strings. # used to connect two parameters before and after, and convert 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;
}
The output is
[Leshy @ leshy SRC] $./A. Out
Token 9 = 10

Additionally, when # define is used for definition, you can use a slash ("\") to continue the line ("
_ ") Same.
For example:
# Define Add1 (x, y) (x + y)
It can also be expressed:
# Define Add1 (x, y )\
(X + y)

Three common usage
1.
C: # define x 100 ----> C +++: const int x = 100;

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

3.
# Ifndef null
# Define null (void *) 0)