thanks to
SuperPlayeR, I saw the define 's especial usage like this :
(1)#define Conn(x,y) x##y
(2)#define ToChar(x) #@x
(3)#define ToString(x) #x
for (1) Conn(1,3) --> 13
Conn("ksk","l") --> "kskl"
for (2) if x 's length less than 3 , it returns the rightest const char else error
E.G ToChar(123) --> '3'
ToChar(1234) -->Error
for (3) ToString (1234) -->"1234"