Identifier Not defined in Macro - c++

Question

I was trying the below program
#include <iostream>
using namespace std;
#define MKSTR(x) #x
#define CONCATE( x , y ) (x)##(y)
int main()
{
int xy = 100;
cout << MKSTR(HELLO C++) << endl;
cout << CONCATE(HELLO,C++) << endl;
cout << CONCATE(x,y) << endl;
return 0;
}
and getting the error
error C2065: 'HELLO' : undeclared identifier
. I don't see see why the VS 2012 compiler is expecting the macro argument or is treating them as identifier. Also the MKSTR macro worked fine but the CONCATE macro is giving me trouble. Can't understand why compiler is doing so.

Answer 1

You need to concatenate symbols first, and then expand it in a string, like this (compiles in GCC 4.8.1) :
#include <iostream>
using namespace std;
#define MKSTR(x) #x
#define CONCATE( x , y ) x ## y
#define CONCATESTR( x , y ) MKSTR(x ## y)
int main()
{
int xy = 100;
cout << MKSTR(HELLO C++) << endl;
cout << CONCATESTR(HELLO,C++) << endl;
cout << CONCATE(x,y) << endl;
return 0;
}
output :
HELLO C++
HELLOC++
100
EDIT :
So for the question of why MKSTR(CONCAT(...)) is not working, the reason is because of the expansion order of macros.
Because CONCAT is a parameter of MKSTR and MKSTR uses operator #, the argument is not expanded but instead immediately stringified. You could do this instead to make it work :
#include <iostream>
using namespace std;
#define CONCATE( x , y ) x ## y
#define MKSTR(x) #x
#define MKSTR2(x) MKSTR(x)
#define CONCATESTR( x , y ) MKSTR(x ## y)
int main()
{
int xy = 100;
cout << MKSTR2(HELLO C++) << endl;
cout << MKSTR2(CONCATE(HELLO,C++)) << endl;
cout << CONCATE(x,y) << endl;
return 0;
}
and it will output what you expect.