C++ help concatenating TCHAR - c++

Question

I recently learned of the ## functionality that i can define in the beginning of my code. I'm trying to compile the following code:
#include <windows.h>
#include <tchar.h>
#include <iostream>
#include <stdio.h>
#include <string>
#define paste(x,y) *x##*y
int main()
{
TCHAR *pcCommPort = "COM";
TCHAR *num = "5";
cout << paste(pcCommPort,num);
return 0;
}
and i keep getting the following error:
expression must have arithmetic or unscoped enum type
it's not liking the fact that i'm using pointers in my "define paste" line. Without any pointers, it'll just return the variable "pcCommPort5." what I want is "COM5."
I've tried _tcscat, strcat, strcat_s, visual studio didn't like any of these....

Answer 1

## doesn't concatenate arbitrary things (especially not strings). What it does it is merges symbols together in the parser into a single symbol.
Let's remove one of those * to see what's going on:
#include <iostream>
#define TO_STRING_HELPER(x) #x
#define TO_STRING(x) TO_STRING_HELPER(x)
#define CONCAT(x, y) *x##y
int main() {
char *pcCommPort = "COM";
char *num = "5";
std::cout << TO_STRING(CONCAT(pcCommPort, num)) << std::endl;
}
Output:
*pcCommPortnum
What CONCAT does in this code is:
Expand x into pcCommPort and y into num. This gives the expression *pcCommPort##num.
Concatenate the two symbols pcCommPort and num into one new symbol: pcCommPortnum. Now the expression is *pcCommPortnum (remember, that last part (pcCommPortnum) is all one symbol).
Finish evaluating the full macro as a * followed by the symbol pcCommPortnum. This becomes the expression *pcCommPortnum. Remember, those are two different symbols: * and pcCommPortnum. The two symbols just follow one after the other.
If we were to try to use *x##*y, what the compiler does is this:
Expand x into pcCommPort and y into num. This gives us the expression *pcCommPort##*num.
Concatenate the two symbols pcCommPort and * into one new symbol: pcCommPort*.
Here, the preprocessor hits an error: the single symbol pcCommPort* is not a valid preprocessing token. Remember, it's not two separate symbols at this point (it is not two symbols pcCommPort followed by *). It is one single symbol (which we call a token).
If you want to concatenate two strings, you're way better off using std::string. You can't* do what you're trying to do with the preprocessor.
*Note, though, that consecutive string literals will be merged together by the compiler (i.e. "COM" "5" will be merged into a single string "COM5" by the compiler). But this only works with string literals, so you'd have to #define pcCommPort "COM" and #define num "5", at which point you could do pcCommPort num (without any further macros) and the compiler would evaluate it to the string "COM5". But unless you really know what you're doing, you really should just use std::string.