variable + value macro expansion - c++

Question

I tried without any result.
My code looks like this:
#include "stdafx.h"
#include <iostream>
#define R() ( rand() )
#define H(a,b) ( a ## b )
#define S(a) ( # a )
#define CAT() H(S(distinct_name_), R())
int main(int argc, _TCHAR* argv[])
{
std::cout << CAT() << std::endl;
std::cout << CAT() << std::endl;
std::cout << CAT() << std::endl;
return 0;
}
I would like to get a result like this:
distinct_name_12233
distinct_name_147
distinct_name_435
as a result of concatenating
distinct_name_ (##) rand()
Right now I am getting an error:
term does not evaluate to a function taking 1 arguments.
Is this achievable ??
EDIT:
I finally succeeded after couple of hours. The preprocessor still does strange things I cannot understand completely. Here it goes:
#include "stdafx.h"
#include <iostream>
class profiler
{
public:
void show()
{
std::cout << "distinct_instance" << std::endl;
}
};
#define XX __LINE__
#define H(a,b) ( a ## b )
#define CAT(r) H(distinct_name_, r)
#define GET_DISTINCT() CAT(XX)
#define PROFILE() \
profiler GET_DISTINCT() ;\
GET_DISTINCT().show() ; \
int main(int argc, _TCHAR* argv[])
{
PROFILE()
PROFILE()
return 0;
}
And the output is:
distinct_instance
distinct_instance
Thanks #Kinopiko for __LINE__ hint. :)

Answer 1

No, you can't do that. Macros are a compile-time thing and functions are called only at run time, so there's no way you could get a random number from rand() into your macro expansion.

Answer 2

I see that a lot of people have already correctly answered this question, but as an alternative suggestion, if your preprocessor implements __TIME__ or __LINE__ you could get a result quite like what you want, with a line number or time concatenated, rather than a random number.

Answer 3

What you are actually getting is...
std::cout << distinct_name_rand() << std::endl;
distinct_name_rand() isn't a function, so it fails with a compile error.
Macros don't execute functions during compile time.

Answer 4

You must pass run-time computed value to the macro since macro are evaluated at compile-time.
Try:
#define H(a,b) ( a ## b )
#define S(a) ( # a )
#define CAT(r) H(S(distinct_name_), r)
std::cout << CAT(rand()) << std::endl;