Question

I want to create a unique variable name on the fly.

This is my code:

int call(int i)
{
    return i;
}
 
#define XCAT3(a, b, c)    a ## b ## c
 
#define CALL_2(arg, place, line) int XCAT3(cl, place, line) = call(arg);
 
#define CALL_1(arg)    CALL_2(arg, __FUNCTION__, __LINE__)
 
int main(int argc, char* argv[])
{
    CALL_1(1); /* this is line 20 */
    return 0;
}

This does work in GCC (http://ideone.com/p4BKQ) but unfortunately not in Visual Studio 2010 or 2012.

The error message is:

test.cpp(20): error C2143: syntax error : missing ‘;’ before ‘function_string’

test.cpp(20): error C2143: syntax error : missing ‘;’ before ‘constant’

test.cpp(20): error C2106: ‘=’ : left operand must be l-value

How can I create an on-the-fly unique variable name with C++?

Solution:

#define CAT2(a,b) a##b
#define CAT(a,b) CAT2(a,b)
#define UNIQUE_ID CAT(_uid_,__COUNTER__)

int main(int argc, char* argv[])
{
    int UNIQUE_ID = 1;
    int UNIQUE_ID = 2;
    return 0;
}

Answer 1

For a unique identifier, many implementations offer the __COUNTER__ preprocessor macro, that expands to an ever increasing number with every use.

#define CAT(a,b) CAT2(a,b) // force expand
#define CAT2(a,b) a##b // actually concatenate
#define UNIQUE_ID() CAT(_uid_,__COUNTER__)

auto UNIQUE_ID() = call(1); // may be _uid_0
auto UNIQUE_ID() = call(2); // may be _uid_1