Question

On executing the following code using ‘Macro Definitions Set 1’, I encounter the error “C2065: ‘C1’ : undeclared identifier”.

While using ‘Macro Definitions Set 2’, the code runs to give an output of 2.

I am guessing it has something to do with preprocessor tokenization. Please Explain.

#include <iostream>

// Macro Definitions Set 1
#define A       1
#define D(n)    B(n)
#define B(n)    C##n
#define CA      2

// Macro Definitions Set 2
//#define A     1
//#define D(n)  C##n
//#define CA        2

int main ()
{

    printf ("%d", D(A));
}

Answer 1

The macro parameter is substituted unless it is before or after ##, or after #.

So in Set 1, D(A) becomes B(1) after substitution of D(n), which is substituted to C1 after rescan.

In Set 2, D(A) becomes CA, where A is not substituted as it is after ##, and CA becomes 2 after rescan.