Why doesn’t the preprocessor cause two adjacent minus signs to be a decrement? [duplicate]

Question

Consider the following code:

#include <stdio.h>
#define A -B
#define B -C
#define C 5

int main()
{
  printf("The value of A is %d\n", A); 
  return 0;
}

Here preprocessing should take place in the following manner:

1. first A should get replaced with -B
2. then B should get replaced with -C thus expression resulting into --C
3. then C should get replaced with 5 thus expression resulting into --5

So the resultant expression should give a compilation error( lvalue error ). But the correct answer is 5, how can the output be 5?

Please help me in this.

Answer 1

It preprocesses to (note the space):

int main()
{
  printf("The value of A is %d\n", - -5);
  return 0;
}

The preprocessor pastes tokens, not strings. It won't create -- out of two adjacent - tokens unless you force token concatenation with ##:

#define CAT_(A,B) A##B
#define CAT(A,B) CAT_(A,B)

#define A CAT(-,B)

#define B -C
#define C 5

int main()
{
  printf("The value of A is %d\n", A); /* A is --5 here—no space */ 
  return 0;
}