Can I include preprocessor directive inside my function [duplicate]

I have read that I am not able to include preprocessor directives inside my function. But when I implement it, it works. Here are my code:

#include <iostream>

#define MY_CONSTANT 41

void myFunction(int x) {

    #if x == 42
       std::cout << "Value is 42." << " "<<x<<std::endl;
    #else
       std::cout << "Value is not 42." <<" "<<x<< std::endl;
    #endif
                       }

int main() {
  myFunction(42);
  return 0;
           }

'The output was not like what I expected. It is: Value is not 42, 42. Could you explain me what really happened here. Why the directive come to the else statement.'