What would be a proper invalid value for a pointer?

Question

Suppose I have this code. Your basic \"if the caller doesn\'t provide a value, calculate value\" scenario.

void fun(const char* ptr = NULL)
{
   if (ptr==NULL) {         


        

Answer 1

void fun()
{
   // calculate what ptr value should be
   const char* ptr = /*...*/;

   // now handle ptr normally
   fun(ptr);
}