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

If you want a special value that corresponds to no useful argument, make one.

header file:

extern const char special_value;

void fun(const char* ptr=&special_value);

implementation:

const char special_value;

void fun(const char* ptr)
{
    if (ptr == &special_value) ....
}