Can I return NULL for vector<double> function? [duplicate]

Question

I have the following function:

/* Calculate if there is an intersection with given intial position and 
  direction */
vector<double> intersection(vector<double> startPos, vector<double> direction)
{
   if(there is intersection)
      return (intersection coordinates);
   else {
      return NULL;
   }
} 

Could I do this and check against NULL if an intersection exists:

vector<double> v = intersection(pos, dir);
if(v == NULL)
   /* Do something */
else
   /* Do something else */

If this is not allowed/bad coding practice, what is another way I may go about this?

Answer 1

NULL really is only a concept of pointers. Since we have a container we can check something else, namely, whether or not the container is empty. If it is then we know we have no elements and if it is not then we know there is stuff to process. That lets you write code like

vector<double> intersection(vector<double> startPos, vector<double> direction)
{
    if(there is intersection)
        return (intersection coordinates);
    else {
        return {}; // this means return a default constructed instance
    }
} 

and then you can use it like

vector<double> v = intersection(pos, dir);
if(v.empty())
    /* Do something */
else
    /* Do something else */

---

Also note that if you want to get a set intersection you can use std::set_intersection and use it like

#include <iostream>
#include <vector>
#include <algorithm>
#include <iterator>
int main()
{
    std::vector<int> v1{1,2,3,4,5,6,7,8};
    std::vector<int> v2{        5,  7,  9,10};
    std::sort(v1.begin(), v1.end());
    std::sort(v2.begin(), v2.end());     
    std::vector<int> v_intersection;     
    std::set_intersection(v1.begin(), v1.end(),
                          v2.begin(), v2.end(),
                          std::back_inserter(v_intersection));
    for(int n : v_intersection)
        std::cout << n << ' ';
}

Output:

5 7