confusion about setting null pointers

The only thing you can set to a nullptr is, well, a pointer.

CourseList is not a pointer. That's the problem. That's why you can't set it to a null pointer.

CourseList is an array of pointers. What you can do, instead, is to set every pointer in a CourseList to a null pointer:

for (auto &p:CourseList) p=nullptr; 

Your CourseList is a group of 100 elements and not just a single element. So you will need to iterate all the 100 and assign each element to be a null pointer. When this Course* courseList[100] = {}; does the thing.

Course * courseList[100] = {}; 

If you to initialize this array in your constructor use

std::fill(courseList, courseList + 100, nullptr); 

or use std::array<Course *, 100> courseList; instead of Course * courseList[100];

CourseList* courseList[100] does not create a pointer to an array, it creates an array of pointers. Depending on what you actually want you should be using one of the following

An array of CourseLists

CourseList courseList[100]; // possibly with = {} courseList[0].attr = whatever; // usage 

Better than the above, but similar, a std::array of CourseLists

std::array<CourseList, 100> courseList; // possibly with = {} courseList[0].attr = whatever; 

Or the closest to what it seems you want, a pointer to a std::array of CourseLists. This is probably not best, but nonetheless, here it is:

std::array<CourseList, 100>* courseList = nullptr; courseList = new std::array<CourseList, 100>{}; // prefer unique_ptr courseList->at(0).attr = whatever; 

If you don't know the size at compile time, or want to pass around the object, your best bet is to use a std::vector

std::vector<CourseList> courseList(100); courseList[0].attr = whatever; 

Finally, the weirdest option would be to actually define a pointer to an array:

CourseList (*courseList)[100] = nullptr; 

Addressing the update: you can just use default initializers in your class like so:

class ScmApp { private: CourseList* courseList[100] = {}; int noOfCourses = 0; }; 

But it seems like you should be using a growable container, std::vector and calling push_back to repeatedly add courses. What I see is a very hard-to-maintain keeping track of who is in what class.

If it is absolutely necessary that you take an existing array and set everything to nullptr then you can use std::fill

std::fill(std::begin(courseList), std::end(courseList), nullptr);