Null reference exception c++ - c++-cli

Question

I keep getting this error in the console:
Unhandled exception: System.NullReferenceException
Here's the code:
class Car {
public:
int X;
int Y;
};
class SpecificCar : public Car {
};
class Container {
public:
int AmountOfCars = 0;
Car **cars = nullptr;
void AddCar(Car *ptr);
};
void Container::AddCar(Car *ptr) {
if(AmountOfCars == 0) {
cars[0] = ptr; //Debbuger says that the problem in question is located here
AmountOfCars++;
}
int main() {
Container container;
Car *ptr = new SpecificCar;
ptr->X = 1;
ptr->Y = 5;
container.AddCar(ptr);
}

Answer 1

While your Container by design isn't storing Cars, it still has to store pointers to cars. You'll have to come up with a method. The Standard offers std::vector<Car> as well as std::vector<Car*> but you're free to come up with anything else. Still, if you don't want the Standard methods, it's really up to you what else you want to do.

Answer 2

Car **cars is not a dynamic container, it's a pointer to a memory region. What you did there is just utterly wrong. You still have to allocate an array of pointers to be able to fill data there, such as
cars = new Car*[5];
With that you can address with indices from 0 to 4 inside array cars[]. Yet again this is not dynamic, your best bet is an std::vector<Car*>, if you want to go your own ways then malloc()/realloc(), maybe linked listing if you really want to bother with it.

Answer 3

The problem is that, in class Container, you defined a member cars initialized to nullptr.
The best way to fix the issue is to use a std::vector<Car*> for cars. If you absolutely don't want to use a vector (why ?), in class Container, you may replace:
Car **cars = nullptr;
by something like:
static const int MAX_AMOUNT_OF_CARS = 100;
Car* cars[MAX_AMOUNT_OF_CARS];
which will define a proper array of Car*; then, you will be able to use cars[0], cars[i], ...

Answer 4

I figure you're trying to teach yourself about memory management. I've rewritten your class and AddCar() to be more what you want. Accessing or removing a car and deleting the container are left as an exercise for the student. (Look at this as pseudo-code. I haven't compiled or run it.)
class Container
{
Car ** cars_ = nullptr;
int capacity_ = 0; // how much room we have for car pointers
int AmountOfCars_ = 0; // how many car pointers we actually contain
public:
int AmountOfCars() const { return AmountOfCars_; }
void AddCar(Car *ptr);
};
void Container::AddCar(Car *ptr)
{
if ( AmountOfCars_ + 1 > capacity_ ) // ensure we have capacity for another Car *
{
if ( capacity_ == 0 ) // if we have none set to 2, so we'll initially allocate room for 4
capacity_ = 2;
int newcapacity = capacity_ * 2; // double the capacity
Cars ** newcars = new Car*[ newcapacity ]; // allocate a new pointer array
memcpy( newcars, cars_, capacity_ * sizeof(Car*) ); // we're just moving pointers
delete cars_; // get rid of the old pointer array
cars_ = newcars; // point to the new pointer array
capacity_ = newcapacity; // update the capacity
}
++AmountOfCars_; // increase the number of cars
cars[ AmountOfCars_ ] = ptr; // and copy the pointer into the slot
}