Assigning values to pointers in C++

Question

This is a snippet of code I use:

void Move::AddToMovesList(Location* &list, int row, int col) {
    // If the list is empty, create the first item
    if (list == NULL)
        list = new Location(row, col);
    // List exists, so append
    else
        list->Add(row, col);
}

If list is NULL, a new Location should be created and the pointer list should point to that new location. That's the behavior I'd expect from this code, but right before gdb exits this function I noticed that list is still NULL. What am I doing wrong here?

I used the ampersand in Location* &list to make sure that I can permanently (vs. locally) change the supplied pointer.

Answer 1

Personally I would not use pointer at all.

class Location
{
    public:
        void add(int row, int col)
        {
            data.push_back(std::make_pair(row,col));
        }
        bool isEmpty()  const {return data.empty(); }
    private:
        std::vector<std::pair<int,int> > data;
};


class Move
{
    public:
        // Pass a reference to the list.
        // No special case processing if the list is empty.
        // No problems with ownership.
        // No problems with lifespan associated with new/delete
        void addToMoveList(Location& list, int row, int col)
        {
            list.add(row, col);
        }
};

int main()
{
    Location    list;  // Don't use new if you can declare a local variable.
    Move        move;

    move.addToMoveList(list, 10, 2);
}

Answer 2

Let's not reinvent the wheel here... Know your libraries.

If you use the STL list container (or any other container) then you don't need to bother with null pointers.