I have a class Grid declared as follows:

class Grid
{
public:
    Grid(int length_x, int length_y);
    ~Grid();

    Position *at(int x, int y);

    private:
        int length_x, length_y;
        std::vector<std::unique_ptr<Position>> grid;
};

Its most important member variable is the vector of unique_ptr<Position>, which I'm using to simulate a 2-dimensional array whose size is determined at runtime. The class declaration for the Position is as follows:

class Position {
public:
    Position(int x, int y);
    ~Position();

    int getX() { return x; };
    int getY() { return y; };

private:
    int x, y;
};

In the Grid's constructor, I want to create the desired number of Positions and add them to the vector<unique_ptr<Position>. Here's what I've tried so far:

Grid::Grid(int length_x, int length_y) 
    : length_x(length_x), length_y(length_y)
{
    grid.resize(length_x * length_y);   

    for (int x = 0; x < length_x; x++) {
        for (int y = 0; y < length_y; y++) {
            std::unique_ptr<Position> temp = std::make_unique<Position>(x, y);
            grid.push_back(std::move(temp));
        }
    }
}

I've also tried using emplace_back() instead of push_back(), as well as replacing the lines inside the for-loop with

grid.push_back(std::make_unique<Position>(x, y));  

and also

grid.emplace_back(std::make_unique<Position>(x, y));

Nothing has worked, because when I try accessing any of the Positions' coordinates by iterating through the vector and calling Position::toString(), I always get the following error:

First-chance exception at 0x0087D8A3 in CombatSim.exe: 0xC0000005: Access violation reading location 0x00000000. Unhandled exception at 0x0087D8A3 in CombatSim.exe: 0xC0000005: Access violation reading location 0x00000000.

As far as I know, I could have one of four problems:

1) I'm adding the unique_ptr to the created Position object to the vector incorrectly

2) I'm using the wrong method to dynamically create Position objects.

3) All of the above.

4) Something I don't know about.

Here's the rest of the code:

Position::Position(int x, int y) 
    : x(x), y(y)
{
}

std::string Position::toString()
{
    std::string result = "Position at (" + std::to_string(x) + ", " + std::to_string(y) + ")";

    return result;
}