Drag and Drop objects using mouse

The code from the Drag and Drop tutorial

    Camera cam;
    Vector2 pos;
    bool holding;

    void Start()
    {
        cam = Camera.main;
    }

    void Update()
    {
        if (holding)
        {
            pos = cam.ScreenToWorldPoint(Input.mousePosition);
            transform.position = pos;
        }
    }

    private void OnMouseDown()
    {
        holding = true;
    }

    private void OnMouseUp()
    {
        holding = false;
    }