access to variable in c++ inner class

Question

I'm using cairomm, opencv4, box2d(not revealed in below code) to make physics education video.

My plan is like this. Many Sobject(Scientific obect) constitute a Scene. So I decided to use inner-class.(I don't know it's best design, I'm newbie to programming)

My problem is in below.

cv::Mat Surface2Mat(Cairo::RefPtr<Cairo::ImageSurface> surface)
{
    cv::Mat Frame(surface->get_height(), surface->get_width(), CV_8UC4, surface->get_data(), surface->get_stride());
    cv::cvtColor(Frame, Frame, cv::ColorConversionCodes::COLOR_BGRA2BGR);
    return Frame;
}

class Scene
{
private:
    int surface_width = 1920;
    int surface_height = 1080;
    int second = 4;

    cv::Size resolution = {surface_width, surface_height};
    const int mp4 = cv::VideoWriter::fourcc('a', 'v', 'c', '1');
    cv::VideoWriter video;
    double FPS;

public:
    class Sobject
    {
    private:
        double width;
        double height;
        double color[3] = {1, 1, 1};

    public:
        Cairo::RefPtr<Cairo::Context> cr;
        Cairo::RefPtr<Cairo::ImageSurface> surface;
        Sobject()
        {
            surface = Cairo::ImageSurface::create(Cairo::Format::FORMAT_ARGB32, 1920, 1080);
            cr = Cairo::Context::create(surface);
        }
        void
        set_position(double x = 0, double y = 0)
        {
            cr->move_to(960, 540);
        }
        void drawCircle()
        {
            cr->arc(0, 0, 40, 0.0, 2 * M_PI);
        }
    };

    Scene(int width = 1920, int height = 1080)
    {

        video.open("Scene.mp4", mp4, FPS = 30, resolution, true);
    }
    ~Scene()
    {
        video.release();
    }
    void write()
    {
        for (int i = 0; i < (FPS * second); i++)
        {
            video.write(Surface2Mat(surface));
        }
    }
};

The "video.write(Surface2Mat(surface));" at last makes problem. How can I access to the surface?

Answer 1

You have not created an instance of Sobject. You can do something like:

class Scene
{
....
public:
    class Sobject
    {...};

    Sobject mSobject; // can have it public or private depending on other requirements

    ....

    void write()
    {
        for (int i = 0; i < (FPS * second); i++)
        {
            // now that you have an instance of Sobject, you can use surface from it
            video.write(Surface2Mat(mSobject.surface));
        }
    }