Question

I have a problem with my SDL program. My goal is to make a dot move along a line. I have all the coordinates saved in a data file. So I just wanted to read them from the file and display the dot at the right position.
The dot class (which is named linefollower) looks like this.

class Linefollower
{
private:
    int x, y;
    char orientation;

public:
    //Initializes the variables
    Linefollower();

    void set(int m_x, int m_y, char m_orietnation);

    void show();

    char get_orientation();
};

Linefollower::Linefollower()
{
    x = 0;
    y = 0;
    orientation = 'E';
}

void Linefollower::set(int m_x, int m_y, char m_orientation)
{
    x = m_x;
    y = m_y;
    orientation = m_orientation;
}

void Linefollower::show()
{
    //Show the linefollower
    apply_surface(x, y, linefollower, screen );
}

char Linefollower::get_orientation()
{
    return orientation;
}

The apply_surface function.

void apply_surface( int x, int y, SDL_Surface * source, SDL_Surface* destination)
{
//Temporary rectangle to hold the offsets
SDL_Rect offset;

//Get the offsets
offset.x = x;
offset.y = y;

//Blit the surface
SDL_BlitSurface( source, NULL, destination, &offset);
}

The loop which ought to display the animation looks like this.

//While the user hasn't quit
    while( quit == false )
    {

        //Apply the surface to the screen
        apply_surface( 0, 0, image, screen );

        fin.read((char*) &my_linefollower, sizeof my_linefollower);
        if(my_linefollower.get_orientation() == 'Q')
            break;


        my_linefollower.show();

        //Upadate the screen
        if( SDL_Flip( screen ) == -1 )
        {
            return 1;
        }

        SDL_Delay(200);

    }

Now I was expecting, that I get a moving dot on the screen, but the only thing which I got is the background (image) for a few seconds untill the if(my_linefollower.get_orientation() == 'Q')
break; was true. What do I do wrong?

PS: I guess it is worth noticing that I am a beginner in SDL and I took most of the code from a tutorial. Learning it exactly would be a waste of time for me, since it is unlikely that I am going to use it again any time soon.

Answer 1

First, you should change your offset in apply_surface to something like this:

SDL_Rect offset = { x, y, 0, 0 };

SDL_Rect doesn’t have a constructor to set your members to 0 by default, so you get uninitialized memory for your width and height.

Also, you should check what linefollower contains, if it’s a valid SDL_Surface. Removing your file reading code and manually controlling a Linefollower will allow you to easily find where the error comes from.

Use a debugger to validate your x and y coordinates.

Other than that, you code should work, although your window will be unresponsive because you’re not pumping events through SDL_PollEvent.