int mouseX,mouseY;
    void Unit::handle_input()
    {
        if(occur.type == SDL_MOUSEBUTTONDOWN)
        {
            mouseX = occur.button.x;
            mouseY = occur.button.y;
        }
    }
Pass mouseX and mouseY as reference to the handle_input function.

    void Unit::show()
    {
        int BoxX;
        int BoxY;
        Box.x = BoxX;
        Box.y = BoxY;
        SDL_Rect unitOffset;
        unitOffset.x = BoxX;
        unitOffset.y = BoxY;
        SDL_BlitSurface(Unit,NULL,screen,unitOffset);
    }
What's happening here? You create two ints and assign them to the previous ones? You know they are zero-initialized? So basically `BoxX, BoxY, Box.x, Box.y, unitOffset.x, unitOffset.y` are all zeros.