The GDK drawing API described here is now deprecated, in favour of Cairo. See the DrawingArea chapter for information about the Cairo API.
      Gdk graphics contexts (Gdk::GC) are a
      server-side resource. They contain information that describes how
      drawing is to be done. This provides for fewer arguments to the
      drawing methods, and less communication between the client and the
      server. The following example shows  how to set up a graphics context
      with a foreground color of red for drawing.
  
Gdk::GC some_gc; some_gc.create(get_window()); Gdk::Color some_color; Gdk::Colormap some_colormap(Gdk::Colormap::get_system()); some_color.set_red(65535); some_color.set_green(0); some_color.set_blue(0); some_colormap.alloc(some_color); some_gc.set_foreground(some_color);
      The first two lines create the graphics context and assign it to the
      appropriate widget. The get_window() method is
      a part of the Gtk::Widget class, so if you put
      this code into a derived widget's implementation then you can call it
      just as it is, otherwise you'd use
      some_widget.get_window().
  
      The next two lines create the Gdk::Color and
      Gdk::Colormap. After setting the color values
      you then need to allocate the color. The system figures out what to
      do in this case. The colormap contains information about how colors
      can be displayed on your screen, and is able to allocate the
      requested color. For example, on a display of only 256 colors the
      exact color requested may not be available, so the closest color to
      the one requested will be used instead. The final line sets the color
      as the foreground color.
  
      There are a number of attributes that can be set for a graphics
      context. There's the foreground and background color.  When drawing
      lines, you can set the thickness of the line with
      set_line_width(). Whether a solid or dashed line
      is drawn can be set with set_line_style(). The
      size and proportions of the dashes are set with
      set_dashes. How two lines join together, whether
      round or pointed or beveled off, is set with
      set_join_style().  Other things that can be set
      within a graphics context include font style, stippling and tiling
      for the filling of solid polygons.
  
      Graphics contexts are central to drawing with Gdk, because nearly all Gdk
      drawing functions and many Pango functions take a
      Gdk::GC object as an argument.  So although Cairo
      has largely superceded many Gdk drawing functions, you're still likely to
      run into Gdk::GC objects quite often, so it's
      important to know what they are and how they're used.