In addition to drawing straight lines Cairo allows you to easily
            draw curved lines (technically a cubic Bézier spline) using the
            Cairo::Context::curve_to() and
            Cairo::Context::rel_curve_to() functions.
            These functions take coordinates for a destination point as well as
            coordinates for two 'control' points.  This is best explained using
            an example, so let's dive in.
        
This simple application draws a curve with Cairo and displays the control points for each end of the curve.
File: myarea.h
#ifndef GTKMM_EXAMPLE_MYAREA_H
#define GTKMM_EXAMPLE_MYAREA_H
#include <gtkmm/drawingarea.h>
class MyArea : public Gtk::DrawingArea
{
public:
  MyArea();
  virtual ~MyArea();
protected:
  //Override default signal handler:
  virtual bool on_expose_event(GdkEventExpose* event);
};
#endif // GTKMM_EXAMPLE_MYAREA_H
File: main.cc
#include "myarea.h"
#include <gtkmm/main.h>
#include <gtkmm/window.h>
int main(int argc, char** argv)
{
   Gtk::Main kit(argc, argv);
   Gtk::Window win;
   win.set_title("DrawingArea");
   MyArea area;
   win.add(area);
   area.show();
   Gtk::Main::run(win);
   return 0;
}
File: myarea.cc
#include "myarea.h"
#include <cairomm/context.h>
MyArea::MyArea()
{
}
MyArea::~MyArea()
{
}
bool MyArea::on_expose_event(GdkEventExpose* event)
{
  // This is where we draw on the window
  Glib::RefPtr<Gdk::Window> window = get_window();
  if(window)
  {
    Gtk::Allocation allocation = get_allocation();
    const int width = allocation.get_width();
    const int height = allocation.get_height();
    double x0=0.1, y0=0.5, // start point
           x1=0.4, y1=0.9,  // control point #1
           x2=0.6, y2=0.1,  // control point #2
           x3=0.9, y3=0.5;  // end point
    Cairo::RefPtr<Cairo::Context> cr = window->create_cairo_context();
    // clip to the area indicated by the expose event so that we only redraw
    // the portion of the window that needs to be redrawn
    cr->rectangle(event->area.x, event->area.y,
            event->area.width, event->area.height);
    cr->clip();
    // scale to unit square (0 to 1 with and height)
    cr->scale(width, height);
    cr->set_line_width(0.05);
    // draw curve
    cr->move_to(x0, y0);
    cr->curve_to(x1, y1, x2, y2, x3, y3);
    cr->stroke();
    // show control points
    cr->set_source_rgba(1, 0.2, 0.2, 0.6);
    cr->move_to(x0, y0);
    cr->line_to (x1, y1);
    cr->move_to(x2, y2);
    cr->line_to (x3, y3);
    cr->stroke();
  }
  return true;
}
            The only difference between this example and the straight line
            example is in the on_expose_event() function,
            but there are a few new concepts and functions introduced here, so
            let's examine them briefly.
        
            Note that we clip to the area that needs re-exposing just as we did
            in the last example.  After clipping, however, we make a call to
            Cairo::Context::scale(), passing in the width
            and height of the drawing area.  This scales the user-space
            coordinate system such that the the width and height of the widget
            are both equal to 1.0 'units'.  There's no particular reason to
            scale the coordinate system in this case, but sometimes it can make
            drawing operations easier.
        
            The call to Cairo::Context::curve_to() should
            be fairly self-explanatory.  The first pair of coordinates define
            the control point for the beginning of the curve.  The second set
            of coordinates define the control point for the end of the curve,
            and the last set of coordinates define the destination point.  To
            make the concept of control points a bit easier to visualize, a
            line has been draw from each control point to the end-point on the
            curve that it is associated with.  Note that these control point
            lines are both translucent.  This is achieved with a variant of
            set_source_rgb() called
            set_source_rgba().  This function takes a
            fourth argument specifying the alpha value of the color (valid
            values are between 0 and 1).