context->save();
context->translate(x, y);
context->scale(width / 2.0, height / 2.0);
context->arc(0.0, 0.0, 1.0, 0.0, 2 * M_PI);
context->restore();

Here's an example of a simple program that draws an arc, a circle and an ellipse into a drawing area.

Source Code

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();
    int lesser = MIN(width, height);

    // coordinates for the center of the window
    int xc, yc;
    xc = width / 2;
    yc = height / 2;

    Cairo::RefPtr<Cairo::Context> cr = window->create_cairo_context();
    cr->set_line_width(lesser * 0.02);  // outline thickness changes
                                        // with window size

    // 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();

    // first draw a simple unclosed arc
    cr->save();
    cr->arc(width / 3.0, height / 4.0, lesser / 4.0, -(M_PI / 5.0), M_PI);
    cr->close_path();   // line back to start point
    cr->set_source_rgb(0.0, 0.8, 0.0);
    cr->fill_preserve();
    cr->restore();  // back to opaque black
    cr->stroke();   // outline it

    // now draw a circle
    cr->save();
    cr->arc(xc, yc, lesser / 4.0, 0.0, 2.0 * M_PI); // full circle
    cr->set_source_rgba(0.0, 0.0, 0.8, 0.6);    // partially translucent
    cr->fill_preserve();
    cr->restore();  // back to opaque black
    cr->stroke();

    // and finally an ellipse
    double ex, ey, ew, eh;
    // center of ellipse
    ex = xc;
    ey = 3.0 * height / 4.0;
    // ellipse dimensions
    ew = 3.0 * width / 4.0;
    eh = height / 3.0;

    cr->save();

    cr->translate(ex, ey);  // make (ex, ey) == (0, 0)
    cr->scale(ew / 2.0, eh / 2.0);  // for width: ew / 2.0 == 1.0
                                    // for height: eh / 2.0 == 1.0

    cr->arc(0.0, 0.0, 1.0, 0.0, 2 * M_PI);  // 'circle' centered at (0, 0)
                                            // with 'radius' of 1.0

    cr->set_source_rgba(0.8, 0.0, 0.0, 0.7);
    cr->fill_preserve();
    cr->restore();  // back to opaque black
    cr->stroke();
  }

  return true;
}

There are a couple of things to note about this example code. Again, the only real difference between this example and the previous ones is the on_expose_event() function, so we'll limit our focus to that function. In addition, the first part of the function is nearly identical to the previous examples, so we'll skip that portion.

Note that in this case, we've expressed nearly everything in terms of the height and width of the window, including the width of the lines. Because of this, when you resize the window, everything scales with the window. Also note that there are three drawing sections in the function and each is wrapped with a save()/restore() pair so that we're back at a known state after each drawing.

The section for drawing an arc introduces one new function, close_path(). This function will in effect draw a straight line from the current point back to the first point in the path. There is a significant difference between calling close_path() and manually drawing a line back to the starting point, however. If you use close_path(), the lines will be nicely joined together. If you use line_to() instead, the lines will end at the same point, but Cairo won't do any special joining.