By deriving directly from Gtk::Widget you can do all the drawing for your widget directly, instead of just arranging child widgets. For instance, a Gtk::Label draws the text of the label, but does not do this by using other widgets.
This example implements a widget which draws a square.
File: examplewindow.h
#ifndef GTKMM_EXAMPLEWINDOW_H
#define GTKMM_EXAMPLEWINDOW_H
#include <gtkmm.h>
#include "mywidget.h"
class ExampleWindow : public Gtk::Window
{
public:
  ExampleWindow();
  virtual ~ExampleWindow();
protected:
  //Signal handlers:
  virtual void on_button_quit();
  //Child widgets:
  Gtk::VBox m_VBox;
  MyWidget m_MyWidget;
  Gtk::HButtonBox m_ButtonBox;
  Gtk::Button m_Button_Quit;
};
#endif //GTKMM_EXAMPLEWINDOW_H
File: mywidget.h
#ifndef GTKMM_CUSTOM_WIDGET_MYWIDGET_H
#define GTKMM_CUSTOM_WIDGET_MYWIDGET_H
#include <gtkmm/widget.h>
class MyWidget : public Gtk::Widget
{
public:
  MyWidget();
  virtual ~MyWidget();
protected:
  //Overrides:
  virtual void on_size_request(Gtk::Requisition* requisition);
  virtual void on_size_allocate(Gtk::Allocation& allocation);
  virtual void on_map();
  virtual void on_unmap();
  virtual void on_realize();
  virtual void on_unrealize();
  virtual bool on_expose_event(GdkEventExpose* event);
  Glib::RefPtr<Gdk::Window> m_refGdkWindow;
  Glib::RefPtr<Gdk::GC> m_refGC;
};
#endif //GTKMM_CUSTOM_WIDGET_MYWIDGET_H
File: examplewindow.cc
#include <iostream>
#include "examplewindow.h"
ExampleWindow::ExampleWindow()
: m_Button_Quit("Quit")
{
  set_title("Custom Widget example");
  set_border_width(6);
  set_default_size(400, 200);
  add(m_VBox);
  m_VBox.pack_start(m_MyWidget, Gtk::PACK_EXPAND_WIDGET);
  m_MyWidget.show();
  m_VBox.pack_start(m_ButtonBox, Gtk::PACK_SHRINK);
  m_ButtonBox.pack_start(m_Button_Quit, Gtk::PACK_SHRINK);
  m_ButtonBox.set_border_width(6);
  m_ButtonBox.set_layout(Gtk::BUTTONBOX_END);
  m_Button_Quit.signal_clicked().connect( sigc::mem_fun(*this, &ExampleWindow::on_button_quit) );
  show_all_children();
}
ExampleWindow::~ExampleWindow()
{
}
void ExampleWindow::on_button_quit()
{
  hide();
}
File: main.cc
#include <gtkmm/main.h>
#include "examplewindow.h"
int main(int argc, char *argv[])
{
  Gtk::Main kit(argc, argv);
  ExampleWindow window;
  Gtk::Main::run(window); //Shows the window and returns when it is closed.
  return 0;
}
File: mywidget.cc
#include <iostream>
#include "mywidget.h"
MyWidget::MyWidget()
{
  set_flags(Gtk::NO_WINDOW);
  //add_events(Gdk::EXPOSURE_MASK);
}
MyWidget::~MyWidget()
{
}
void MyWidget::on_size_request(Gtk::Requisition* requisition)
{
  //Initialize the output parameter:
  *requisition = Gtk::Requisition();
  //Discover the total amount of minimum space needed by this widget.
  
  //Let's make this simple example widget always need 50 by 50:
  requisition->height = 50;
  requisition->width = 50;
}
void MyWidget::on_size_allocate(Gtk::Allocation& allocation)
{
  //Do something with the space that we have actually been given:
  //(We will not be given heights or widths less than we have requested, though we might get more)
  //Use the offered allocation for this container:
  set_allocation(allocation);
  if(m_refGdkWindow)
    m_refGdkWindow->move_resize( allocation.get_x(), allocation.get_y(), allocation.get_width(), allocation.get_height() );
}
void MyWidget::on_map()
{
  //Call base class:
  Gtk::Widget::on_map();
}
void MyWidget::on_unmap()
{
  //Call base class:
  Gtk::Widget::on_unmap();
}
void MyWidget::on_realize()
{
  //Call base class:
  Gtk::Widget::on_realize();
  //TODO: Not sure whether this is necessary:
  set_flags(Gtk::REALIZED);
  set_window( get_parent_window() );
  GtkWidget* cWidget = (GtkWidget*)gobj();
  cWidget->style = gtk_style_attach(cWidget->style, get_window()->gobj());
 
  if(!m_refGdkWindow)
  {
    //Create the GdkWindow:
    GdkWindowAttr attributes;
    memset(&attributes, 0, sizeof(attributes));
    Gtk::Allocation allocation = get_allocation();
    //Set initial position and size of the Gdk::Window:
    attributes.x = allocation.get_x();
    attributes.y = allocation.get_y();
    attributes.width = allocation.get_width();
    attributes.height = allocation.get_height();
    attributes.window_type = GDK_WINDOW_CHILD;
    attributes.wclass = GDK_INPUT_OUTPUT;
    m_refGdkWindow = Gdk::Window::create(
      get_window() /* parent */,
      &attributes,
      GDK_WA_X | GDK_WA_Y);
    //Allocate a GC for use in on_expose_event():
    m_refGC = Gdk::GC::create(m_refGdkWindow);
  }
}
void MyWidget::on_unrealize()
{
  m_refGdkWindow.clear();
  m_refGC.clear();
  //Call base class:
  Gtk::Widget::on_unrealize();
}
bool MyWidget::on_expose_event(GdkEventExpose* /* event */)
{
  if(m_refGdkWindow)
  {
    //Draw on the Gdk::Window:
    Glib::RefPtr<Gdk::Colormap> colormap = get_default_colormap ();
    Gdk::Color color_blue("blue");
    Gdk::Color color_red("red");
    colormap->alloc_color(color_blue);
    colormap->alloc_color(color_red);
    m_refGdkWindow->set_background( color_red );
    m_refGdkWindow->clear();
    m_refGC->set_foreground( color_blue );
    m_refGdkWindow->draw_line(m_refGC, 1, 1, 100, 100);
  }
  return true;
}