Table of Contents
gtkmm makes it very easy to derive new widgets by inheriting from an existing widget class, either by deriving from a container and adding child widgets, or by deriving from a single-item widget, and changing its behaviour. But you might occasionally find that no suitable starting point already exists. In this case, you can implement a widget from scratch.
When deriving from Gtk::Container, you should override the following virtual methods:
    
get_request_mode_vfunc(): Return what Gtk::SizeRequestMode is preferred by the container.
get_preferred_width_vfunc(): Calculate the minimum and natural width of the container.
get_preferred_height_vfunc(): Calculate the minimum and natural height of the container.
get_preferred_width_for_height_vfunc(): Calculate the minimum and natural width of the container, if it would be given the specified height.
get_preferred_height_for_width_vfunc(): Calculate the minimum and natural height of the container, if it would be given the specified width.
on_size_allocate(): Position the child widgets, given the height and width that the container has actually been given.
forall_vfunc(): Call the same callback for each of the children.
on_add(): Add a child widget to the container.
on_remove(): Remove a child widget from the container.
child_type_vfunc(): Return what type of child can be added.
The get_request_mode_vfunc(),
        get_preferred_width_vfunc(),
        get_preferred_height_vfunc(),
        get_preferred_width_for_height_vfunc(),
        get_preferred_height_for_width_vfunc(), and
        on_size_allocate() virtual methods control the
        layout of the child widgets. For instance, if your container has 2
        child widgets, with one below the other, your
        get_request_mode_vfunc() might request
        height-for-width layout. Then your
        get_preferred_width_vfunc()
        might report the maximum of the widths of the child widgets, and
        get_preferred_height_for_width_vfunc()
        might report the sum of their heights. If you want padding between
        the child widgets then you would add that to the width and height too.
        Your widget's container will use this result to ensure that your widget
        gets enough space, and not less. By examining each widget's parent, and
        its parent, this logic will eventually decide the size of the top-level
        window.
You are not guaranteed to get the Gtk::SizeRequestMode
        that you request. Therefore all four of the
        get_preferred_xxx_vfunc() methods must return
        sensible values.
on_size_allocate() receives the actual
       height and width that the parent container has decided to give to your
       widget. This might be more than the minimum, or even more than the natural
       size, for instance if the
       top-level window has been expanded. You might choose to ignore the extra
       space and leave a blank area, or you might choose to expand your child
       widgets to fill the space, or you might choose to expand the padding
       between your widgets. It's your container, so you decide. Don't forget to
       call set_allocation() inside your
       on_size_allocate() implementation to actually use the
       allocated space that has been offered by the parent container.
Unless your container is a top-level window that derives from
      Gtk::Window, you should probably also call
      Gtk::Widget::set_has_window(false) in your
      constructor. This means that your container does not create its own
      Gdk::Window, but uses its parent's
      window. (Note the difference between Gtk::Window
      and Gdk::Window.) If your container does need
      its own Gdk::Window, and does not derive from
      Gtk::Window, you must also override the
      on_realize() method as described in the
      Custom Widgets section.
      And unless your container draws directly onto the underlying
      Gdk::Window, you should probably call
      set_redraw_on_allocate(false) to improve
      performance.
By overriding forall_vfunc() you can allow
      applications to operate on all of the container's child widgets. For
      instance, show_all_children() uses this to find all
      the child widgets and show them.
Although your container might have its own method to set the child
      widgets, you should still provide an implementation for the virtual
      on_add() and on_remove()
      methods from the base class, so that the add() and remove() methods will
      do something appropriate if they are called.
Your implementation of the child_type_vfunc()
      method should report the type of widget that may be added to your
      container, if it is not yet full. This is usually
      Gtk::Widget::get_type() to indicate that the
      container may contain any class derived from
      Gtk::Widget. If the container may not contain any
      more widgets, then this method should return
      G_TYPE_NONE.
This example implements a container with two child widgets, one above
        the other. Of course, in this case it would be far simpler just to use
        a vertical Gtk::Box.
File: examplewindow.h (For use with gtkmm 3, not gtkmm 2)
#ifndef GTKMM_EXAMPLEWINDOW_H
#define GTKMM_EXAMPLEWINDOW_H
#include <gtkmm.h>
#include "mycontainer.h"
class ExampleWindow : public Gtk::Window
{
public:
  ExampleWindow();
  virtual ~ExampleWindow();
protected:
  //Signal handlers:
  void on_button_quit();
  //Child widgets:
  Gtk::Box m_VBox;
  MyContainer m_MyContainer;
  Gtk::Button m_Button_One;
  Gtk::Label m_Label_Two;
  Gtk::ButtonBox m_ButtonBox;
  Gtk::Button m_Button_Quit;
};
#endif //GTKMM_EXAMPLEWINDOW_H
File: mycontainer.h (For use with gtkmm 3, not gtkmm 2)
#ifndef GTKMM_CUSTOM_CONTAINER_MYCONTAINER_H
#define GTKMM_CUSTOM_CONTAINER_MYCONTAINER_H
#include <gtkmm/container.h>
class MyContainer : public Gtk::Container
{
public:
  MyContainer();
  virtual ~MyContainer();
  void set_child_widgets(Gtk::Widget& child_one, Gtk::Widget& child_two);
protected:
  //Overrides:
  Gtk::SizeRequestMode get_request_mode_vfunc() const override;
  void get_preferred_width_vfunc(int& minimum_width, int& natural_width) const override;
  void get_preferred_height_for_width_vfunc(int width, int& minimum_height, int& natural_height) const override;
  void get_preferred_height_vfunc(int& minimum_height, int& natural_height) const override;
  void get_preferred_width_for_height_vfunc(int height, int& minimum_width, int& natural_width) const override;
  void on_size_allocate(Gtk::Allocation& allocation) override;
  void forall_vfunc(gboolean include_internals, GtkCallback callback, gpointer callback_data) override;
  void on_add(Gtk::Widget* child) override;
  void on_remove(Gtk::Widget* child) override;
  GType child_type_vfunc() const override;
  Gtk::Widget* m_child_one;
  Gtk::Widget* m_child_two;
};
#endif //GTKMM_CUSTOM_CONTAINER_MYCONTAINER_H
File: main.cc (For use with gtkmm 3, not gtkmm 2)
#include "examplewindow.h"
#include <gtkmm/application.h>
int main(int argc, char *argv[])
{
  auto app = Gtk::Application::create(argc, argv, "org.gtkmm.example");
  ExampleWindow window;
  //Shows the window and returns when it is closed.
  return app->run(window);
}
File: examplewindow.cc (For use with gtkmm 3, not gtkmm 2)
#include <iostream>
#include "examplewindow.h"
ExampleWindow::ExampleWindow()
: m_VBox(Gtk::ORIENTATION_VERTICAL),
  m_Button_One("Child One"),
  m_Label_Two("Child 2", Gtk::ALIGN_END, Gtk::ALIGN_CENTER),
  m_Button_Quit("Quit")
{
  set_title("Custom Container example");
  set_border_width(6);
  set_default_size(400, 200);
  add(m_VBox);
  //Add the child widgets to the custom container:
  m_MyContainer.set_child_widgets(m_Button_One, m_Label_Two);
  m_VBox.pack_start(m_MyContainer, Gtk::PACK_EXPAND_WIDGET);
  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: mycontainer.cc (For use with gtkmm 3, not gtkmm 2)
#include <iostream>
#include <algorithm> // std::max
#include "mycontainer.h"
MyContainer::MyContainer()
: m_child_one(nullptr), m_child_two(nullptr)
{
  set_has_window(false);
  set_redraw_on_allocate(false);
}
MyContainer::~MyContainer()
{
/*
  // These calls to Gtk::Widget::unparent() are necessary if MyContainer is
  // deleted before its children. But if you use a version of gtkmm where bug
  // https://bugzilla.gnome.org/show_bug.cgi?id=605728
  // has not been fixed (gtkmm 3.7.10 or earlier) and the children are deleted
  // before the container, these calls can make the program crash.
  // That's because on_remove() is not called, when the children are deleted.
  if (m_child_one)
    m_child_one->unparent();
  if (m_child_two)
    m_child_two->unparent();
*/
}
void MyContainer::set_child_widgets(Gtk::Widget& child_one,
        Gtk::Widget& child_two)
{
  m_child_one = &child_one;
  m_child_two = &child_two;
  m_child_one->set_parent(*this);
  m_child_two->set_parent(*this);
}
//This example container is a simplified VBox with at most two children.
Gtk::SizeRequestMode MyContainer::get_request_mode_vfunc() const
{
  return Gtk::SIZE_REQUEST_HEIGHT_FOR_WIDTH;
}
//Discover the total amount of minimum space and natural space needed by
//this container and its children.
void MyContainer::get_preferred_width_vfunc(int& minimum_width, int& natural_width) const
{
  int child_minimum_width[2] = {0, 0};
  int child_natural_width[2] = {0, 0};
  if(m_child_one && m_child_one->get_visible())
    m_child_one->get_preferred_width(child_minimum_width[0], child_natural_width[0]);
  if(m_child_two && m_child_two->get_visible())
    m_child_two->get_preferred_width(child_minimum_width[1], child_natural_width[1]);
  //Request a width equal to the width of the widest visible child.
  minimum_width = std::max(child_minimum_width[0], child_minimum_width[1]);
  natural_width = std::max(child_natural_width[0], child_natural_width[1]);
}
void MyContainer::get_preferred_height_for_width_vfunc(int width,
   int& minimum_height, int& natural_height) const
{
  int child_minimum_height[2] = {0, 0};
  int child_natural_height[2] = {0, 0};
  int nvis_children = 0;
  if(m_child_one && m_child_one->get_visible())
  {
    ++nvis_children;
    m_child_one->get_preferred_height_for_width(width, child_minimum_height[0],
                                                child_natural_height[0]);
  }
  if(m_child_two && m_child_two->get_visible())
  {
    ++nvis_children;
    m_child_two->get_preferred_height_for_width(width, child_minimum_height[1],
                                                child_natural_height[1]);
  }
  //The allocated height will be divided equally among the visible children.
  //Request a height equal to the number of visible children times the height
  //of the highest child.
  minimum_height = nvis_children * std::max(child_minimum_height[0],
                                            child_minimum_height[1]);
  natural_height = nvis_children * std::max(child_natural_height[0],
                                            child_natural_height[1]);
}
void MyContainer::get_preferred_height_vfunc(int& minimum_height, int& natural_height) const
{
  int child_minimum_height[2] = {0, 0};
  int child_natural_height[2] = {0, 0};
  int nvis_children = 0;
  if(m_child_one && m_child_one->get_visible())
  {
    ++nvis_children;
    m_child_one->get_preferred_height(child_minimum_height[0], child_natural_height[0]);
  }
  if(m_child_two && m_child_two->get_visible())
  {
    ++nvis_children;
    m_child_two->get_preferred_height(child_minimum_height[1], child_natural_height[1]);
  }
  //The allocated height will be divided equally among the visible children.
  //Request a height equal to the number of visible children times the height
  //of the highest child.
  minimum_height = nvis_children * std::max(child_minimum_height[0],
                                            child_minimum_height[1]);
  natural_height = nvis_children * std::max(child_natural_height[0],
                                            child_natural_height[1]);
}
void MyContainer::get_preferred_width_for_height_vfunc(int height,
   int& minimum_width, int& natural_width) const
{
  int child_minimum_width[2] = {0, 0};
  int child_natural_width[2] = {0, 0};
  int nvis_children = 0;
  //Get number of visible children.
  if(m_child_one && m_child_one->get_visible())
    ++nvis_children;
  if(m_child_two && m_child_two->get_visible())
    ++nvis_children;
  if(nvis_children > 0)
  {
    //Divide the height equally among the visible children.
    const int height_per_child = height / nvis_children;
    if(m_child_one && m_child_one->get_visible())
      m_child_one->get_preferred_width_for_height(height_per_child,
                   child_minimum_width[0], child_natural_width[0]);
    if(m_child_two && m_child_two->get_visible())
      m_child_two->get_preferred_width_for_height(height_per_child,
                   child_minimum_width[1], child_natural_width[1]);
  }
  //Request a width equal to the width of the widest child.
  minimum_width = std::max(child_minimum_width[0], child_minimum_width[1]);
  natural_width = std::max(child_natural_width[0], child_natural_width[1]);
}
void MyContainer::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);
  //Get number of visible children.
  int nvis_children = 0;
  if(m_child_one && m_child_one->get_visible())
    ++nvis_children;
  if(m_child_two && m_child_two->get_visible())
    ++nvis_children;
  if(nvis_children <= 0)
    return;
  //Assign space to the children:
  Gtk::Allocation child_allocation_one;
  Gtk::Allocation child_allocation_two;
  //Place the first child at the top-left:
  child_allocation_one.set_x( allocation.get_x() );
  child_allocation_one.set_y( allocation.get_y() );
  //Make it take up the full width available:
  child_allocation_one.set_width( allocation.get_width() );
  if(m_child_one && m_child_one->get_visible())
  {
    //Divide the height equally among the visible children.
    child_allocation_one.set_height( allocation.get_height() / nvis_children);
    m_child_one->size_allocate(child_allocation_one);
  }
  else
    child_allocation_one.set_height(0);
  //Place the second child below the first child:
  child_allocation_two.set_x( allocation.get_x() );
  child_allocation_two.set_y( allocation.get_y() +
          child_allocation_one.get_height());
  //Make it take up the full width available:
  child_allocation_two.set_width( allocation.get_width() );
  //Make it take up the remaining height:
  child_allocation_two.set_height( allocation.get_height() -
          child_allocation_one.get_height());
  if(m_child_two && m_child_two->get_visible())
    m_child_two->size_allocate(child_allocation_two);
}
void MyContainer::forall_vfunc(gboolean, GtkCallback callback, gpointer callback_data)
{
  if(m_child_one)
    callback(m_child_one->gobj(), callback_data);
  if(m_child_two)
    callback(m_child_two->gobj(), callback_data);
}
void MyContainer::on_add(Gtk::Widget* child)
{
  if(!m_child_one)
  {
    m_child_one = child;
    m_child_one->set_parent(*this);
  }
  else if(!m_child_two)
  {
    m_child_two = child;
    m_child_two->set_parent(*this);
  }
}
void MyContainer::on_remove(Gtk::Widget* child)
{
  if(child)
  {
    const bool visible = child->get_visible();
    bool found = false;
    if(child == m_child_one)
    {
      m_child_one = nullptr;
      found = true;
    }
    else if(child == m_child_two)
    {
      m_child_two = nullptr;
      found = true;
    }
    if(found)
    {
      child->unparent();
      if(visible)
        queue_resize();
    }
  }
}
GType MyContainer::child_type_vfunc() const
{
  //If there is still space for one widget, then report the type of widget that
  //may be added.
  if(!m_child_one || !m_child_two)
    return Gtk::Widget::get_type();
  else
  {
    //No more widgets may be added.
    return G_TYPE_NONE;
  }
}