We've now learned enough to look at a real example. In accordance with an ancient tradition of computer science, we now introduce Hello World, a la gtkmm:

Source Code

File: helloworld.h (For use with gtkmm 3, not gtkmm 2)

#ifndef GTKMM_EXAMPLE_HELLOWORLD_H
#define GTKMM_EXAMPLE_HELLOWORLD_H

#include <gtkmm/button.h>
#include <gtkmm/window.h>

class HelloWorld : public Gtk::Window
{

public:
  HelloWorld();
  virtual ~HelloWorld();

protected:
  //Signal handlers:
  void on_button_clicked();

  //Member widgets:
  Gtk::Button m_button;
};

#endif // GTKMM_EXAMPLE_HELLOWORLD_H

File: helloworld.cc (For use with gtkmm 3, not gtkmm 2)

#include "helloworld.h"
#include <iostream>

HelloWorld::HelloWorld()
: m_button("Hello World")   // creates a new button with label "Hello World".
{
  // Sets the border width of the window.
  set_border_width(10);

  // When the button receives the "clicked" signal, it will call the
  // on_button_clicked() method defined below.
  m_button.signal_clicked().connect(sigc::mem_fun(*this,
              &HelloWorld::on_button_clicked));

  // This packs the button into the Window (a container).
  add(m_button);

  // The final step is to display this newly created widget...
  m_button.show();
}

HelloWorld::~HelloWorld()
{
}

void HelloWorld::on_button_clicked()
{
  std::cout << "Hello World" << std::endl;
}

File: main.cc (For use with gtkmm 3, not gtkmm 2)

#include "helloworld.h"
#include <gtkmm/application.h>

int main (int argc, char *argv[])
{
  auto app = Gtk::Application::create(argc, argv, "org.gtkmm.example");

  HelloWorld helloworld;

  //Shows the window and returns when it is closed.
  return app->run(helloworld);
}

Try to compile and run it before going on. You should see something like this:

Pretty thrilling, eh? Let's examine the code. First, the HelloWorld class:

class HelloWorld : public Gtk::Window
{

public:
  HelloWorld();
  virtual ~HelloWorld();

protected:
  //Signal handlers:
  virtual void on_button_clicked();

  //Member widgets:
  Gtk::Button m_button;
};

This class implements the "Hello World" window. It's derived from Gtk::Window, and has a single Gtk::Button as a member. We've chosen to use the constructor to do all of the initialisation work for the window, including setting up the signals. Here it is, with the comments omitted:

HelloWorld::HelloWorld()
:
  m_button ("Hello World")
{
  set_border_width(10);
  m_button.signal_clicked().connect(sigc::mem_fun(*this,
    &HelloWorld::on_button_clicked));
  add(m_button);.
  m_button.show();
}

Notice that we've used an initialiser statement to give the m_button object the label "Hello World".

Next we call the Window's set_border_width() method. This sets the amount of space between the sides of the window and the widget it contains.

We then hook up a signal handler to m_button's clicked signal. This prints our friendly greeting to stdout.

Next, we use the Window's add() method to put m_button in the Window. (add() comes from Gtk::Container, which is described in the chapter on container widgets.) The add() method places the Widget in the Window, but it doesn't display the widget. gtkmm widgets are always invisible when you create them - to display them, you must call their show() method, which is what we do in the next line.

Now let's look at our program's main() function. Here it is, without comments:

int main(int argc, char** argv)
{
  Glib::RefPtr<Gtk::Application> app = Gtk::Application::create(argc, argv, "org.gtkmm.example");

  HelloWorld helloworld;
  return app->run(helloworld);
}

First we instantiate an object stored in a RefPtr smartpointer called app. This is of type Gtk::Application. Every gtkmm program must have one of these. We pass our command-line arguments to its create() method. It takes the arguments it wants, and leaves you the rest, as we described earlier.

Next we make an object of our HelloWorld class, whose constructor takes no arguments, but it isn't visible yet. When we call Gtk::Application::run(), giving it the helloworld Window, it shows the Window and starts the gtkmm event loop. During the event loop gtkmm idles, waiting for actions from the user, and responding appropriately. When the user closes the Window, run() will return, causing the final line of our main() function be to executed. The application will then finish.