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<tr><th colspan="3" align="center">Hello World in <span class="application">gtkmm</span>
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="sec-helloworld"></a>Hello World in <span class="application">gtkmm</span>
</h2></div></div></div>
<p>
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 <span class="application">gtkmm</span>:
</p>
<p><a class="ulink" href="https://gitlab.gnome.org/GNOME/gtkmm-documentation/tree/master/examples/book/helloworld" target="_top">Source Code</a></p>
<p>File: <code class="filename">helloworld.h</code> (For use with gtkmm 4)</p>
<pre class="programlisting"><code class="code">#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();
~HelloWorld() override;
protected:
//Signal handlers:
void on_button_clicked();
//Member widgets:
Gtk::Button m_button;
};
#endif // GTKMM_EXAMPLE_HELLOWORLD_H
</code></pre>
<p>File: <code class="filename">helloworld.cc</code> (For use with gtkmm 4)</p>
<pre class="programlisting"><code class="code">#include "helloworld.h"
#include <iostream>
HelloWorld::HelloWorld()
: m_button("Hello World") // creates a new button with label "Hello World".
{
// Sets the margin around the button.
m_button.set_margin(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).
set_child(m_button);
}
HelloWorld::~HelloWorld()
{
}
void HelloWorld::on_button_clicked()
{
std::cout << "Hello World" << std::endl;
}
</code></pre>
<p>File: <code class="filename">main.cc</code> (For use with gtkmm 4)</p>
<pre class="programlisting"><code class="code">#include "helloworld.h"
#include <gtkmm/application.h>
int main(int argc, char* argv[])
{
auto app = Gtk::Application::create("org.gtkmm.example");
//Shows the window and returns when it is closed.
return app->make_window_and_run<HelloWorld>(argc, argv);
}
</code></pre>
<p>
Try to compile and run it before going on. You should see something like this:
</p>
<div class="figure">
<a name="figure-helloworld"></a><p class="title"><b>Figure 3.1. Hello World</b></p>
<div class="figure-contents">
<div class="screenshot">
<div class="mediaobject"><img src="figures/helloworld.png" alt="Hello World"></div>
</div>
</div>
</div>
<br class="figure-break">
<p>
Pretty thrilling, eh? Let's examine the code. First, the
<code class="classname">HelloWorld</code> class:
</p>
<pre class="programlisting"><code class="code">class HelloWorld : public Gtk::Window
{
public:
HelloWorld();
~HelloWorld() override;
protected:
//Signal handlers:
void on_button_clicked();
//Member widgets:
Gtk::Button m_button;
};</code></pre>
<p>
This class implements the "Hello World" window. It's derived from
<code class="classname">Gtk::Window</code>, and has a single <code class="classname">Gtk::Button</code> as a member.
We've chosen to use the
constructor to do all of the initialization work for the window,
including setting up the signals. Here it is, with the comments
omitted:
</p>
<pre class="programlisting"><code class="code">HelloWorld::HelloWorld()
: m_button("Hello World")
{
m_button.set_margin(10);
m_button.signal_clicked().connect(sigc::mem_fun(*this,
&HelloWorld::on_button_clicked));
set_child(m_button);
}</code></pre>
<p>
Notice that we've used an initializer statement to give the <code class="literal">m_button</code>
object the label "Hello World".
</p>
<p>
Next we call the Button's <code class="methodname">set_margin()</code> method. This sets
the amount of space around the button.
</p>
<p>
We then hook up a signal handler to <code class="literal">m_button</code>'s <code class="literal">clicked</code> signal.
This prints our friendly greeting to <code class="literal">stdout</code>.
</p>
<p>
Next, we use the Window's <code class="methodname">set_child()</code> method to put
<code class="literal">m_button</code> in the Window. The <code class="methodname">set_child()</code>
method places the Widget in the Window.
</p>
<p>
Now let's look at our program's <code class="function">main()</code> function. Here it is,
without comments:
</p>
<pre class="programlisting"><code class="code">int main(int argc, char* argv[])
{
auto app = Gtk::Application::create("org.gtkmm.example");
return app->make_window_and_run<HelloWorld>(argc, argv);
}</code></pre>
<p>
First we instantiate an object stored in a <code class="classname">Glib::RefPtr</code> smartpointer called <code class="literal">app</code>. This is of type
<code class="classname">Gtk::Application</code>. Every <span class="application">gtkmm</span> program must have one of these.
</p>
<p>
Next we call <code class="methodname">make_window_and_run()</code> which creates an object
of our <code class="classname">HelloWorld</code> class, shows that Window and starts the <span class="application">gtkmm</span>
<span class="emphasis"><em>event loop</em></span>. During the event loop <span class="application">gtkmm</span> idles, waiting for actions
from the user, and responding appropriately.
When the user closes the Window, <code class="methodname">make_window_and_run()</code> will return,
causing our <code class="function">main()</code> function to return. The application will then finish.
</p>
<p>
Like the simple example we showed earlier, this Hello World program does not use
the command-line parameters.
</p>
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