title: Understanding a Gtk::Button example code line
body: <p>I'm reading the official gnome tutorial for gtkmm, the one about buttons specifically <a href="https://developer-old.gnome.org/gtkmm-tutorial/stable/sec-helloworld.html.es" rel="nofollow noreferrer">Hello world in gtkmm</a></p>
  <p>I understand almost all of the code but this part in the helloworld.cc part:</p>
  <pre><code>HelloWorld::HelloWorld()
  : m_button(&quot;Hello World&quot;)   // creates a new button with label &quot;Hello World&quot;.
  </code></pre>
  <p>If I'm right thats the class constructor but I don't understand the &quot;: m_button&quot; line. I have never seen something like that before.</p>
  <p>Thanks for all!</p>