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<div class="appendix">
<div class="titlepage"><div><div><h1 class="title">
<a name="chapter-signals"></a>Appendix B. Signals</h1></div></div></div>
<div class="toc">
<p><b>Table of Contents</b></p>
<ul class="toc">
<li><span class="section"><a href="chapter-signals.html#sec-connecting-signal-handlers">Connecting signal handlers</a></span></li>
<li><span class="section"><a href="sec-writing-signal-handlers.html">Writing signal handlers</a></span></li>
<li><span class="section"><a href="sec-disconnecting-signal-handlers.html">Disconnecting signal handlers</a></span></li>
<li><span class="section"><a href="sec-overriding-default-signal-handlers.html">Overriding default signal handlers</a></span></li>
<li><span class="section"><a href="sec-binding-extra-arguments.html">Binding extra arguments</a></span></li>
<li><span class="section"><a href="sec-eventsignals.html">Event signals</a></span></li>
<li><span class="section"><a href="sec-exceptions-in-signal-handlers.html">Exceptions in signal handlers</a></span></li>
</ul>
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<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="sec-connecting-signal-handlers"></a>Connecting signal handlers</h2></div></div></div>
<p>
<span class="application">gtkmm</span> widget classes have signal accessor methods, such as
<code class="methodname">Gtk::Button::signal_clicked()</code>, which allow you to connect
your signal handler. Thanks to the flexibility of
<span class="application">libsigc++</span>, the callback library used by <span class="application">gtkmm</span>, the
signal handler can be almost any kind of function, but you will probably want
to use a class method. Among <span class="application">GTK</span> C coders, these
signal handlers are often named callbacks.
</p>
<p>
Here's an example of a signal handler being connected to a signal:
</p>
<pre class="programlisting"><code class="code">
#include <gtkmm/button.h>
void on_button_clicked()
{
std::cout << "Hello World" << std::endl;
}
class some_class
{
public:
some_class
{
button.signal_clicked().connect(sigc::ptr_fun(&on_button_clicked));
}
private:
Gtk::Button button {"Hello World"};
};
</code></pre>
<p>
There's rather a lot to think about in this (non-functional) code.
First let's identify the parties involved:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
<p>
The signal handler is <code class="methodname">on_button_clicked()</code>.
</p>
</li>
<li class="listitem">
<p>
We're hooking it up to the <code class="classname">Gtk::Button</code> object called
<code class="varname">button</code>.
</p>
</li>
<li class="listitem">
<p>
When the Button emits its <code class="literal">clicked</code> signal,
<code class="methodname">on_button_clicked()</code> will be called.
</p>
</li>
</ul></div>
<p>
Now let's look at the connection again:
</p>
<pre class="programlisting"><code class="code"> ...
button.signal_clicked().connect(sigc::ptr_fun(&on_button_clicked));
...
</code></pre>
<p>
Note that we don't pass a pointer to <code class="methodname">on_button_clicked()</code>
directly to the signal's <code class="methodname">connect()</code> method. Instead, we
call <code class="function">sigc::ptr_fun()</code>, and pass the result to
<code class="methodname">connect()</code>.
</p>
<p>
<code class="function">sigc::ptr_fun()</code> generates a <code class="classname">sigc::slot</code>.
A slot is an object which
looks and feels like a function, but is actually an object. These are also
known as function objects, or functors.
<code class="function">sigc::ptr_fun()</code> generates a slot for a standalone function or static method.
<code class="function">sigc::mem_fun()</code> generates a slot for a member method of a particular instance.
</p>
<p>
A C++ lambda expression is a functor which can be implicitly converted to a
<code class="classname">sigc::slot</code> in the call to <code class="methodname">connect()</code>.
A lambda expression can be used instead of <code class="function">sigc::ptr_fun()</code>.
It's also possible to use a lambda expression instead of <code class="function">sigc::mem_fun()</code>,
but then you won't get automatic disconnection of the signal handler when a
<code class="classname">sigc::trackable</code>-derived object goes out of scope.
</p>
<p>
Here's a slightly larger example of slots in action:
</p>
<pre class="programlisting"><code class="code">
#include <gtkmm/button.h>
void on_button_clicked()
{
std::cout << "Hello World" << std::endl;
}
class some_class
{
public:
some_class
{
button.signal_clicked().connect(sigc::ptr_fun(&on_button_clicked));
button.signal_clicked().connect(sigc::mem_fun(*this, &some_class::on_button_clicked));
}
void on_button_clicked();
private:
Gtk::Button button {"Hello World"};
};
</code></pre>
<p>
The first call to <code class="methodname">connect()</code> is just like the one we saw
last time; nothing new here.</p>
<p>The next is more interesting.
<code class="function">sigc::mem_fun()</code> is called with two arguments. The first
argument is <em class="parameter"><code>*this</code></em>, which is the object that our
new slot will be pointing at. The second argument is a pointer to one of its
methods. This particular version of <code class="function">sigc::mem_fun()</code>
creates a slot which will, when "called", call the pointed-to method of the
specified object.
</p>
<p>
Another thing to note about this example is that we made the call to
<code class="methodname">connect()</code> twice for the same signal object. This is
perfectly fine - when the button is clicked, both signal handlers will be
called.
</p>
<p>
We just told you that the button's <code class="literal">clicked</code> signal is expecting
to call a method with no arguments. All signals have
requirements like this - you can't hook a function with two arguments
to a signal expecting none (unless you use an adapter, such as
<code class="function">sigc::bind()</code>, of course). Therefore, it's important to
know what type of signal handler you'll be expected to connect to a given
signal.
</p>
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