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<a name="sec-signals-comparison"></a>Appendix D. Comparison with other signalling systems</h1></div></div></div>
<p>
(An aside: <span class="application">GTK</span> calls this scheme "signalling"; the
sharp-eyed reader with GUI toolkit experience will note that this same design
is often
seen under the name of "broadcaster-listener" (e.g., in Metrowerks'
PowerPlant framework for the Macintosh). It works in much the same
way: one sets up <code class="literal">broadcasters</code>, and then connects
<code class="literal">listeners</code> to them; the broadcaster keeps a list of the
objects listening to it, and when someone gives the broadcaster a
message, it calls all of its objects in its list with the message. In
<span class="application">gtkmm</span>, signal objects play the role of broadcasters, and slots
play the role of listeners - sort of. More on this later.)
</p>
<p>
<span class="application">gtkmm</span> signal handlers are strongly-typed, whereas
<span class="application">GTK</span> C code allows you to connect a callback with
the wrong number and type of arguments, leading to a segfault at runtime. And,
unlike <span class="application">Qt</span>, <span class="application">gtkmm</span> achieves this without modifying
the C++ language.</p>
<p>
Re. Overriding signal handlers: You can do this in the straight-C world of GTK too; that's what GTK's
object system is for. But in GTK, you have to go through some
complicated procedures to get object-oriented features like
inheritance and overloading. In C++, it's simple, since those
features are supported in the language itself; you can let the
compiler do the dirty work.
</p>
<p>
This is one of the places where the beauty of C++ really comes out.
One wouldn't think of subclassing a GTK widget simply to override its
action method; it's just too much trouble. In GTK, you almost always
use signals to get things done, unless you're writing a new widget.
But because overriding methods is so easy in C++, it's entirely
practical - and sensible - to subclass a button for that purpose.
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