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<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Chapter 1. Introduction</title><meta name="generator" content="DocBook XSL Stylesheets V1.75.2" /><link rel="home" href="index.html" title="LibSigC++" /><link rel="up" href="index.html" title="LibSigC++" /><link rel="prev" href="index.html" title="LibSigC++" /><link rel="next" href="ch02.html" title="Chapter 2. Connecting your code to signals" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter 1. Introduction</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="index.html">Prev</a> </td><th width="60%" align="center"> </th><td width="20%" align="right"> <a accesskey="n" href="ch02.html">Next</a></td></tr></table><hr /></div><div class="chapter" title="Chapter 1. Introduction"><div class="titlepage"><div><div><h2 class="title"><a id="sec-introduction"></a>Chapter 1. Introduction</h2></div></div></div><div class="toc"><p><b>Table of Contents</b></p><dl><dt><span class="sect1"><a href="ch01.html#id3074155">Motivation</a></span></dt></dl></div><div class="sect1" title="Motivation"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="id3074155"></a>Motivation</h2></div></div></div><p>There are many situations in which it is desirable to decouple code that
detects an event, and the code that deals with it. This is especially common in
GUI programming, where a toolkit might provide user interface elements such as
clickable buttons but, being a generic toolkit, doesn't know how an individual
application using that toolkit should handle the user clicking on it.</p><p>In C the callbacks are generally handled by the application calling a
'register' function and passing a pointer to a function and a <code class="literal">void *</code>
argument, eg.</p><pre class="programlisting">
void clicked(void *data);
button * okbutton = create_button("ok");
static char somedata[] = "This is some data I want the clicked() function to have";
register_click_handler(okbutton, clicked, somedata);
</pre><p>When clicked, the toolkit will call <code class="literal">clicked()</code> with the data pointer passed
to the <code class="literal">register_click_handler</code> function.</p><p>This works in C, but is not typesafe. There is no compile-time way of
ensuring that <code class="literal">clicked()</code> isn't expecting a struct of some sort instead of a
<code class="literal">char *</code>.</p><p>As C++ programmers, we want type safety. We also want to be able to use
things other than free-standing functions as callbacks.</p><p>LibSigC++ provides the concept of a slot, which holds a reference to one of
the things that can be used as a callback:
</p><div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">free-standing functions as in the example</li><li class="listitem">pointers to objects that define operator()</li><li class="listitem">pointer-to-member and instance of object to invoke that on (the
object must inherit from <code class="literal">SigC::Object</code>)</li></ul></div><p>All of which can take different numbers and types of arguments.</p><p>To make it easier to construct these, an overloaded template called <code class="literal">slot</code>
is provided. slot takes an argument (or, where necessary arguments) and returns
a generic Slot type that can be invoked with <code class="literal">operator()</code>.</p><p>For the other side of the fence, LibSigC++ provides <code class="literal">signal</code>s, to which the
client can attach <code class="literal">slot</code>s. When the <code class="literal">signal</code> is emitted, all the connected
<code class="literal">slot</code>s are called back.</p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="index.html">Prev</a> </td><td width="20%" align="center"> </td><td width="40%" align="right"> <a accesskey="n" href="ch02.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">LibSigC++ </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 2. Connecting your code to signals</td></tr></table></div></body></html>
