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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>Marshallers</title><meta name="generator" content="DocBook XSL Stylesheets V1.75.2" /><link rel="home" href="index.html" title="LibSigC++" /><link rel="up" href="ch04.html" title="Chapter 4. Advanced topics" /><link rel="prev" href="ch04s02.html" title="Retyping" /><link rel="next" href="ch05.html" title="Chapter 5. Reference" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Marshallers</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ch04s02.html">Prev</a> </td><th width="60%" align="center">Chapter 4. Advanced topics</th><td width="20%" align="right"> <a accesskey="n" href="ch05.html">Next</a></td></tr></table><hr /></div><div class="sect1" title="Marshallers"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="id3095282"></a>Marshallers</h2></div></div></div><p>When I first mentioned return values, I said that more advanced handling of
multiple return values was possible with <code class="literal">Marshallers</code>.</p><p>A Marshaller is a class that gets fed all the return values as they're
returned. It can do a couple of things:
</p><div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">It can stop the emit process at any point, causing no further slots
to be called</li><li class="listitem">It can return a value, of any type</li></ul></div><p>For example, if each <code class="literal">slot</code> returned an <code class="literal">int</code>, we could use a marshaller return
the average value as a <code class="literal">double</code>. Or we could return all values in a
<code class="literal">std::vector<int></code>, or maybe stop as soon as the first slot returns 5.</p><p>As an example, here's the averaging marshaller:</p><pre class="programlisting">
class Averager
{
public:
// we must typedef InType and OutType for the SigC library
typedef double OutType;
typedef int InType;
Averager()
: total_(0), number_(0)
{}
OutType value() { return (double)total_/(double)number_; } // avoid integer division
static OutType default_value() { return 0; }
// This is the function called for each return value.
// If it returns 'true' it stops here.
bool marshal(InType newval)
{
total_ += newval; // total of values
++number_; // count of values
return false; // continue emittion process
};
private:
int total_;
int number_;
};
</pre><p>To use this, we pass the type as an extra template argument when defining
the <code class="literal">Signal</code>, eg.</p><pre class="programlisting">
SigC::Signal0<int,Averager> mysignal;
</pre><p>Now we can do:</p><pre class="programlisting">
double average_of_all_connected_slots = mysignal();
</pre><p>Each connected <code class="literal">slot</code> will be called, its value passed to an instance of
<code class="literal">Averager</code> and that <code class="literal">Averager</code>'s <code class="literal">value()</code> will be returned.</p><p>In the downloadable examples, this is example6.cc.</p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ch04s02.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="ch04.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="ch05.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Retyping </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 5. Reference</td></tr></table></div></body></html>
