<?xml version="1.0"?>
<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN" "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" >
<book>
<bookinfo>
<title>LibSigC++</title>
<author>
<firstname>Ainsley</firstname>
<surname>Pereira</surname>
</author>
<pubdate>September 2002</pubdate>
<abstract>
<para>LibSigC++ is a C++ template library implementing typesafe callbacks. This is an intro to LibSigC++.</para>
</abstract>
</bookinfo>
<chapter id="sec-introduction">
<title>Introduction</title>
<sect1>
<title>Motivation</title>
<para>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.</para>
<para>In C the callbacks are generally handled by the application calling a
'register' function and passing a pointer to a function and a <literal remap="tt">void *</literal>
argument, eg.</para>
<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);
</programlisting>
<para>When clicked, the toolkit will call <literal remap="tt">clicked()</literal> with the data pointer passed
to the <literal remap="tt">register_click_handler</literal> function.</para>
<para>This works in C, but is not typesafe. There is no compile-time way of
ensuring that <literal remap="tt">clicked()</literal> isn't expecting a struct of some sort instead of a
<literal remap="tt">char *</literal>.</para>
<para>As C++ programmers, we want type safety. We also want to be able to use
things other than free-standing functions as callbacks.</para>
<para>LibSigC++ provides the concept of a slot, which holds a reference to one of
the things that can be used as a callback:
<itemizedlist>
<listitem>free-standing functions as in the example</listitem>
<listitem>pointers to objects that define operator()</listitem>
<listitem>pointer-to-member and instance of object to invoke that on (the
object must inherit from <literal remap="tt">SigC::Object</literal>)</listitem>
</itemizedlist></para>
<para>All of which can take different numbers and types of arguments.</para>
<para>To make it easier to construct these, an overloaded template called <literal remap="tt">slot</literal>
is provided. slot takes an argument (or, where necessary arguments) and returns
a generic Slot type that can be invoked with <literal remap="tt">operator()</literal>.</para>
<para>For the other side of the fence, LibSigC++ provides <literal remap="tt">signal</literal>s, to which the
client can attach <literal remap="tt">slot</literal>s. When the <literal remap="tt">signal</literal> is emitted, all the connected
<literal remap="tt">slot</literal>s are called back.</para>
</sect1>
</chapter>
<chapter id="sec-connecting">
<title>Connecting your code to signals</title>
<sect1>
<title>A simple example</title>
<para>The terminology for signals and slots has come under some
criticism for not being as intuitive as it maybe could have been, but with a little
experience it won't cause a problem. Honest.</para>
<para>So to get some experience, lets look at a simple example...</para>
<para>Lets say you and I are writing an application which informs the user when
aliens land in the car park. To keep the design nice and clean, and allow for
maximum portability to different interfaces, we decide to use LibSigC++ to
split the project in two parts.</para>
<para>I will write the <literal remap="tt">AlienDetector</literal> class, and you will write the code to inform
the user. (Well, OK, I'll write both, but we're pretending, remember?)</para>
<para>Here's my class:</para>
<programlisting>
class AlienDetector
{
public:
AlienDetector();
void run();
SigC::Signal0<void> detected;
};
</programlisting>
<para>(I'll explain the type of detected later.)</para>
<para>Here's your code that uses it:</para>
<programlisting>
void warn_people()
{
cout << "There are aliens in the carpark!" << endl;
}
int main()
{
AlienDetector mydetector;
mydetector.detected.connect( SigC::slot(warn_people) );
mydetector.run();
return 0;
}
</programlisting>
<para>Pretty simple really - you call the <literal remap="tt">connect()</literal> method on the signal to
connect your function. <literal remap="tt">connect()</literal> takes a <literal remap="tt">slot</literal> parameter (remember slots
are capable of holding any type of callback), so you convert your
<literal remap="tt">warn_people()</literal> function to a slot using the <literal remap="tt">slot()</literal> function.</para>
<para>To compile this example from the downloadable example code, use:</para>
<programlisting>g++ example1.cc -o eg1 `pkg-config --cflags --libs sigc++-1.2`</programlisting>
<para>Note that those `` characters are backticks, not single quotes. Run it with</para>
<programlisting>./eg1</programlisting>
<para>(Try not to panic when the aliens land!)</para>
</sect1>
<sect1>
<title>Using a member function</title>
<para>Suppose you found a more sophisticated alien alerter class on the web,
such as this:</para>
<programlisting>
class AlienAlerter : public SigC::Object
{
public:
AlienAlerter(char const* servername);
void alert();
private:
// ...
};
</programlisting>
<para>(Handily it derives from <literal remap="tt">SigC::Object</literal> already. This isn't quite so
unlikely as you might think; all appropriate bits of the popular gtkmm library do so,
for example.)</para>
<para>You could rewrite your code as follows:</para>
<programlisting>
int main()
{
AlienDetector mydetector;
AlienAlerter myalerter("localhost"); // added
mydetector.detected.connect( SigC::slot(myalerter, &AlienAlerter::alert) ); // changed
mydetector.run();
return 0;
}
</programlisting>
<para>Note that only 2 lines are different - one to create an instance of the
class, and the line to connect the method to the signal.</para>
<para>This code is in example2.cc, which can be compiled in the same way as
example1.cc</para>
</sect1>
<sect1>
<title>Signals with parameters</title>
<para>Functions taking no parameters and returning void are quite useful,
especially when they're members of classes that can store unlimited amounts of
safely typed data, but they're not sufficient for everything.</para>
<para>What if aliens don't land in the carpark, but somewhere else? Let's modify
the example so that the callback function takes a <literal remap="tt">std::string</literal> with the location
in which aliens were detected.</para>
<para>I change my class to:</para>
<programlisting>
class AlienDetector
{
public:
AlienDetector();
void run();
SigC::Signal1<void, std::string> detected; // changed
};
</programlisting>
<para>The only line I had to change was the signal line (in <literal remap="tt">run()</literal> I need to change
my code to supply the argument when I emit the signal too, but that's not shown
here).</para>
<para>The name of the type is '<literal remap="tt">SignalN</literal>', where N is the number of arguments that
the slots should take. The template parameters are the return type, then the
argument types.</para>
<para>Obviously LibSigC++ doesn't define an infinite number of <literal remap="tt">Signal</literal> templates,
but it does define <literal remap="tt">Signal0</literal>-<literal remap="tt">Signal5</literal>, which should be enough for most people. (If
you know M4, you can tweak it to provide more if you really need to.)</para>
<para>The types in the function signature are in the same order as the template
parameters, eg:</para>
<programlisting>
SigC::Signal1<void, std::string>
void function(std::string foo);
</programlisting>
<para>So now you can update your alerter (for simplicity, lets go back to the
free-standing function version):</para>
<programlisting>
void warn_people(std::string where)
{
cout << "There are aliens in " << where << "!" << endl;
}
int main()
{
AlienDetector mydetector;
mydetector.detected.connect( SigC::slot(warn_people) );
mydetector.run();
return 0;
}
</programlisting>
<para>Easy.</para>
</sect1>
<sect1>
<title>Disconnecting</title>
<para>If you decide you no longer want your code to be called whenever a signal is
emitted, you must remember the return value of <literal remap="tt">connect()</literal>, which we've been
ignoring until now.</para>
<para><literal remap="tt">connect()</literal> returns a <literal remap="tt">SigC::Connection</literal> object, which has a member
<literal remap="tt">disconnect()</literal>. This does just what you think it does.</para>
</sect1>
</chapter>
<chapter id="sec-writing">
<title>Writing your own signals</title>
<sect1>
<title>Quick recap</title>
<para>If all you want to do is use gtkmm, and connect your functionality to its
signals, you can probably stop reading here.</para>
<para>You might benefit from reading on anyway though, as this section is going to
be quite simple, and the 'Rebinding' technique from the next section is
occasionally useful.</para>
<para>We've already covered the way the types of signals are made up, but lets
recap:</para>
<para>A signal is an instance of a template, named <literal remap="tt">SigC::SignalN</literal> where
N is the number of arguments taken, 0-5. The template arguments are the types,
in the order they appear in the function signature that can be connected to that
signal; that is the return type, then the argument types.</para>
<para>To provide a signal for people to connect to, you must make available an
instance of that <literal remap="tt">SigC::Signal</literal>. In <literal remap="tt">AlienDetector</literal> this was done
with a public data member. That's not considered good practice usually, so you
might want to consider making a member function that returns the signal by
reference. (This is what gtkmm does.)</para>
<para>Once you've done this, all you have to do is emit the signal when you're
ready. Look at the code for <literal remap="tt">AlienDetector::run()</literal>:</para>
<programlisting>
void AlienDetector::run()
{
sleep(3); // wait for aliens
detected.emit(); // panic!
}
</programlisting>
<para>As a shortcut, <literal remap="tt">Signal</literal> defines <literal remap="tt">operator()</literal> as a synonym for
<literal remap="tt">emit()</literal>, so you could just write <literal remap="tt">detected();</literal> as in the second
example version:</para>
<programlisting>
void AlienDetector::run()
{
sleep(3); // wait for aliens
detected("the carpark"); // this is the std::string version, looks like
// they landed in the carpark afterall.
}
</programlisting>
</sect1>
<sect1>
<title>What about return values?</title>
<para>If you only ever have one slot connected to a signal, or if you only care
about the return value of the last registered one, it's quite straightforward:</para>
<programlisting>
SigC::Signal0<int> somesignal;
int a_return_value;
a_return_value = somesignal();
</programlisting>
<para>If you care about every return value things are a little more complicated.
See the section on Marshallers for more info.</para>
</sect1>
</chapter>
<chapter id="sec-advanced">
<title>Advanced topics</title>
<sect1>
<title>Rebinding</title>
<para>Suppose you already have a function that you want to be called back when a
signal is emitted, but it takes the wrong argument types. For example, lets try
to attach the <literal remap="tt">warn_people(std::string)</literal> function to the detected signal
from the first example, which didn't supply a location string.</para>
<para>Just trying to connect it with:</para>
<programlisting>
myaliendetector.detected.connect(SigC::slot(warn_people));
</programlisting>
<para>results in a compile-time error, because the types don't match. This is good!
This is typesafety at work. In the C way of doing things, this would have just
died at runtime after trying to print a random bit of memory as the location -
ick!</para>
<para>We have to make up a location string, and bind it to the function, so that
when detected is emitted with no arguments, something adds it in before
<literal remap="tt">warn_people</literal> is actually called.</para>
<para>We could write it ourselves - it's not hard:</para>
<programlisting>
void warn_people_wrapper() // note this is the signature that 'detected' expects
{
warn_people("the carpark");
}
</programlisting>
<para>but after our first million or so we might start looking for a better way. As
it happens, LibSigC++ has one.</para>
<programlisting>
SigC::bind(slot, arg);
</programlisting>
<para>binds arg as the argument to slot, and returns a new slot of the same return
type, but with one fewer arguments.</para>
<para>Now we can write:</para>
<programlisting>
myaliendetector.detected.connect(SigC::bind( SigC::slot(warn_people), "the carpark" ) );
</programlisting>
<para>If the input slot has multiple args, the rightmost one is bound.</para>
<para>The return type can also be bound with <literal remap="tt">bind_return(slot, returnvalue);</literal> though
this is not so commonly useful.</para>
<para>So if we can attach the new <literal remap="tt">warn_people()</literal> to the old detector, can we attach
the old <literal remap="tt">warn_people</literal> (the one that didn't take an argument) to the new detector?</para>
<para>Of course, we just need to hide the extra argument. This can be done with
<literal remap="tt">SigC::hide</literal>, eg.</para>
<programlisting>
myaliendetector.detected.connect( SigC::hide<std::string>( SigC::slot(warn_people) ) );
</programlisting>
<para>The template arguments are the types to hide (from the right only - you can't
hide the first argument of 3, for example, only the last).</para>
<para><literal remap="tt">hide_return</literal> effectively makes the return type void.</para>
</sect1>
<sect1>
<title>Retyping</title>
<para>A similar topic is retyping. Perhaps you have a signal that takes an <literal remap="tt">int</literal>, but
you want to connect a function that takes a <literal remap="tt">double</literal>.</para>
<para>This can be achieved with the <literal remap="tt">retype</literal> template. <literal remap="tt">retype</literal> has template arguments
just like <literal remap="tt">Signal</literal> - return value, signal types.</para>
<para>It's a function template that takes a <literal remap="tt">slot</literal>, and returns a <literal remap="tt">slot</literal>. eg.</para>
<programlisting>
void dostuff(double foo)
{
}
SigC::Signal1<void,int> asignal;
asignal.connect( retype<void,int>( slot(&dostuff) ) );
</programlisting>
<para>If you only want to change the return type, you can use <literal remap="tt">retype_return</literal>.
<literal remap="tt">retype_return</literal> needs only one template argument.</para>
</sect1>
<sect1>
<title>Marshallers</title>
<para>When I first mentioned return values, I said that more advanced handling of
multiple return values was possible with <literal remap="tt">Marshallers</literal>.</para>
<para>A Marshaller is a class that gets fed all the return values as they're
returned. It can do a couple of things:
<itemizedlist>
<listitem>It can stop the emit process at any point, causing no further slots
to be called</listitem>
<listitem>It can return a value, of any type</listitem>
</itemizedlist></para>
<para>For example, if each <literal remap="tt">slot</literal> returned an <literal remap="tt">int</literal>, we could use a marshaller return
the average value as a <literal remap="tt">double</literal>. Or we could return all values in a
<literal remap="tt">std::vector<int></literal>, or maybe stop as soon as the first slot returns 5.</para>
<para>As an example, here's the averaging marshaller:</para>
<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_;
};
</programlisting>
<para>To use this, we pass the type as an extra template argument when defining
the <literal remap="tt">Signal</literal>, eg.</para>
<programlisting>
SigC::Signal0<int,Averager> mysignal;
</programlisting>
<para>Now we can do:</para>
<programlisting>
double average_of_all_connected_slots = mysignal();
</programlisting>
<para>Each connected <literal remap="tt">slot</literal> will be called, its value passed to an instance of
<literal remap="tt">Averager</literal> and that <literal remap="tt">Averager</literal>'s <literal remap="tt">value()</literal> will be returned.</para>
<para>In the downloadable examples, this is example6.cc.</para>
</sect1>
</chapter>
<chapter id="sec-reference">
<title>Reference</title>
<para>See the reference documentation <ulink url="http://libsigc.sourceforge.net/libsigc1_2/reference/">online</ulink></para>
</chapter>
</book>
