<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE section PUBLIC "-//Boost//DTD BoostBook XML V1.0//EN"
  "http://www.boost.org/tools/boostbook/dtd/boostbook.dtd">
<section last-revision="$Date: 2004/11/18 23:15:15 $" id="signals.tutorial">
  <title>Tutorial</title>

  <using-namespace name="boost"/>
  <using-namespace name="boost::signals"/>
  <using-class name="boost::signalN"/>

  <section>
    <title>How to Read this Tutorial</title>
<para>This tutorial is not meant to be read linearly. Its top-level
structure roughly separates different concepts in the library
(e.g., handling calling multiple slots, passing values to and from
slots) and in each of these concepts the basic ideas are presented
first and then more complex uses of the library are described
later. Each of the sections is marked <emphasis>Beginner</emphasis>,
<emphasis>Intermediate</emphasis>, or <emphasis>Advanced</emphasis> to help guide the
reader. The <emphasis>Beginner</emphasis> sections include information that all
library users should know; one can make good use of the Signals
library after having read only the <emphasis>Beginner</emphasis> sections. The
<emphasis>Intermediate</emphasis> sections build on the <emphasis>Beginner</emphasis>
sections with slightly more complex uses of the library. Finally,
the <emphasis>Advanced</emphasis> sections detail very advanced uses of the
Signals library, that often require a solid working knowledge of
the <emphasis>Beginner</emphasis> and <emphasis>Intermediate</emphasis> topics; most users
will not need to read the <emphasis>Advanced</emphasis> sections.</para>
</section>

<section><title>Compatibility Note</title> 

<para>Boost.Signals has two syntactical forms: the preferred form and
the compatibility form. The preferred form fits more closely with the
C++ language and reduces the number of separate template parameters
that need to be considered, often improving readability; however, the
preferred form is not supported on all platforms due to compiler
bugs. The compatible form will work on all compilers supported by
Boost.Signals. Consult the table below to determine which syntactic
form to use for your compiler. Users of Boost.Function, please note
that the preferred syntactic form in Signals is equivalent to that of
Function's preferred syntactic form.</para>

<para>If your compiler does not appear in this list, please try the
preferred syntax and report your results to the Boost list so that
we can keep this table up-to-date.</para>

  <informaltable>
    <tgroup cols="2" align="left">
      <thead>
        <row>
          <entry>Preferred syntax</entry>
          <entry>Portable syntax</entry>
        </row>
      </thead>
      <tbody>
        <row>
          <entry>
            <itemizedlist>
              <listitem><para>GNU C++ 2.95.x, 3.0.x, 3.1.x</para></listitem>
              <listitem><para>Comeau C++ 4.2.45.2</para></listitem>
              <listitem><para>SGI MIPSpro 7.3.0</para></listitem>
              <listitem><para>Intel C++ 5.0, 6.0</para></listitem>
              <listitem><para>Compaq's cxx 6.2</para></listitem>
              <listitem><para>Microsoft Visual C++ 7.1</para></listitem>
            </itemizedlist>
          </entry>
          <entry>
            <itemizedlist>
              <listitem><para><emphasis>Any compiler supporting the preferred syntax</emphasis></para></listitem>
              <listitem><para>Microsoft Visual C++ 6.0, 7.0</para></listitem>
              <listitem><para>Borland C++ 5.5.1</para></listitem>
              <listitem><para>Sun WorkShop 6 update 2 C++ 5.3</para></listitem>
              <listitem><para>Metrowerks CodeWarrior 8.1</para></listitem>
            </itemizedlist>
          </entry>
        </row>
      </tbody>
    </tgroup>
  </informaltable>
</section>

<section><title>Hello, World! (Beginner)</title>
<para>The following example writes "Hello, World!" using signals and
slots. First, we create a signal <code>sig</code>, a signal that
takes no arguments and has a void return value. Next, we connect
the <code>hello</code> function object to the signal using the
<code>connect</code> method. Finally, use the signal
<code>sig</code> like a function to call the slots, which in turns
invokes <code>HelloWorld::operator()</code> to print "Hello,
World!".</para>
  <informaltable>
    <tgroup cols="2" align="left">
      <thead>
        <row>
          <entry>Preferred syntax</entry>
          <entry>Portable syntax</entry>
        </row>
      </thead>
      <tbody>
        <row>
          <entry>
<programlisting>
struct HelloWorld 
{
  void operator()() const 
  { 
    std::cout &lt;&lt; "Hello, World!" &lt;&lt; std::endl;
  } 
};

// ...

// Signal with no arguments and a void return value
<classname>boost::signal</classname>&lt;void ()&gt; sig;

// Connect a HelloWorld slot
HelloWorld hello;
sig.<methodname>connect</methodname>(hello);

// Call all of the slots
sig();
</programlisting>
</entry>
<entry>
<programlisting>
struct HelloWorld 
{
  void operator()() const 
  { 
    std::cout &lt;&lt; "Hello, World!" &lt;&lt; std::endl;
  } 
};

// ...

// Signal with no arguments and a void return value
<classname alt="boost::signalN">boost::signal0</classname>&lt;void&gt; sig;

// Connect a HelloWorld slot
HelloWorld hello;
sig.<methodname>connect</methodname>(hello);

// Call all of the slots
sig();
</programlisting>
</entry>
          </row>
        </tbody>
      </tgroup>
    </informaltable>
</section>

<section><title>Calling multiple slots</title>
<section><title>Connecting multiple slots (Beginner)</title>
<para>Calling a single slot from a signal isn't very interesting, so
we can make the Hello, World program more interesting by splitting
the work of printing "Hello, World!" into two completely separate
slots. The first slot will print "Hello" and may look like
this:</para>
<programlisting>
struct Hello 
{
  void operator()() const
  {
    std::cout &lt;&lt; "Hello";
  }
};
</programlisting>
<para>The second slot will print ", World!" and a newline, to complete
the program. The second slot may look like this:</para>
<programlisting>
struct World
{
  void operator()() const
  {
    std::cout &lt;&lt; ", World!" &lt;&lt; std::endl;
  }
};
</programlisting>
<para>Like in our previous example, we can create a signal
<code>sig</code> that takes no arguments and has a
<code>void</code> return value. This time, we connect both a
<code>hello</code> and a <code>world</code> slot to the same
signal, and when we call the signal both slots will be called.</para>
  <informaltable>
    <tgroup cols="2" align="left">
      <thead>
        <row>
          <entry>Preferred syntax</entry>
          <entry>Portable syntax</entry>
        </row>
      </thead>
      <tbody>
        <row>
          <entry>
<programlisting>
<classname>boost::signal</classname>&lt;void ()&gt; sig;

sig.<methodname>connect</methodname>(Hello());
sig.<methodname>connect</methodname>(World());

sig();
</programlisting>
</entry>
<entry>
<programlisting>
<classname alt="boost::signalN">boost::signal0</classname>&lt;void&gt; sig;

sig.<methodname>connect</methodname>(Hello());
sig.<methodname>connect</methodname>(World());

sig();
</programlisting>
</entry>
            </row>
          </tbody>
        </tgroup>
      </informaltable>
<para>By default, slots are called in first-in first-out (FIFO) order,
so the output of this program will be as expected:</para>
<programlisting>
Hello, World!
</programlisting>
</section>

<section><title>Ordering slot call groups (Intermediate)</title>
<para>Slots are free to have side effects, and that can mean that some
slots will have to be called before others even if they are not connected in that order. The Boost.Signals
library allows slots to be placed into groups that are ordered in
some way. For our Hello, World program, we want "Hello" to be
printed before ", World!", so we put "Hello" into a group that must
be executed before the group that ", World!" is in. To do this, we
can supply an extra parameter at the beginning of the
<code>connect</code> call that specifies the group. Group values
are, by default, <code>int</code>s, and are ordered by the integer
&lt; relation. Here's how we construct Hello, World:</para>
  <informaltable>
    <tgroup cols="2" align="left">
      <thead>
        <row>
          <entry>Preferred syntax</entry>
          <entry>Portable syntax</entry>
        </row>
      </thead>
      <tbody>
        <row>
          <entry>
<programlisting>
<classname>boost::signal</classname>&lt;void ()&gt; sig;
sig.<methodname>connect</methodname>(1, World());
sig.<methodname>connect</methodname>(0, Hello());
sig();
</programlisting>
</entry>
            <entry>
<programlisting>
<classname alt="boost::signalN">boost::signal0</classname>&lt;void&gt; sig;
sig.<methodname>connect</methodname>(1, World());
sig.<methodname>connect</methodname>(0, Hello());
sig();
</programlisting>
</entry>
            </row>
          </tbody>
        </tgroup>
      </informaltable>

<para>This program will correctly print "Hello, World!", because the
<code>Hello</code> object is in group 0, which precedes group 1 where
the <code>World</code> object resides. The group
parameter is, in fact, optional. We omitted it in the first Hello,
World example because it was unnecessary when all of the slots are
independent. So what happens if we mix calls to connect that use the
group parameter and those that don't? The "unnamed" slots (i.e., those
that have been connected without specifying a group name) can be
placed at the front or back of the slot list (by passing
<code>boost::signals::at_front</code> or <code>boost::signals::at_back</code>
as the last parameter to <code><methodname
alt="boost::signalN::connect">connect</methodname></code>, respectively), and defaults to the end of the list. When
a group is specified, the final parameter describes where the slot
will be placed within the group ordering. If we add a new slot
to our example like this:</para>
<programlisting>
struct GoodMorning
{
  void operator()() const
  {
    std::cout &lt;&lt; "... and good morning!" &lt;&lt; std::endl;
  }
};

sig.<methodname>connect</methodname>(GoodMorning());
</programlisting>
<para>... we will get the result we wanted:</para>
<programlisting>
Hello, World!
... and good morning!
</programlisting>
</section>
</section>

<section><title>Passing values to and from slots</title>
<section><title>Slot Arguments (Beginner)</title>
<para>Signals can propagate arguments to each of the slots they call.
For instance, a signal that propagates mouse motion events might
want to pass along the new mouse coordinates and whether the mouse
buttons are pressed.</para>
<para>As an example, we'll create a signal that passes two
<code>float</code> arguments to its slots. Then we'll create a few
slots that print the results of various arithmetic operations on
these values.</para>
<programlisting>
void print_sum(float x, float y)
{
  std::cout &lt;&lt; "The sum is " &lt;&lt; x+y &lt;&lt; std::endl;
}

void print_product(float x, float y)
{
  std::cout &lt;&lt; "The product is " &lt;&lt; x*y &lt;&lt; std::endl;
}

void print_difference(float x, float y)
{
  std::cout &lt;&lt; "The difference is " &lt;&lt; x-y &lt;&lt; std::endl;
}

void print_quotient(float x, float y)
{
  std::cout &lt;&lt; "The quotient is " &lt;&lt; x/y &lt;&lt; std::endl;
}
</programlisting>

  <informaltable>
    <tgroup cols="2" align="left">
      <thead>
        <row>
          <entry>Preferred syntax</entry>
          <entry>Portable syntax</entry>
        </row>
      </thead>
      <tbody>
        <row>
          <entry>
<programlisting>
<classname>boost::signal</classname>&lt;void (float, float)&gt; sig;

sig.<methodname>connect</methodname>(&amp;print_sum);
sig.<methodname>connect</methodname>(&amp;print_product);
sig.<methodname>connect</methodname>(&amp;print_difference);
sig.<methodname>connect</methodname>(&amp;print_quotient);

sig(5, 3);
</programlisting>
</entry>
<entry>
<programlisting>
<classname alt="boost::signalN">boost::signal2</classname>&lt;void, float, float&gt; sig;

sig.<methodname>connect</methodname>(&amp;print_sum);
sig.<methodname>connect</methodname>(&amp;print_product);
sig.<methodname>connect</methodname>(&amp;print_difference);
sig.<methodname>connect</methodname>(&amp;print_quotient);

sig(5, 3);
</programlisting>
</entry>
              </row>
            </tbody>
          </tgroup>
        </informaltable>

<para>This program will print out the following:</para>
<programlisting>
The sum is 8
The difference is 2
The product is 15
The quotient is 1.66667
</programlisting>
<para>So any values that are given to <code>sig</code> when it is
called like a function are passed to each of the slots. We have to
declare the types of these values up front when we create the
signal. The type <code><classname>boost::signal</classname>&lt;void (float,
float)&gt;</code> means that the signal has a <code>void</code>
return value and takes two <code>float</code> values. Any slot
connected to <code>sig</code> must therefore be able to take two
<code>float</code> values.</para>
</section>

<section><title>Signal Return Values (Advanced)</title>
<para>Just as slots can receive arguments, they can also return
values. These values can then be returned back to the caller of the
signal through a <firstterm>combiner</firstterm>. The combiner is a mechanism
that can take the results of calling slots (there many be no
results or a hundred; we don't know until the program runs) and
coalesces them into a single result to be returned to the caller.
The single result is often a simple function of the results of the
slot calls: the result of the last slot call, the maximum value
returned by any slot, or a container of all of the results are some
possibilities.</para>
<para>We can modify our previous arithmetic operations example
slightly so that the slots all return the results of computing the
product, quotient, sum, or difference. Then the signal itself can
return a value based on these results to be printed:</para>
  <informaltable>
    <tgroup cols="2" align="left">
      <thead>
        <row>
          <entry>Preferred syntax</entry>
          <entry>Portable syntax</entry>
        </row>
      </thead>
      <tbody>
        <row>
          <entry>
<programlisting>
float product(float x, float y) { return x*y; }
float quotient(float x, float y) { return x/y; }
float sum(float x, float y) { return x+y; }
float difference(float x, float y) { return x-y; }

<classname>boost::signal</classname>&lt;float (float x, float y)&gt; sig;

sig.<methodname>connect</methodname>(&amp;product);
sig.<methodname>connect</methodname>(&amp;quotient);
sig.<methodname>connect</methodname>(&amp;sum);
sig.<methodname>connect</methodname>(&amp;difference);

std::cout &lt;&lt; sig(5, 3) &lt;&lt; std::endl;
</programlisting>
</entry>
<entry>
<programlisting>
float product(float x, float y) { return x*y; }
float quotient(float x, float y) { return x/y; }
float sum(float x, float y) { return x+y; }
float difference(float x, float y) { return x-y; }

<classname alt="boost::signalN">boost::signal2</classname>&lt;float, float, float&gt; sig;

sig.<methodname>connect</methodname>(&amp;product);
sig.<methodname>connect</methodname>(&amp;quotient);
sig.<methodname>connect</methodname>(&amp;sum);
sig.<methodname>connect</methodname>(&amp;difference);

std::cout &lt;&lt; sig(5, 3) &lt;&lt; std::endl;
</programlisting>
</entry>
            </row>
          </tbody>
        </tgroup>
      </informaltable>

<para>This example program will output <code>2</code>. This is because the
default behavior of a signal that has a return type
(<code>float</code>, the first template argument given to the
<code><classname>boost::signal</classname></code> class template) is to call all slots and
then return the result returned by the last slot called. This
behavior is admittedly silly for this example, because slots have
no side effects and the result is the last slot connect.</para>
<para>A more interesting signal result would be the maximum of the
values returned by any slot. To do this, we create a custom
combiner that looks like this:</para>
<programlisting>
template&lt;typename T&gt;
struct maximum
{
  typedef T result_type;

  template&lt;typename InputIterator&gt;
  T operator()(InputIterator first, InputIterator last) const
  {
    // If there are no slots to call, just return the
    // default-constructed value
    if (first == last)
      return T();

    T max_value = *first++;
    while (first != last) {
      if (max_value &lt; *first)
        max_value = *first;
      ++first;
    }
  
    return max_value;
  }
};
</programlisting>
<para>The <code>maximum</code> class template acts as a function
object. Its result type is given by its template parameter, and
this is the type it expects to be computing the maximum based on
(e.g., <code>maximum&lt;float&gt;</code> would find the maximum
<code>float</code> in a sequence of <code>float</code>s). When a
<code>maximum</code> object is invoked, it is given an input
iterator sequence <code>[first, last)</code> that includes the
results of calling all of the slots. <code>maximum</code> uses this
input iterator sequence to calculate the maximum element, and
returns that maximum value.</para>
<para>We actually use this new function object type by installing it
as a combiner for our signal. The combiner template argument
follows the signal's calling signature:</para>
  <informaltable>
    <tgroup cols="2" align="left">
      <thead>
        <row>
          <entry>Preferred syntax</entry>
          <entry>Portable syntax</entry>
        </row>
      </thead>
      <tbody>
        <row>
          <entry>
<programlisting>
<classname>boost::signal</classname>&lt;float (float x, float y), 
              maximum&lt;float&gt; &gt; sig;
</programlisting>
</entry>
<entry>
<programlisting>
<classname alt="boost::signalN">boost::signal2</classname>&lt;float, float, float, 
               maximum&lt;float&gt; &gt; sig;
</programlisting>
</entry>
            </row>
          </tbody>
        </tgroup>
      </informaltable>

<para>Now we can connect slots that perform arithmetic functions and
use the signal:</para>
<programlisting>
sig.<methodname>connect</methodname>(&amp;quotient);
sig.<methodname>connect</methodname>(&amp;product);
sig.<methodname>connect</methodname>(&amp;sum);
sig.<methodname>connect</methodname>(&amp;difference);

std::cout &lt;&lt; sig(5, 3) &lt;&lt; std::endl;
</programlisting>
<para>The output of this program will be <code>15</code>, because
regardless of the order in which the slots are connected, the product
of 5 and 3 will be larger than the quotient, sum, or
difference.</para>
<para>In other cases we might want to return all of the values
computed by the slots together, in one large data structure. This
is easily done with a different combiner:</para>
<programlisting>
template&lt;typename Container&gt;
struct aggregate_values
{
  typedef Container result_type;

  template&lt;typename InputIterator&gt;
  Container operator()(InputIterator first, InputIterator last) const
  {
    return Container(first, last);
  }
};
</programlisting>
<para>
Again, we can create a signal with this new combiner: 
</para>
  <informaltable>
    <tgroup cols="2" align="left">
      <thead>
        <row>
          <entry>Preferred syntax</entry>
          <entry>Portable syntax</entry>
        </row>
      </thead>
      <tbody>
        <row>
          <entry>
<programlisting>
<classname>boost::signal</classname>&lt;float (float, float), 
    aggregate_values&lt;std::vector&lt;float&gt; &gt; &gt; sig;

sig.<methodname>connect</methodname>(&amp;quotient);
sig.<methodname>connect</methodname>(&amp;product);
sig.<methodname>connect</methodname>(&amp;sum);
sig.<methodname>connect</methodname>(&amp;difference);

std::vector&lt;float&gt; results = sig(5, 3);
std::copy(results.begin(), results.end(), 
    std::ostream_iterator&lt;float&gt;(cout, " "));
</programlisting>
</entry>
<entry>
<programlisting>
<classname alt="boost::signalN">boost::signal2</classname>&lt;float, float, float,
    aggregate_values&lt;std::vector&lt;float&gt; &gt; &gt; sig;

sig.<methodname>connect</methodname>(&amp;quotient);
sig.<methodname>connect</methodname>(&amp;product);
sig.<methodname>connect</methodname>(&amp;sum);
sig.<methodname>connect</methodname>(&amp;difference);

std::vector&lt;float&gt; results = sig(5, 3);
std::copy(results.begin(), results.end(), 
    std::ostream_iterator&lt;float&gt;(cout, " "));
</programlisting>
</entry>
            </row>
          </tbody>
        </tgroup>
      </informaltable>

<para>The output of this program will contain 15, 8, 1.6667, and 2. It
is interesting here that
the first template argument for the <code>signal</code> class,
<code>float</code>, is not actually the return type of the signal.
Instead, it is the return type used by the connected slots and will
also be the <code>value_type</code> of the input iterators passed
to the combiner. The combiner itself is a function object and its
<code>result_type</code> member type becomes the return type of the
signal.</para>
<para>The input iterators passed to the combiner transform dereference
operations into slot calls. Combiners therefore have the option to
invoke only some slots until some particular criterion is met. For
instance, in a distributed computing system, the combiner may ask
each remote system whether it will handle the request. Only one
remote system needs to handle a particular request, so after a
remote system accepts the work we do not want to ask any other
remote systems to perform the same task. Such a combiner need only
check the value returned when dereferencing the iterator, and
return when the value is acceptable. The following combiner returns
the first non-NULL pointer to a <code>FulfilledRequest</code> data
structure, without asking any later slots to fulfill the
request:</para>
<programlisting>
struct DistributeRequest {
  typedef FulfilledRequest* result_type;

  template&lt;typename InputIterator&gt;
  result_type operator()(InputIterator first, InputIterator last) const
  {
    while (first != last) {
      if (result_type fulfilled = *first)
        return fulfilled;
      ++first;
    }
    return 0;
  }
};
</programlisting>
</section>
</section>

<section><title>Connection Management</title>
<section><title>Disconnecting Slots (Beginner)</title>
<para>Slots aren't expected to exist indefinately after they are
connected. Often slots are only used to receive a few events and
are then disconnected, and the programmer needs control to decide
when a slot should no longer be connected.</para>
<para>The entry point for managing connections explicitly is the
<code><classname>boost::signals::connection</classname></code> class. The
<code><classname>connection</classname></code> class uniquely represents the connection
between a particular signal and a particular slot. The
<code><methodname alt="connection::connected">connected</methodname>()</code> method checks if the signal and slot are
still connected, and the <code><methodname alt="connection::disconnect">disconnect()</methodname></code> method
disconnects the signal and slot if they are connected before it is
called. Each call to the signal's <code>connect()</code> method
returns a connection object, which can be used to determine if the
connection still exists or to disconnect the signal and slot.</para>
<programlisting>
boost::signals::connection c = sig.<methodname>connect</methodname>(HelloWorld());
if (c.<methodname>connected</methodname>()) {
<emphasis>// c is still connected to the signal</emphasis>
  sig(); <emphasis>// Prints "Hello, World!"</emphasis>
}

c.disconnect(); <emphasis>// Disconnect the HelloWorld object</emphasis>
assert(!c.<methodname>connected</methodname>()); <emphasis>c isn't connected any more</emphasis>

sig(); <emphasis>// Does nothing: there are no connected slots</emphasis>
</programlisting>
</section>

<section><title>Scoped connections (Intermediate)</title>
<para>The <code>boost::signals::scoped_connection</code> class
references a signal/slot connection that will be disconnected when
the <code>scoped_connection</code> class goes out of scope. This
ability is useful when a connection need only be temporary,
e.g.,</para>
<programlisting>
{
  boost::signals::scoped_connection c = sig.<methodname>connect</methodname>(ShortLived());
  sig(); <emphasis>// will call ShortLived function object</emphasis>
}
sig(); <emphasis>// ShortLived function object no longer connected to sig</emphasis>
</programlisting>
</section>

<section><title>Disconnecting equivalent slots (Intermediate)</title>
<para>One can disconnect slots that are equivalent to a given function
object using a form of the
<code><methodname>disconnect</methodname></code> method, so long as
the type of the function object has an accessible <code>==</code>
operator. For instance:

</para>
  <informaltable>
    <tgroup cols="2" align="left">
      <thead>
        <row>
          <entry>Preferred syntax</entry>
          <entry>Portable syntax</entry>
        </row>
      </thead>
      <tbody>
        <row>
          <entry>
<programlisting>
void foo();
void bar();

signal&lt;void()&gt; sig;

sig.connect(&amp;foo);
sig.connect(&amp;bar);

// disconnects foo, but not bar
sig.disconnect(&amp;foo);
</programlisting>
</entry>
<entry>
<programlisting>
void foo();
void bar();

signal0&lt;void&gt; sig;

sig.connect(&amp;foo);
sig.connect(&amp;bar);

// disconnects foo, but not bar
sig.disconnect(&amp;foo);
</programlisting>
</entry>
            </row>
          </tbody>
        </tgroup>
      </informaltable>

</section>

<section><title>Automatic connection management (Intermediate)</title>
<para>Boost.Signals can automatically track the lifetime of objects
involved in signal/slot connections, including automatic
disconnection of slots when objects involved in the slot call are
destroyed. For instance, consider a simple news delivery service,
where clients connect to a news provider that then sends news to
all connected clients as information arrives. The news delivery
service may be constructed like this: </para>
  <informaltable>
    <tgroup cols="2" align="left">
      <thead>
        <row>
          <entry>Preferred syntax</entry>
          <entry>Portable syntax</entry>
        </row>
      </thead>
      <tbody>
        <row>
          <entry>
<programlisting>
class NewsItem { /* ... */ };

boost::signal&lt;void (const NewsItem&amp;)&gt; deliverNews;
</programlisting>
</entry>
<entry>
<programlisting>
class NewsItem { /* ... */ };

boost::signal1&lt;void, const NewsItem&amp;&gt; deliverNews;
</programlisting>
</entry>
            </row>
          </tbody>
        </tgroup>
      </informaltable>

<para>Clients that wish to receive news updates need only connect a
function object that can receive news items to the
<code>deliverNews</code> signal. For instance, we may have a
special message area in our application specifically for news,
e.g.,:</para>
<programlisting>
struct NewsMessageArea : public MessageArea
{
public:
  // ...

  void displayNews(const NewsItem&amp; news) const
  {
    messageText = news.text();
    update();
  }
};

// ...
NewsMessageArea newsMessageArea = new NewsMessageArea(/* ... */);
// ...
deliverNews.<methodname>connect</methodname>(boost::bind(&amp;NewsMessageArea::displayNews, 
                                newsMessageArea, _1));
</programlisting>
<para>However, what if the user closes the news message area,
destroying the <code>newsMessageArea</code> object that
<code>deliverNews</code> knows about? Most likely, a segmentation
fault will occur. However, with Boost.Signals one need only make
<code>NewsMessageArea</code> <emphasis>trackable</emphasis>, and the slot
involving <code>newsMessageArea</code> will be disconnected when
<code>newsMessageArea</code> is destroyed. The
<code>NewsMessageArea</code> class is made trackable by deriving
publicly from the <code>boost::signals::trackable</code> class,
e.g.:</para>
<programlisting>
struct NewsMessageArea : public MessageArea, public boost::signals::trackable
{
  // ...
};
</programlisting>
<para>At this time there is a significant limitation to the use of
<code>trackable</code> objects in making slot connections: function
objects built using Boost.Bind are understood, such that pointers
or references to <code>trackable</code> objects passed to
<code>boost::bind</code> will be found and tracked.</para>
<para><emphasis role="bold">Warning</emphasis>: User-defined function objects and function
objects from other libraries (e.g., Boost.Function or Boost.Lambda)
do not implement the required interfaces for <code>trackable</code>
object detection, and <emphasis>will silently ignore any bound trackable
objects</emphasis>. Future versions of the Boost libraries will address
this limitation.</para>
</section>

<section><title>When can disconnections occur? (Intermediate)</title>
<para>Signal/slot disconnections occur when any of these conditions
occur:</para>
<itemizedlist>
<listitem><para>The connection is explicitly disconnected via the connection's
<code>disconnect</code> method directly, or indirectly via the
signal's <code>disconnect</code> method or
<code>scoped_connection</code>'s destructor.</para></listitem>
<listitem><para>A <code>trackable</code> object bound to the slot is
destroyed.</para></listitem>
<listitem><para>The signal is destroyed.</para></listitem></itemizedlist>
<para>These events can occur at any time without disrupting a signal's
calling sequence. If a signal/slot connection is disconnected at
any time during a signal's calling sequence, the calling sequence
will still continue but will not invoke the disconnected slot.
Additionally, a signal may be destroyed while it is in a calling
sequence, and which case it will complete its slot call sequence
but may not be accessed directly.</para>
<para>Signals may be invoked recursively (e.g., a signal A calls a
slot B that invokes signal A...). The disconnection behavior does
not change in the recursive case, except that the slot calling
sequence includes slot calls for all nested invocations of the
signal.</para>
</section>

<section><title>Passing slots (Intermediate)</title>
<para>Slots in the Boost.Signals library are created from arbitrary
function objects, and therefore have no fixed type. However, it is
commonplace to require that slots be passed through interfaces that
cannot be templates. Slots can be passed via the
<code>slot_type</code> for each particular signal type and any
function object compatible with the signature of the signal can be
passed to a <code>slot_type</code> parameter. For instance:</para>
  <informaltable>
    <tgroup cols="2" align="left">
      <thead>
        <row>
          <entry>Preferred syntax</entry>
          <entry>Portable syntax</entry>
        </row>
      </thead>
      <tbody>
        <row>
          <entry>
<programlisting>
class Button 
{
  typedef boost::signal&lt;void (int x, int y)&gt; OnClick;

public:
  void doOnClick(const OnClick::slot_type&amp; slot);

private:
  OnClick onClick;
};

void Button::doOnClick(
      const OnClick::slot_type&amp; slot
    )
{
  onClick.<methodname>connect</methodname>(slot);
}

void printCoordinates(long x, long y)
{
  std::cout &lt;&lt; "(" &lt;&lt; x &lt;&lt; ", " &lt;&lt; y &lt;&lt; ")\n";
}

void f(Button&amp; button)
{
  button.doOnClick(&amp;printCoordinates);
}
</programlisting>
</entry>
<entry>
<programlisting>
class Button 
{
  typedef <classname alt="boost::signalN">boost::signal2</classname>&lt;void,int,int&gt; OnClick;

public:
  void doOnClick(const OnClick::slot_type&amp; slot);

private:
  OnClick onClick;
};

void Button::doOnClick(
      const OnClick::slot_type&amp; slot
    )
{
  onClick.<methodname>connect</methodname>(slot);
}

void printCoordinates(long x, long y)
{
  std::cout &lt;&lt; "(" &lt;&lt; x &lt;&lt; ", " &lt;&lt; y &lt;&lt; ")\n";
}

void f(Button&amp; button)
{
  button.doOnClick(&amp;printCoordinates);
}
</programlisting>
</entry>
            </row>
          </tbody>
        </tgroup>
      </informaltable>

<para>The <code>doOnClick</code> method is now functionally equivalent
to the <code>connect</code> method of the <code>onClick</code>
signal, but the details of the <code>doOnClick</code> method can be
hidden in an implementation detail file.</para>
</section>
</section>

<section>
  <title>Linking against the Signals library</title>

  <para>Part of the Boost.Signals library is compiled into a binary
  library that must be linked into your application to use Signals. To
  build this library, execute the command <command>bjam</command> in
  either the top-level Boost directory or in
  <code>libs/signals/build</code>. On Unix, the directory
  <code>libs/signals/build/bin-stage</code> will then contain
  libraries named, e.g., <code>libboost_signals.a</code> that can be
  linked in your program with <code>-lboost_signals</code>.</para>

  <para>On Windows, with Microsoft Visual C++ or Borland C++, the
  linking process is nearly automatic. As with the
  <libraryname>Regex</libraryname> library, the libraries in
  <code>libs\signals\build\bin-stage</code> will have mangled names
  and will be automatically be including in the link process. To link
  against the Signals library binary dynamically (e.g., using the
  Signals DLL), define <code>BOOST_SIGNALS_DYN_LINK</code> when
  building your application; to link statically, define
  <code>BOOST_SIGNALS_STATIC_LINK</code>. </para>
</section>

</section>
