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<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="signals.tutorial"></a>Tutorial</h2></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="tutorial.html#idp203953016">How to Read this Tutorial</a></span></dt>
<dt><span class="section"><a href="tutorial.html#idp203956424">Compatibility Note</a></span></dt>
<dt><span class="section"><a href="tutorial.html#idp203963176">Hello, World! (Beginner)</a></span></dt>
<dt><span class="section"><a href="tutorial.html#idp203972552">Calling multiple slots</a></span></dt>
<dt><span class="section"><a href="tutorial.html#idp203997352">Passing values to and from slots</a></span></dt>
<dt><span class="section"><a href="tutorial.html#idp204068504">Connection Management</a></span></dt>
<dt><span class="section"><a href="tutorial.html#idp204114952">Example: Document-View</a></span></dt>
<dt><span class="section"><a href="tutorial.html#idp204122600">Linking against the Signals library</a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="idp203953016"></a>How to Read this Tutorial</h3></div></div></div>
<p>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 <span class="emphasis"><em>Beginner</em></span>,
<span class="emphasis"><em>Intermediate</em></span>, or <span class="emphasis"><em>Advanced</em></span> to help guide the
reader. The <span class="emphasis"><em>Beginner</em></span> sections include information that all
library users should know; one can make good use of the Signals
library after having read only the <span class="emphasis"><em>Beginner</em></span> sections. The
<span class="emphasis"><em>Intermediate</em></span> sections build on the <span class="emphasis"><em>Beginner</em></span>
sections with slightly more complex uses of the library. Finally,
the <span class="emphasis"><em>Advanced</em></span> sections detail very advanced uses of the
Signals library, that often require a solid working knowledge of
the <span class="emphasis"><em>Beginner</em></span> and <span class="emphasis"><em>Intermediate</em></span> topics; most users
will not need to read the <span class="emphasis"><em>Advanced</em></span> sections.</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="idp203956424"></a>Compatibility Note</h3></div></div></div>
<p>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.</p>
<p>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.</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th align="left">Preferred syntax</th>
<th align="left">Portable syntax</th>
</tr></thead>
<tbody><tr>
<td align="left">
            <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>GNU C++ 2.95.x, 3.0.x, 3.1.x</p></li>
<li class="listitem"><p>Comeau C++ 4.2.45.2</p></li>
<li class="listitem"><p>SGI MIPSpro 7.3.0</p></li>
<li class="listitem"><p>Intel C++ 5.0, 6.0</p></li>
<li class="listitem"><p>Compaq's cxx 6.2</p></li>
<li class="listitem"><p>Microsoft Visual C++ 7.1</p></li>
</ul></div>
          </td>
<td align="left">
            <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p><span class="emphasis"><em>Any compiler supporting the preferred syntax</em></span></p></li>
<li class="listitem"><p>Microsoft Visual C++ 6.0, 7.0</p></li>
<li class="listitem"><p>Borland C++ 5.5.1</p></li>
<li class="listitem"><p>Sun WorkShop 6 update 2 C++ 5.3</p></li>
<li class="listitem"><p>Metrowerks CodeWarrior 8.1</p></li>
</ul></div>
          </td>
</tr></tbody>
</table></div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="idp203963176"></a>Hello, World! (Beginner)</h3></div></div></div>
<p>The following example writes "Hello, World!" using signals and
slots. First, we create a signal <code class="computeroutput">sig</code>, a signal that
takes no arguments and has a void return value. Next, we connect
the <code class="computeroutput">hello</code> function object to the signal using the
<code class="computeroutput">connect</code> method. Finally, use the signal
<code class="computeroutput">sig</code> like a function to call the slots, which in turns
invokes <code class="computeroutput">HelloWorld::operator()</code> to print "Hello,
World!".</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th align="left">Preferred syntax</th>
<th align="left">Portable syntax</th>
</tr></thead>
<tbody><tr>
<td align="left">
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-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
<code class="computeroutput"><a class="link" href="../boost/signal.html" title="Class template signal">boost::signal</a></code>&lt;void ()&gt; sig;

// Connect a HelloWorld slot
HelloWorld hello;
sig.<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></code>(hello);

// Call all of the slots
sig();
</pre>
</td>
<td align="left">
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-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
<code class="computeroutput"><a class="link" href="../boost/signalN.html" title="Class template signalN">boost::signal0</a></code>&lt;void&gt; sig;

// Connect a HelloWorld slot
HelloWorld hello;
sig.<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></code>(hello);

// Call all of the slots
sig();
</pre>
</td>
</tr></tbody>
</table></div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="idp203972552"></a>Calling multiple slots</h3></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="tutorial.html#idp203972840">Connecting multiple slots (Beginner)</a></span></dt>
<dt><span class="section"><a href="tutorial.html#idp203983944">Ordering slot call groups (Intermediate)</a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="idp203972840"></a>Connecting multiple slots (Beginner)</h4></div></div></div>
<p>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:</p>
<pre class="programlisting">
struct Hello 
{
  void operator()() const
  {
    std::cout &lt;&lt; "Hello";
  }
};
</pre>
<p>The second slot will print ", World!" and a newline, to complete
the program. The second slot may look like this:</p>
<pre class="programlisting">
struct World
{
  void operator()() const
  {
    std::cout &lt;&lt; ", World!" &lt;&lt; std::endl;
  }
};
</pre>
<p>Like in our previous example, we can create a signal
<code class="computeroutput">sig</code> that takes no arguments and has a
<code class="computeroutput">void</code> return value. This time, we connect both a
<code class="computeroutput">hello</code> and a <code class="computeroutput">world</code> slot to the same
signal, and when we call the signal both slots will be called.</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th align="left">Preferred syntax</th>
<th align="left">Portable syntax</th>
</tr></thead>
<tbody><tr>
<td align="left">
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting">
<code class="computeroutput"><a class="link" href="../boost/signal.html" title="Class template signal">boost::signal</a></code>&lt;void ()&gt; sig;

sig.<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></code>(Hello());
sig.<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></code>(World());

sig();
</pre>
</td>
<td align="left">
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting">
<code class="computeroutput"><a class="link" href="../boost/signalN.html" title="Class template signalN">boost::signal0</a></code>&lt;void&gt; sig;

sig.<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></code>(Hello());
sig.<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></code>(World());

sig();
</pre>
</td>
</tr></tbody>
</table></div>
<p>By default, slots are called in first-in first-out (FIFO) order,
so the output of this program will be as expected:</p>
<pre class="programlisting">
Hello, World!
</pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="idp203983944"></a>Ordering slot call groups (Intermediate)</h4></div></div></div>
<p>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 class="computeroutput">connect</code> call that specifies the group. Group values
are, by default, <code class="computeroutput">int</code>s, and are ordered by the integer
&lt; relation. Here's how we construct Hello, World:</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th align="left">Preferred syntax</th>
<th align="left">Portable syntax</th>
</tr></thead>
<tbody><tr>
<td align="left">
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting">
<code class="computeroutput"><a class="link" href="../boost/signal.html" title="Class template signal">boost::signal</a></code>&lt;void ()&gt; sig;
sig.<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></code>(1, World());
sig.<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></code>(0, Hello());
sig();
</pre>
</td>
<td align="left">
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting">
<code class="computeroutput"><a class="link" href="../boost/signalN.html" title="Class template signalN">boost::signal0</a></code>&lt;void&gt; sig;
sig.<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></code>(1, World());
sig.<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></code>(0, Hello());
sig();
</pre>
</td>
</tr></tbody>
</table></div>
<p>This program will correctly print "Hello, World!", because the
<code class="computeroutput">Hello</code> object is in group 0, which precedes group 1 where
the <code class="computeroutput">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 class="computeroutput">boost::signals::at_front</code> or <code class="computeroutput">boost::signals::at_back</code>
as the last parameter to <code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></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:</p>
<pre class="programlisting">
struct GoodMorning
{
  void operator()() const
  {
    std::cout &lt;&lt; "... and good morning!" &lt;&lt; std::endl;
  }
};

sig.<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></code>(GoodMorning());
</pre>
<p>... we will get the result we wanted:</p>
<pre class="programlisting">
Hello, World!
... and good morning!
</pre>
</div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="idp203997352"></a>Passing values to and from slots</h3></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="tutorial.html#idp203997656">Slot Arguments (Beginner)</a></span></dt>
<dt><span class="section"><a href="tutorial.html#idp204012392">Signal Return Values (Advanced)</a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="idp203997656"></a>Slot Arguments (Beginner)</h4></div></div></div>
<p>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.</p>
<p>As an example, we'll create a signal that passes two
<code class="computeroutput">float</code> arguments to its slots. Then we'll create a few
slots that print the results of various arithmetic operations on
these values.</p>
<pre class="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;
}
</pre>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th align="left">Preferred syntax</th>
<th align="left">Portable syntax</th>
</tr></thead>
<tbody><tr>
<td align="left">
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting">
<code class="computeroutput"><a class="link" href="../boost/signal.html" title="Class template signal">boost::signal</a></code>&lt;void (float, float)&gt; sig;

sig.<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></code>(&amp;print_sum);
sig.<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></code>(&amp;print_product);
sig.<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></code>(&amp;print_difference);
sig.<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></code>(&amp;print_quotient);

sig(5, 3);
</pre>
</td>
<td align="left">
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting">
<code class="computeroutput"><a class="link" href="../boost/signalN.html" title="Class template signalN">boost::signal2</a></code>&lt;void, float, float&gt; sig;

sig.<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></code>(&amp;print_sum);
sig.<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></code>(&amp;print_product);
sig.<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></code>(&amp;print_difference);
sig.<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></code>(&amp;print_quotient);

sig(5, 3);
</pre>
</td>
</tr></tbody>
</table></div>
<p>This program will print out the following:</p>
<pre class="programlisting">
The sum is 8
The product is 15
The difference is 2
The quotient is 1.66667
</pre>
<p>So any values that are given to <code class="computeroutput">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 class="computeroutput"><a class="link" href="../boost/signal.html" title="Class template signal">boost::signal</a>&lt;void (float,
float)&gt;</code> means that the signal has a <code class="computeroutput">void</code>
return value and takes two <code class="computeroutput">float</code> values. Any slot
connected to <code class="computeroutput">sig</code> must therefore be able to take two
<code class="computeroutput">float</code> values.</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="idp204012392"></a>Signal Return Values (Advanced)</h4></div></div></div>
<p>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 <em class="firstterm">combiner</em>. 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.</p>
<p>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:</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th align="left">Preferred syntax</th>
<th align="left">Portable syntax</th>
</tr></thead>
<tbody><tr>
<td align="left">
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-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; }

<code class="computeroutput"><a class="link" href="../boost/signal.html" title="Class template signal">boost::signal</a></code>&lt;float (float x, float y)&gt; sig;

sig.<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></code>(&amp;product);
sig.<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></code>(&amp;quotient);
sig.<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></code>(&amp;sum);
sig.<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></code>(&amp;difference);

std::cout &lt;&lt; sig(5, 3) &lt;&lt; std::endl;
</pre>
</td>
<td align="left">
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-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; }

<code class="computeroutput"><a class="link" href="../boost/signalN.html" title="Class template signalN">boost::signal2</a></code>&lt;float, float, float&gt; sig;

sig.<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></code>(&amp;product);
sig.<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></code>(&amp;quotient);
sig.<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></code>(&amp;sum);
sig.<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></code>(&amp;difference);

std::cout &lt;&lt; sig(5, 3) &lt;&lt; std::endl;
</pre>
</td>
</tr></tbody>
</table></div>
<p>This example program will output <code class="computeroutput">2</code>. This is because the
default behavior of a signal that has a return type
(<code class="computeroutput">float</code>, the first template argument given to the
<code class="computeroutput"><a class="link" href="../boost/signal.html" title="Class template signal">boost::signal</a></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.</p>
<p>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:</p>
<pre class="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;
  }
};
</pre>
<p>The <code class="computeroutput">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 class="computeroutput">maximum&lt;float&gt;</code> would find the maximum
<code class="computeroutput">float</code> in a sequence of <code class="computeroutput">float</code>s). When a
<code class="computeroutput">maximum</code> object is invoked, it is given an input
iterator sequence <code class="computeroutput">[first, last)</code> that includes the
results of calling all of the slots. <code class="computeroutput">maximum</code> uses this
input iterator sequence to calculate the maximum element, and
returns that maximum value.</p>
<p>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:</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th align="left">Preferred syntax</th>
<th align="left">Portable syntax</th>
</tr></thead>
<tbody><tr>
<td align="left">
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting">
<code class="computeroutput"><a class="link" href="../boost/signal.html" title="Class template signal">boost::signal</a></code>&lt;float (float x, float y), 
              maximum&lt;float&gt; &gt; sig;
</pre>
</td>
<td align="left">
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting">
<code class="computeroutput"><a class="link" href="../boost/signalN.html" title="Class template signalN">boost::signal2</a></code>&lt;float, float, float, 
               maximum&lt;float&gt; &gt; sig;
</pre>
</td>
</tr></tbody>
</table></div>
<p>Now we can connect slots that perform arithmetic functions and
use the signal:</p>
<pre class="programlisting">
sig.<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></code>(&amp;quotient);
sig.<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></code>(&amp;product);
sig.<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></code>(&amp;sum);
sig.<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></code>(&amp;difference);

std::cout &lt;&lt; sig(5, 3) &lt;&lt; std::endl;
</pre>
<p>The output of this program will be <code class="computeroutput">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.</p>
<p>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:</p>
<pre class="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);
  }
};
</pre>
<p>
Again, we can create a signal with this new combiner: 
</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th align="left">Preferred syntax</th>
<th align="left">Portable syntax</th>
</tr></thead>
<tbody><tr>
<td align="left">
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting">
<code class="computeroutput"><a class="link" href="../boost/signal.html" title="Class template signal">boost::signal</a></code>&lt;float (float, float), 
    aggregate_values&lt;std::vector&lt;float&gt; &gt; &gt; sig;

sig.<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></code>(&amp;quotient);
sig.<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></code>(&amp;product);
sig.<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></code>(&amp;sum);
sig.<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></code>(&amp;difference);

std::vector&lt;float&gt; results = sig(5, 3);
std::copy(results.begin(), results.end(), 
    std::ostream_iterator&lt;float&gt;(cout, " "));
</pre>
</td>
<td align="left">
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting">
<code class="computeroutput"><a class="link" href="../boost/signalN.html" title="Class template signalN">boost::signal2</a></code>&lt;float, float, float,
    aggregate_values&lt;std::vector&lt;float&gt; &gt; &gt; sig;

sig.<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></code>(&amp;quotient);
sig.<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></code>(&amp;product);
sig.<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></code>(&amp;sum);
sig.<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></code>(&amp;difference);

std::vector&lt;float&gt; results = sig(5, 3);
std::copy(results.begin(), results.end(), 
    std::ostream_iterator&lt;float&gt;(cout, " "));
</pre>
</td>
</tr></tbody>
</table></div>
<p>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 class="computeroutput">signal</code> class,
<code class="computeroutput">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 class="computeroutput">value_type</code> of the input iterators passed
to the combiner. The combiner itself is a function object and its
<code class="computeroutput">result_type</code> member type becomes the return type of the
signal.</p>
<p>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 class="computeroutput">FulfilledRequest</code> data
structure, without asking any later slots to fulfill the
request:</p>
<pre class="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;
  }
};
</pre>
</div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="idp204068504"></a>Connection Management</h3></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="tutorial.html#idp204068760">Disconnecting Slots (Beginner)</a></span></dt>
<dt><span class="section"><a href="tutorial.html#idp204076136">Blocking Slots (Beginner)</a></span></dt>
<dt><span class="section"><a href="tutorial.html#idp204080888">Scoped connections (Intermediate)</a></span></dt>
<dt><span class="section"><a href="tutorial.html#idp204083880">Disconnecting equivalent slots (Intermediate)</a></span></dt>
<dt><span class="section"><a href="tutorial.html#idp204088984">Automatic connection management (Intermediate)</a></span></dt>
<dt><span class="section"><a href="tutorial.html#idp204101176">When can disconnections occur? (Intermediate)</a></span></dt>
<dt><span class="section"><a href="tutorial.html#idp204105400">Passing slots (Intermediate)</a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="idp204068760"></a>Disconnecting Slots (Beginner)</h4></div></div></div>
<p>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.</p>
<p>The entry point for managing connections explicitly is the
<code class="computeroutput"><a class="link" href="../boost/signals/connection.html" title="Class connection">boost::signals::connection</a></code> class. The
<code class="computeroutput"><a class="link" href="../boost/signals/connection.html" title="Class connection">connection</a></code> class uniquely represents the connection
between a particular signal and a particular slot. The
<code class="computeroutput"><a class="link" href="../boost/signals/connection.html#idp58326232-bb">connected</a>()</code> method checks if the signal and slot are
still connected, and the <code class="computeroutput"><a class="link" href="../boost/signals/connection.html#idp46282360-bb">disconnect()</a></code> method
disconnects the signal and slot if they are connected before it is
called. Each call to the signal's <code class="computeroutput">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.</p>
<pre class="programlisting">
boost::signals::connection c = sig.<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></code>(HelloWorld());
if (c.<code class="computeroutput">connected</code>()) {
<span class="emphasis"><em>// c is still connected to the signal</em></span>
  sig(); <span class="emphasis"><em>// Prints "Hello, World!"</em></span>
}

c.disconnect(); <span class="emphasis"><em>// Disconnect the HelloWorld object</em></span>
assert(!c.<code class="computeroutput">connected</code>()); <span class="emphasis"><em>c isn't connected any more</em></span>

sig(); <span class="emphasis"><em>// Does nothing: there are no connected slots</em></span>
</pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="idp204076136"></a>Blocking Slots (Beginner)</h4></div></div></div>
<p>Slots can be temporarily "blocked", meaning that they will be
ignored when the signal is invoked but have not been disconnected. The
<code class="computeroutput">block</code> member function
temporarily blocks a slot, which can be unblocked via
<code class="computeroutput">unblock</code>. Here is an example of
blocking/unblocking slots:</p>
<pre class="programlisting">
boost::signals::connection c = sig.<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></code>(HelloWorld());
sig(); <span class="emphasis"><em>// Prints "Hello, World!"</em></span>

c.<code class="computeroutput">block</code>(); <span class="emphasis"><em>// block the slot</em></span>
assert(c.<code class="computeroutput">blocked</code>());
sig(); <span class="emphasis"><em>// No output: the slot is blocked</em></span>

c.<code class="computeroutput">unblock</code>(); <span class="emphasis"><em>// unblock the slot</em></span>
sig(); <span class="emphasis"><em>// Prints "Hello, World!"</em></span>
</pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="idp204080888"></a>Scoped connections (Intermediate)</h4></div></div></div>
<p>The <code class="computeroutput">boost::signals::scoped_connection</code> class
references a signal/slot connection that will be disconnected when
the <code class="computeroutput">scoped_connection</code> class goes out of scope. This
ability is useful when a connection need only be temporary,
e.g.,</p>
<pre class="programlisting">
{
  boost::signals::scoped_connection c = sig.<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></code>(ShortLived());
  sig(); <span class="emphasis"><em>// will call ShortLived function object</em></span>
}
sig(); <span class="emphasis"><em>// ShortLived function object no longer connected to sig</em></span>
</pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="idp204083880"></a>Disconnecting equivalent slots (Intermediate)</h4></div></div></div>
<p>One can disconnect slots that are equivalent to a given function
object using a form of the
<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60883096-bb">disconnect</a></code> method, so long as
the type of the function object has an accessible <code class="computeroutput">==</code>
operator. For instance:

</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th align="left">Preferred syntax</th>
<th align="left">Portable syntax</th>
</tr></thead>
<tbody><tr>
<td align="left">
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-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);
</pre>
</td>
<td align="left">
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-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);
</pre>
</td>
</tr></tbody>
</table></div>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="idp204088984"></a>Automatic connection management (Intermediate)</h4></div></div></div>
<p>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: </p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th align="left">Preferred syntax</th>
<th align="left">Portable syntax</th>
</tr></thead>
<tbody><tr>
<td align="left">
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting">
class NewsItem { /* ... */ };

boost::signal&lt;void (const NewsItem&amp;)&gt; deliverNews;
</pre>
</td>
<td align="left">
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting">
class NewsItem { /* ... */ };

boost::signal1&lt;void, const NewsItem&amp;&gt; deliverNews;
</pre>
</td>
</tr></tbody>
</table></div>
<p>Clients that wish to receive news updates need only connect a
function object that can receive news items to the
<code class="computeroutput">deliverNews</code> signal. For instance, we may have a
special message area in our application specifically for news,
e.g.,:</p>
<pre class="programlisting">
struct NewsMessageArea : public MessageArea
{
public:
  // ...

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

// ...
NewsMessageArea newsMessageArea = new NewsMessageArea(/* ... */);
// ...
deliverNews.<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></code>(boost::bind(&amp;NewsMessageArea::displayNews, 
                                newsMessageArea, _1));
</pre>
<p>However, what if the user closes the news message area,
destroying the <code class="computeroutput">newsMessageArea</code> object that
<code class="computeroutput">deliverNews</code> knows about? Most likely, a segmentation
fault will occur. However, with Boost.Signals one need only make
<code class="computeroutput">NewsMessageArea</code> <span class="emphasis"><em>trackable</em></span>, and the slot
involving <code class="computeroutput">newsMessageArea</code> will be disconnected when
<code class="computeroutput">newsMessageArea</code> is destroyed. The
<code class="computeroutput">NewsMessageArea</code> class is made trackable by deriving
publicly from the <code class="computeroutput">boost::signals::trackable</code> class,
e.g.:</p>
<pre class="programlisting">
struct NewsMessageArea : public MessageArea, public boost::signals::trackable
{
  // ...
};
</pre>
<p>At this time there is a significant limitation to the use of
<code class="computeroutput">trackable</code> objects in making slot connections: function
objects built using Boost.Bind are understood, such that pointers
or references to <code class="computeroutput">trackable</code> objects passed to
<code class="computeroutput">boost::bind</code> will be found and tracked.</p>
<p><span class="bold"><strong>Warning</strong></span>: 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 class="computeroutput">trackable</code>
object detection, and <span class="emphasis"><em>will silently ignore any bound trackable
objects</em></span>. Future versions of the Boost libraries will address
this limitation.</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="idp204101176"></a>When can disconnections occur? (Intermediate)</h4></div></div></div>
<p>Signal/slot disconnections occur when any of these conditions
occur:</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>The connection is explicitly disconnected via the connection's
<code class="computeroutput">disconnect</code> method directly, or indirectly via the
signal's <code class="computeroutput">disconnect</code> method or
<code class="computeroutput">scoped_connection</code>'s destructor.</p></li>
<li class="listitem"><p>A <code class="computeroutput">trackable</code> object bound to the slot is
destroyed.</p></li>
<li class="listitem"><p>The signal is destroyed.</p></li>
</ul></div>
<p>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.</p>
<p>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.</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="idp204105400"></a>Passing slots (Intermediate)</h4></div></div></div>
<p>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 class="computeroutput">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 class="computeroutput">slot_type</code> parameter. For instance:</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th align="left">Preferred syntax</th>
<th align="left">Portable syntax</th>
</tr></thead>
<tbody><tr>
<td align="left">
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-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.<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></code>(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);
}
</pre>
</td>
<td align="left">
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting">
class Button 
{
  typedef <code class="computeroutput"><a class="link" href="../boost/signalN.html" title="Class template signalN">boost::signal2</a></code>&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.<code class="computeroutput"><a class="link" href="../boost/signalN.html#idp60505848-bb">connect</a></code>(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);
}
</pre>
</td>
</tr></tbody>
</table></div>
<p>The <code class="computeroutput">doOnClick</code> method is now functionally equivalent
to the <code class="computeroutput">connect</code> method of the <code class="computeroutput">onClick</code>
signal, but the details of the <code class="computeroutput">doOnClick</code> method can be
hidden in an implementation detail file.</p>
</div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="idp204114952"></a>Example: Document-View</h3></div></div></div>
<p>Signals can be used to implement flexible Document-View
  architectures. The document will contain a signal to which each of
  the views can connect. The following <code class="computeroutput">Document</code> class
  defines a simple text document that supports mulitple views. Note
  that it stores a single signal to which all of the views will be
  connected.</p>
<pre class="programlisting">class Document
{
public:
    typedef boost::signal&lt;void (bool)&gt;  signal_t;
    typedef boost::signals::connection  connection_t;

public:
    Document()
    {}

    connection_t connect(signal_t::slot_function_type subscriber)
    {
        return m_sig.connect(subscriber);
    }

    void disconnect(connection_t subscriber)
    {
        subscriber.disconnect();
    }

    void append(const char* s)
    {
        m_text += s;
        m_sig(true);
    }

    const std::string&amp; getText() const
    {
        return m_text;
    }

private:
    signal_t    m_sig;
    std::string m_text;
};</pre>
<p>Next, we can define a <code class="computeroutput">View</code> base class from which
  views can derive. This isn't strictly required, but it keeps the
  Document-View logic separate from the logic itself. Note that the
  constructor just connects the view to the document and the
  destructor disconnects the view.</p>
<pre class="programlisting">
class View
{
public:
    View(Document&amp; m)
        : m_document(m)
    {
        m_connection = m_document.connect(boost::bind(&amp;View::refresh, this, _1));
    }

    virtual ~View()
    {
        m_document.disconnect(m_connection);
    }

    virtual void refresh(bool bExtended) const = 0;

protected:
    Document&amp;               m_document;

private:
    Document::connection_t  m_connection;
};
  </pre>
<p>Finally, we can begin to define views. The
  following <code class="computeroutput">TextView</code> class provides a simple view of the
    document text.</p>
<pre class="programlisting">class TextView : public View
{
public:
    TextView(Document&amp; doc)
        : View(doc)
    {}

    virtual void refresh(bool bExtended) const
    {
        std::cout &lt;&lt; "TextView: " &lt;&lt; m_document.getText() &lt;&lt; std::endl;
    }
};</pre>
<p>Alternatively, we can provide a view of the document
    translated into hex values using the <code class="computeroutput">HexView</code>
    view:</p>
<pre class="programlisting">class HexView : public View
{
public:
    HexView(Document&amp; doc)
        : View(doc)
    {}

    virtual void refresh(bool bExtended) const
    {
        const std::string&amp;  s = m_document.getText();

        std::cout &lt;&lt; "HexView:";

        for (std::string::const_iterator it = s.begin(); it != s.end(); ++it)
            std::cout &lt;&lt; ' ' &lt;&lt; std::hex &lt;&lt; static_cast&lt;int&gt;(*it);

        std::cout &lt;&lt; std::endl;
    }
};</pre>
<p>To tie the example together, here is a
  simple <code class="computeroutput">main</code> function that sets up two views and then
    modifies the document:</p>
<pre class="programlisting">int main(int argc, char* argv[])
{
    Document    doc;
    TextView    v1(doc);
    HexView     v2(doc);

    doc.append(argc == 2 ? argv[1] : "Hello world!");
    return 0;
}</pre>
<p>The complete example source, contributed by Keith MacDonald,
    is available in <a href="../../../libs/signals/example/doc_view.cpp" target="_top"><code class="computeroutput">libs/signals/example/doc_view.cpp</code></a>.</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="idp204122600"></a>Linking against the Signals library</h3></div></div></div>
<p>Part of the Boost.Signals library is compiled into a binary
  library that must be linked into your application to use
  Signals. Please refer to
    the <a href="../../../more/getting_started.html" target="_top">Getting Started</a>
  guide. You will need to link against the <code class="computeroutput">boost_signals</code>
  library.</p>
</div>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"><p><small>Last revised: January 29, 2007 at 20:05:29 +0000</small></p></td>
<td align="right"><div class="copyright-footer">Copyright &#169; 2001-2004 Douglas Gregor<p>Use, modification and distribution is subject to the Boost
    Software License, Version 1.0. (See accompanying file
    <code class="filename">LICENSE_1_0.txt</code> or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)</p>
</div></td>
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