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</div>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="signals2.tutorial"></a>Tutorial</h2></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="tutorial.html#idp524171872">How to Read this Tutorial</a></span></dt>
<dt><span class="section"><a href="tutorial.html#idp524178656">Hello, World! (Beginner)</a></span></dt>
<dt><span class="section"><a href="tutorial.html#idp524186288">Calling Multiple Slots</a></span></dt>
<dt><span class="section"><a href="tutorial.html#idp524214416">Passing Values to and from Slots</a></span></dt>
<dt><span class="section"><a href="tutorial.html#idp524266176">Connection Management</a></span></dt>
<dt><span class="section"><a href="tutorial.html#signals2.tutorial.document-view">Example: Document-View</a></span></dt>
<dt><span class="section"><a href="tutorial.html#signals2.tutorial.extended-slot-type">Giving a Slot Access to its Connection (Advanced)</a></span></dt>
<dt><span class="section"><a href="tutorial.html#signals2.tutorial.signal-mutex-template-parameter">Changing the <code class="computeroutput">Mutex</code> Type of a Signal (Advanced).</a></span></dt>
<dt><span class="section"><a href="tutorial.html#idp524417472">Linking against the Signals2 library</a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="idp524171872"></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 Signals2
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
Signals2 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="idp524178656"></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>
<pre class="programlisting"><code class="computeroutput">struct HelloWorld
{
void operator()() const
{
std::cout << "Hello, World!" << std::endl;
}
};
</code></pre>
<pre class="programlisting"><code class="computeroutput"> // Signal with no arguments and a void return value
boost::signals2::signal<void ()> sig;
// Connect a HelloWorld slot
HelloWorld hello;
sig.connect(hello);
// Call all of the slots
sig();
</code></pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="idp524186288"></a>Calling Multiple Slots</h3></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="tutorial.html#idp524186832">Connecting Multiple Slots (Beginner)</a></span></dt>
<dt><span class="section"><a href="tutorial.html#idp524197008">Ordering Slot Call Groups (Intermediate)</a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="idp524186832"></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"><code class="computeroutput">struct Hello
{
void operator()() const
{
std::cout << "Hello";
}
};
</code></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"><code class="computeroutput">struct World
{
void operator()() const
{
std::cout << ", World!" << std::endl;
}
};
</code></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>
<pre class="programlisting"><code class="computeroutput"> boost::signals2::signal<void ()> sig;
sig.connect(Hello());
sig.connect(World());
sig();
</code></pre>
<p>By default, slots are pushed onto the back of the slot list,
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="idp524197008"></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.Signals2
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
< relation. Here's how we construct Hello, World:</p>
<pre class="programlisting"><code class="computeroutput"> boost::signals2::signal<void ()> sig;
sig.connect(1, World()); // connect with group 1
sig.connect(0, Hello()); // connect with group 0
</code></pre>
<p>Invoking the signal 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::signals2::at_front</code> or <code class="computeroutput">boost::signals2::at_back</code>
as the last parameter to <code class="computeroutput"><a class="link" href="../boost/signals2/signal.html#idp728304368-bb">connect</a></code>, respectively),
and default to the end of the list. When
a group is specified, the final <code class="computeroutput">at_front</code> or <code class="computeroutput">at_back</code>
parameter describes where the slot
will be placed within the group ordering. Ungrouped slots connected with
<code class="computeroutput">at_front</code> will always precede all grouped slots. Ungrouped
slots connected with <code class="computeroutput">at_back</code> will always succeed all
grouped slots.
</p>
<p>
If we add a new slot to our example like this:
</p>
<pre class="programlisting"><code class="computeroutput">struct GoodMorning
{
void operator()() const
{
std::cout << "... and good morning!" << std::endl;
}
};
</code></pre>
<pre class="programlisting"><code class="computeroutput"> // by default slots are connected at the end of the slot list
sig.connect(GoodMorning());
// slots are invoked this order:
// 1) ungrouped slots connected with boost::signals2::at_front
// 2) grouped slots according to ordering of their groups
// 3) ungrouped slots connected with boost::signals2::at_back
sig();
</code></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="idp524214416"></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#idp524214976">Slot Arguments (Beginner)</a></span></dt>
<dt><span class="section"><a href="tutorial.html#idp524227840">Signal Return Values (Advanced)</a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="idp524214976"></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"><code class="computeroutput">void print_args(float x, float y)
{
std::cout << "The arguments are " << x << " and " << y << std::endl;
}
void print_sum(float x, float y)
{
std::cout << "The sum is " << x + y << std::endl;
}
void print_product(float x, float y)
{
std::cout << "The product is " << x * y << std::endl;
}
void print_difference(float x, float y)
{
std::cout << "The difference is " << x - y << std::endl;
}
void print_quotient(float x, float y)
{
std::cout << "The quotient is " << x / y << std::endl;
}
</code></pre>
<pre class="programlisting"><code class="computeroutput"> boost::signals2::signal<void (float, float)> sig;
sig.connect(&print_args);
sig.connect(&print_sum);
sig.connect(&print_product);
sig.connect(&print_difference);
sig.connect(&print_quotient);
sig(5., 3.);
</code></pre>
<p>This program will print out the following:</p>
<pre class="programlisting">The arguments are 5 and 3
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/signals2/signal.html" title="Class template signal">boost::signals2::signal</a><void (float,
float)></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="idp524227840"></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 may 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>
<pre class="programlisting"><code class="computeroutput">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></pre>
<pre class="programlisting">boost::signals2::signal<float (float, float)> sig;</pre>
<pre class="programlisting"><code class="computeroutput"> sig.connect(&product);
sig.connect(&quotient);
sig.connect(&sum);
sig.connect(&difference);
// The default combiner returns a boost::optional containing the return
// value of the last slot in the slot list, in this case the
// difference function.
std::cout << *sig(5, 3) << std::endl;
</code></pre>
<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/signals2/signal.html" title="Class template signal">boost::signals2::signal</a></code> class template) is to call all slots and
then return a <code class="computeroutput">boost::optional</code> containing
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 connected.</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"><code class="computeroutput">// combiner which returns the maximum value returned by all slots
template<typename T>
struct maximum
{
typedef T result_type;
template<typename InputIterator>
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 < *first)
max_value = *first;
++first;
}
return max_value;
}
};
</code></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<float></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>
<pre class="programlisting">
<code class="computeroutput"><a class="link" href="../boost/signals2/signal.html" title="Class template signal">boost::signals2::signal</a></code><float (float x, float y),
maximum<float> > sig;
</pre>
<p>Now we can connect slots that perform arithmetic functions and
use the signal:</p>
<pre class="programlisting"><code class="computeroutput"> sig.connect(&product);
sig.connect(&quotient);
sig.connect(&sum);
sig.connect(&difference);
// Outputs the maximum value returned by the connected slots, in this case
// 15 from the product function.
std::cout << "maximum: " << sig(5, 3) << std::endl;
</code></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"><code class="computeroutput">// aggregate_values is a combiner which places all the values returned
// from slots into a container
template<typename Container>
struct aggregate_values
{
typedef Container result_type;
template<typename InputIterator>
Container operator()(InputIterator first, InputIterator last) const
{
Container values;
while(first != last) {
values.push_back(*first);
++first;
}
return values;
}
};
</code></pre>
<p>
Again, we can create a signal with this new combiner:
</p>
<pre class="programlisting">
<code class="computeroutput"><a class="link" href="../boost/signals2/signal.html" title="Class template signal">boost::signals2::signal</a></code><float (float, float),
aggregate_values<std::vector<float> > > sig;</pre>
<pre class="programlisting"><code class="computeroutput"> sig.connect(&quotient);
sig.connect(&product);
sig.connect(&sum);
sig.connect(&difference);
std::vector<float> results = sig(5, 3);
std::cout << "aggregate values: ";
std::copy(results.begin(), results.end(),
std::ostream_iterator<float>(std::cout, " "));
std::cout << "\n";
</code></pre>
<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<typename InputIterator>
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="idp524266176"></a>Connection Management</h3></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="tutorial.html#idp524266720">Disconnecting Slots (Beginner)</a></span></dt>
<dt><span class="section"><a href="tutorial.html#idp524276320">Blocking Slots (Beginner)</a></span></dt>
<dt><span class="section"><a href="tutorial.html#idp524283152">Scoped Connections (Intermediate)</a></span></dt>
<dt><span class="section"><a href="tutorial.html#idp524294352">Disconnecting Equivalent Slots (Intermediate)</a></span></dt>
<dt><span class="section"><a href="tutorial.html#signals2.tutorial.connection-management">Automatic Connection Management (Intermediate)</a></span></dt>
<dt><span class="section"><a href="tutorial.html#signals2.tutorial.deconstruct">Postconstructors and Predestructors (Advanced)</a></span></dt>
<dt><span class="section"><a href="tutorial.html#idp524348016">When Can Disconnections Occur? (Intermediate)</a></span></dt>
<dt><span class="section"><a href="tutorial.html#idp524355616">Passing Slots (Intermediate)</a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="idp524266720"></a>Disconnecting Slots (Beginner)</h4></div></div></div>
<p>Slots aren't expected to exist indefinitely 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/signals2/connection.html" title="Class connection">boost::signals2::connection</a></code> class. The
<code class="computeroutput">connection</code> class uniquely represents the connection
between a particular signal and a particular slot. The
<code class="computeroutput"><a class="link" href="../boost/signals2/connection.html#idp727882320-bb">connected</a>()</code> method checks if the signal and slot are
still connected, and the <code class="computeroutput"><a class="link" href="../boost/signals2/connection.html#idp727877456-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"><code class="computeroutput"> boost::signals2::connection c = sig.connect(HelloWorld());
std::cout << "c is connected\n";
sig(); // Prints "Hello, World!"
c.disconnect(); // Disconnect the HelloWorld object
std::cout << "c is disconnected\n";
sig(); // Does nothing: there are no connected slots
</code></pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="idp524276320"></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 permanently disconnected.
This is typically used to prevent infinite recursion in cases where
otherwise running a slot would cause the signal it is connected to to be
invoked again. A
<code class="computeroutput"><a class="link" href="../boost/signals2/shared_connection_block.html" title="Class shared_connection_block">boost::signals2::shared_connection_block</a></code> object will
temporarily block a slot. The connection is unblocked by either
destroying or calling
<code class="computeroutput"><a class="link" href="../boost/signals2/shared_connection_block.html#idp728222928-bb">unblock</a></code>
on all the
<code class="computeroutput">shared_connection_block</code> objects that reference the connection.
Here is an example of
blocking/unblocking slots:</p>
<pre class="programlisting"><code class="computeroutput"> boost::signals2::connection c = sig.connect(HelloWorld());
std::cout << "c is not blocked.\n";
sig(); // Prints "Hello, World!"
{
boost::signals2::shared_connection_block block(c); // block the slot
std::cout << "c is blocked.\n";
sig(); // No output: the slot is blocked
} // shared_connection_block going out of scope unblocks the slot
std::cout << "c is not blocked.\n";
sig(); // Prints "Hello, World!"}
</code></pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="idp524283152"></a>Scoped Connections (Intermediate)</h4></div></div></div>
<p>The <code class="computeroutput"><a class="link" href="../boost/signals2/scoped_connection.html" title="Class scoped_connection">boost::signals2::scoped_connection</a></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"><code class="computeroutput"> {
boost::signals2::scoped_connection c(sig.connect(ShortLived()));
sig(); // will call ShortLived function object
} // scoped_connection goes out of scope and disconnects
sig(); // ShortLived function object no longer connected to sig
</code></pre>
<p>
Note, attempts to initialize a scoped_connection with the assignment syntax
will fail due to it being noncopyable. Either the explicit initialization syntax
or default construction followed by assignment from a <code class="computeroutput"><a class="link" href="../boost/signals2/connection.html" title="Class connection">signals2::connection</a></code>
will work:
</p>
<pre class="programlisting">
// doesn't compile due to compiler attempting to copy a temporary scoped_connection object
// boost::signals2::scoped_connection c0 = sig.<code class="computeroutput"><a class="link" href="../boost/signals2/signal.html#idp728304368-bb">connect</a></code>(ShortLived());
// okay
boost::signals2::scoped_connection c1(sig.<code class="computeroutput"><a class="link" href="../boost/signals2/signal.html#idp728304368-bb">connect</a></code>(ShortLived()));
// also okay
boost::signals2::scoped_connection c2;
c2 = sig.<code class="computeroutput"><a class="link" href="../boost/signals2/signal.html#idp728304368-bb">connect</a></code>(ShortLived());
</pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="idp524294352"></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/signals2/signal.html#idp728333920-bb">signal::disconnect</a></code> method, so long as
the type of the function object has an accessible <code class="computeroutput">==</code>
operator. For instance:
</p>
<pre class="programlisting"><code class="computeroutput">void foo() { std::cout << "foo"; }
void bar() { std::cout << "bar\n"; }
</code></pre>
<pre class="programlisting"><code class="computeroutput"><a class="link" href="../boost/signals2/signal.html" title="Class template signal">boost::signals2::signal</a></code><void ()> sig;</pre>
</div>
<pre class="programlisting"><code class="computeroutput"> sig.connect(&foo);
sig.connect(&bar);
sig();
// disconnects foo, but not bar
sig.disconnect(&foo);
sig();
</code></pre>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="signals2.tutorial.connection-management"></a>Automatic Connection Management (Intermediate)</h4></div></div></div>
<p>Boost.Signals2 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>
<pre class="programlisting">
class NewsItem { /* ... */ };
typedef boost::signals2::signal<void (const NewsItem&)> signal_type;
signal_type deliverNews;
</pre>
<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& news) const
{
messageText = news.text();
update();
}
};
// ...
NewsMessageArea *newsMessageArea = new NewsMessageArea(/* ... */);
// ...
deliverNews.<code class="computeroutput"><a class="link" href="../boost/signals2/signal.html#idp728304368-bb">connect</a></code>(boost::bind(&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.Signals2 one may track any object
which is managed by a shared_ptr, by using
<code class="computeroutput"><a class="link" href="../boost/signals2/slot.html#idp728559776-bb">slot::track</a></code>. A slot will automatically
disconnect when any of its tracked objects expire. In
addition, Boost.Signals2 will ensure that no tracked object expires
while the slot it is associated with is in mid-execution. It does so by creating
temporary shared_ptr copies of the slot's tracked objects before executing it.
To track <code class="computeroutput">NewsMessageArea</code>, we use a shared_ptr to manage
its lifetime, and pass the shared_ptr to the slot via its
<code class="computeroutput"><a class="link" href="../boost/signals2/slot.html#idp728559776-bb">slot::track</a></code>
method before connecting it,
e.g.:</p>
<pre class="programlisting">
// ...
boost::shared_ptr<NewsMessageArea> newsMessageArea(new NewsMessageArea(/* ... */));
// ...
deliverNews.<code class="computeroutput"><a class="link" href="../boost/signals2/signal.html#idp728304368-bb">connect</a></code>(signal_type::slot_type(&NewsMessageArea::displayNews,
newsMessageArea.get(), _1).track(newsMessageArea));
</pre>
<p>
Note there is no explicit call to bind() needed in the above example. If the
<code class="computeroutput"><a class="link" href="../boost/signals2/slot.html" title="Class template slot">signals2::slot</a></code> constructor is passed more than one
argument, it will automatically pass all the arguments to <code class="computeroutput">bind</code> and use the
returned function object.
</p>
<p>Also note, we pass an ordinary pointer as the
second argument to the slot constructor, using <code class="computeroutput">newsMessageArea.get()</code>
instead of passing the <code class="computeroutput">shared_ptr</code> itself. If we had passed the
<code class="computeroutput">newsMessageArea</code> itself, a copy of the <code class="computeroutput">shared_ptr</code> would
have been bound into the slot function, preventing the <code class="computeroutput">shared_ptr</code>
from expiring. However, the use of
<code class="computeroutput"><a class="link" href="../boost/signals2/slot.html#idp728559776-bb">slot::track</a></code>
implies we wish to allow the tracked object to expire, and automatically
disconnect the connection when this occurs.
</p>
<p>
<code class="computeroutput">shared_ptr</code> classes other than <code class="computeroutput">boost::shared_ptr</code>
(such as <code class="computeroutput">std::shared_ptr</code>) may also be tracked for connection management
purposes. They are supported by the <code class="computeroutput"><a class="link" href="../boost/signals2/slot.html#idp728573792-bb">slot::track_foreign</a></code> method.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="signals2.tutorial.deconstruct"></a>Postconstructors and Predestructors (Advanced)</h4></div></div></div>
<p>One limitation of using <code class="computeroutput">shared_ptr</code> for tracking is that
an object cannot setup tracking of itself in its constructor. However, it is
possible to set up tracking in a post-constructor which is called after the
object has been created and passed to a <code class="computeroutput">shared_ptr</code>.
The Boost.Signals2
library provides support for post-constructors and pre-destructors
via the <code class="computeroutput"><a class="link" href="../boost/signals2/deconstruct.html" title="Function deconstruct">deconstruct()</a></code> factory function.
</p>
<p>
For most cases, the simplest and most robust way to setup postconstructors
for a class is to define an associated <code class="computeroutput">adl_postconstruct</code> function
which can be found by <code class="computeroutput"><a class="link" href="../boost/signals2/deconstruct.html" title="Function deconstruct">deconstruct()</a></code>,
make the class' constructors private, and give <code class="computeroutput"><a class="link" href="../boost/signals2/deconstruct.html" title="Function deconstruct">deconstruct</a></code>
access to the private constructors by declaring <code class="computeroutput"><a class="link" href="../boost/signals2/deconstruct_access.html" title="Class deconstruct_access">deconstruct_access</a></code>
a friend. This will ensure that objects of the class may only be created
through the <code class="computeroutput"><a class="link" href="../boost/signals2/deconstruct.html" title="Function deconstruct">deconstruct()</a></code> function, and their
associated <code class="computeroutput">adl_postconstruct()</code> function will always be called.
</p>
<p>The <a class="link" href="examples.html#signals2.examples.deconstruct" title="Postconstructors and Predestructors with deconstruct()">examples</a> section
contains several examples of defining classes with postconstructors and
predestructors, and creating objects of these classes using
<code class="computeroutput"><a class="link" href="../boost/signals2/deconstruct.html" title="Function deconstruct">deconstruct()</a></code>
</p>
<p>
Be aware that the postconstructor/predestructor support in Boost.Signals2
is in no way essential to the use of the library. The use of
<code class="computeroutput"><a class="link" href="../boost/signals2/deconstruct.html" title="Function deconstruct">deconstruct</a></code>
is purely optional. One alternative is to
define static factory functions for your classes. The
factory function can create an object, pass ownership of the object to
a <code class="computeroutput">shared_ptr</code>, setup tracking for the object,
then return the <code class="computeroutput">shared_ptr</code>.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="idp524348016"></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>An object tracked by 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>
<p>
Note, even after a connection is disconnected, its's associated slot
may still be in the process of executing. In other words, disconnection
does not block waiting for the connection's associated slot to complete execution.
This situation may occur in a multi-threaded environment if the
disconnection occurs concurrently with signal invocation,
or in a single-threaded environment if a slot disconnects itself.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="idp524355616"></a>Passing Slots (Intermediate)</h4></div></div></div>
<p>Slots in the Boost.Signals2 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>
<pre class="programlisting"><code class="computeroutput">// a pretend GUI button
class Button
{
typedef boost::signals2::signal<void (int x, int y)> OnClick;
public:
typedef OnClick::slot_type OnClickSlotType;
// forward slots through Button interface to its private signal
boost::signals2::connection doOnClick(const OnClickSlotType & slot);
// simulate user clicking on GUI button at coordinates 52, 38
void simulateClick();
private:
OnClick onClick;
};
boost::signals2::connection Button::doOnClick(const OnClickSlotType & slot)
{
return onClick.connect(slot);
}
void Button::simulateClick()
{
onClick(52, 38);
}
void printCoordinates(long x, long y)
{
std::cout << "(" << x << ", " << y << ")\n";
}
</code></pre>
<pre class="programlisting">
<code class="computeroutput"> Button button;
button.doOnClick(&printCoordinates);
button.simulateClick();
</code></pre>
<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="signals2.tutorial.document-view"></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"><code class="computeroutput">class Document
{
public:
typedef boost::signals2::signal<void ()> signal_t;
public:
Document()
{}
/* Connect a slot to the signal which will be emitted whenever
text is appended to the document. */
boost::signals2::connection connect(const signal_t::slot_type &subscriber)
{
return m_sig.connect(subscriber);
}
void append(const char* s)
{
m_text += s;
m_sig();
}
const std::string& getText() const
{
return m_text;
}
private:
signal_t m_sig;
std::string m_text;
};
</code></pre>
<p>
Next, 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"><code class="computeroutput">class TextView
{
public:
TextView(Document& doc): m_document(doc)
{
m_connection = m_document.connect(boost::bind(&TextView::refresh, this));
}
~TextView()
{
m_connection.disconnect();
}
void refresh() const
{
std::cout << "TextView: " << m_document.getText() << std::endl;
}
private:
Document& m_document;
boost::signals2::connection m_connection;
};
</code></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"><code class="computeroutput">class HexView
{
public:
HexView(Document& doc): m_document(doc)
{
m_connection = m_document.connect(boost::bind(&HexView::refresh, this));
}
~HexView()
{
m_connection.disconnect();
}
void refresh() const
{
const std::string& s = m_document.getText();
std::cout << "HexView:";
for (std::string::const_iterator it = s.begin(); it != s.end(); ++it)
std::cout << ' ' << std::hex << static_cast<int>(*it);
std::cout << std::endl;
}
private:
Document& m_document;
boost::signals2::connection m_connection;
};
</code></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"><code class="computeroutput">int main(int argc, char* argv[])
{
Document doc;
TextView v1(doc);
HexView v2(doc);
doc.append(argc == 2 ? argv[1] : "Hello world!");
return 0;
}
</code></pre>
<p>The complete example source, contributed by Keith MacDonald,
is available in the <a class="link" href="examples.html#signals2.examples.document-view" title="Document-View">examples</a> section.
We also provide variations on the program which employ automatic connection management
to disconnect views on their destruction.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="signals2.tutorial.extended-slot-type"></a>Giving a Slot Access to its Connection (Advanced)</h3></div></div></div>
<p>
You may encounter situations where you wish to disconnect or block a slot's
connection from within the slot itself. For example, suppose you have a group
of asynchronous tasks, each of which emits a signal when it completes.
You wish to connect a slot to all the tasks to retrieve their results as
each completes. Once a
given task completes and the slot is run, the slot no longer needs to be
connected to the completed task.
Therefore, you may wish to clean up old connections by having the slot
disconnect its invoking connection when it runs.
</p>
<p>
For a slot to disconnect (or block) its invoking connection, it must have
access to a <code class="computeroutput"><a class="link" href="../boost/signals2/connection.html" title="Class connection">signals2::connection</a></code> object which references
the invoking signal-slot connection. The difficulty is,
the <code class="computeroutput">connection</code> object is returned by the
<code class="computeroutput"><a class="link" href="../boost/signals2/signal.html#idp728304368-bb">signal::connect</a></code>
method, and therefore is not available until after the slot is
already connected to the signal. This can be particularly troublesome
in a multi-threaded environment where the signal may be invoked
concurrently by a different thread while the slot is being connected.
</p>
<p>
Therefore, the signal classes provide
<code class="computeroutput"><a class="link" href="../boost/signals2/signal.html#idp728321344-bb">signal::connect_extended</a></code>
methods, which allow slots which take an extra argument to be connected to a signal.
The extra argument is a <code class="computeroutput"><a class="link" href="../boost/signals2/connection.html" title="Class connection">signals2::connection</a></code> object which refers
to the signal-slot connection currently invoking the slot.
<code class="computeroutput"><a class="link" href="../boost/signals2/signal.html#idp728321344-bb">signal::connect_extended</a></code>
uses slots of the type given by the
<code class="computeroutput"><a class="link" href="../boost/signals2/signal.html#boost.signals2.signal.extended_slot_type">signal::extended_slot_type</a></code>
typedef.
</p>
<p>
The examples section includes an
<a class="link" href="examples.html#signals2.examples.tutorial.extended_slot" title="extended_slot">extended_slot</a>
program which demonstrates the syntax for using
<code class="computeroutput"><a class="link" href="../boost/signals2/signal.html#idp728321344-bb">signal::connect_extended</a></code>.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="signals2.tutorial.signal-mutex-template-parameter"></a>Changing the <code class="computeroutput">Mutex</code> Type of a Signal (Advanced).</h3></div></div></div>
<p>
For most cases the default type of <code class="computeroutput"><a class="link" href="../boost/signals2/mutex.html" title="Class mutex">boost::signals2::mutex</a></code> for
a <code class="computeroutput"><a class="link" href="../boost/signals2/signal.html" title="Class template signal">signals2::signal</a></code>'s <code class="computeroutput">Mutex</code> template type parameter should
be fine. If you wish to use an alternate mutex type, it must be default-constructible
and fulfill the <code class="computeroutput">Lockable</code> concept defined by the Boost.Thread library.
That is, it must have <code class="computeroutput">lock()</code> and <code class="computeroutput">unlock()</code> methods
(the <code class="computeroutput">Lockable</code> concept also includes a <code class="computeroutput">try_lock()</code> method
but this library does not require try locking).
</p>
<p>
The Boost.Signals2 library provides one alternate mutex class for use with <code class="computeroutput">signal</code>:
<code class="computeroutput"><a class="link" href="../boost/signals2/dummy_mutex.html" title="Class dummy_mutex">boost::signals2::dummy_mutex</a></code>. This is a fake mutex for
use in single-threaded programs, where locking a real mutex would be useless
overhead. Other mutex types you could use with <code class="computeroutput">signal</code> include
<code class="computeroutput">boost::mutex</code>, or the <code class="computeroutput">std::mutex</code> from
C++11.
</p>
<p>
Changing a signal's <code class="computeroutput">Mutex</code> template type parameter can be tedious, due to
the large number of template parameters which precede it. The
<code class="computeroutput"><a class="link" href="../boost/signals2/signal_type.html" title="Class template signal_type">signal_type</a></code> metafunction is particularly useful in this case,
since it enables named template type parameters for the <code class="computeroutput"><a class="link" href="../boost/signals2/signal.html" title="Class template signal">signals2::signal</a></code>
class. For example, to declare a signal which takes an <code class="computeroutput">int</code> as
an argument and uses a <code class="computeroutput"><a class="link" href="../boost/signals2/dummy_mutex.html" title="Class dummy_mutex">boost::signals2::dummy_mutex</a></code>
for its <code class="computeroutput">Mutex</code> types, you could write:
</p>
<pre class="programlisting">namespace bs2 = boost::signals2;
using namespace bs2::keywords;
bs2::signal_type<void (int), mutex_type<bs2::dummy_mutex> >::type sig;
</pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="idp524417472"></a>Linking against the Signals2 library</h3></div></div></div>
<p>Unlike the original Boost.Signals library, Boost.Signals2 is currently header-only.
</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: June 12, 2007 at 14:01:23 -0400</small></p></td>
<td align="right"><div class="copyright-footer">Copyright © 2001-2004 Douglas Gregor<br>Copyright © 2007-2009 Frank Mori Hess<p>Distributed under 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>
</tr></table>
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