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<section last-revision="$Date: 2007-06-12 14:01:23 -0400 (Tue, 12 Jun 2007) $" id="signals2.rationale">
  <title>Design Rationale</title>

  <using-namespace name="boost"/>
  <using-namespace name="boost::signals2"/>
  <using-class name="boost::signals2::signalN"/>

  <section>
    <title>User-level Connection Management</title>

    <para> Users need to have fine control over the connection of
    signals to slots and their eventual disconnection. The primary approach
    taken by Boost.Signals2 is to return a
    <code><classname>connection</classname></code> object that enables
    connected/disconnected query, manual disconnection, and an
    automatic disconnection on destruction mode (<classname>scoped_connection</classname>).
    In addition, two other interfaces are supported by the
    <methodname alt="signalN::disconnect">signal::disconnect</methodname> overloaded method:</para>

    <itemizedlist>
      <listitem>
        <para><emphasis role="bold">Pass slot to
        disconnect</emphasis>: in this interface model, the
        disconnection of a slot connected with
        <code>sig.<methodname>connect</methodname>(typeof(sig)::slot_type(slot_func))</code> is
        performed via
        <code>sig.<methodname>disconnect</methodname>(slot_func)</code>. Internally,
        a linear search using slot comparison is performed and the
        slot, if found, is removed from the list. Unfortunately,
        querying connectedness ends up as a
        linear-time operation.</para>
      </listitem>

      <listitem>
        <para><emphasis role="bold">Pass a token to
        disconnect</emphasis>: this approach identifies slots with a
        token that is easily comparable (e.g., a string), enabling
        slots to be arbitrary function objects. While this approach is
        essentially equivalent to the connection approach taken by Boost.Signals2,
        it is possibly more error-prone for several reasons:</para>

        <itemizedlist>
          <listitem>
            <para>Connections and disconnections must be paired, so
            the problem becomes similar to the problems incurred when
            pairing <code>new</code> and <code>delete</code> for
            dynamic memory allocation. While errors of this sort would
            not be catastrophic for a signals and slots
            implementation, their detection is generally
            nontrivial.</para>
          </listitem>

          <listitem>
            <para>If tokens are not unique, two slots may have
            the same name and be indistinguishable. In
            environments where many connections will be made
            dynamically, name generation becomes an additional task
            for the user.</para>
          </listitem>
        </itemizedlist>

        <para> This type of interface is supported in Boost.Signals2
        via the slot grouping mechanism, and the overload of
        <methodname alt="signalN::disconnect">signal::disconnect</methodname>
        which takes an argument of the signal's <code>Group</code> type.</para>
      </listitem>
    </itemizedlist>
  </section>

  <section>
    <title>Automatic Connection Management</title>

    <para>Automatic connection management in Signals2
      depends on the use of <classname>boost::shared_ptr</classname> to
      manage the lifetimes of tracked objects.  This is differs from
      the original Boost.Signals library, which instead relied on derivation
      from the <code><classname>boost::signals::trackable</classname></code> class.
      The library would be
      notified of an object's destruction by the
      <code><classname>boost::signals::trackable</classname></code> destructor.
    </para>
    <para>Unfortunately, the <code><classname>boost::signals::trackable</classname></code>
      scheme cannot be made thread safe due
      to destructor ordering.  The destructor of an class derived from
      <code><classname>boost::signals::trackable</classname></code> will always be
      called before the destructor of the base <code><classname>boost::signals::trackable</classname></code>
      class.  However, for thread-safety the connection between the signal and object
      needs to be disconnected before the object runs its destructors.
      Otherwise, if an object being destroyed
      in one thread is connected to a signal concurrently
      invoking in another thread, the signal may call into
      a partially destroyed object.
    </para>
    <para>We solve this problem by requiring that tracked objects be
      managed by <classname>shared_ptr</classname>.  Slots keep a
      <classname>weak_ptr</classname> to every object the slot depends
      on.  Connections to a slot are disconnected when any of its tracked
      <classname>weak_ptr</classname>s expire.  Additionally, signals
      create their own temporary <classname>shared_ptr</classname>s to
      all of a slot's tracked objects prior to invoking the slot.  This
      insures none of the tracked objects destruct in mid-invocation.
    </para>
    <para>The new connection management scheme has the advantage of being
      non-intrusive.  Objects of any type may be tracked using the
      <classname>shared_ptr/weak_ptr</classname> scheme.  The old
      <code><classname>boost::signals::trackable</classname></code>
      scheme requires the tracked objects to be derived from the <code>trackable</code>
      base class, which is not always practical when interacting
      with classes from 3rd party libraries.
    </para>
  </section>

  <section>
    <title><code>optional_last_value</code> as the Default Combiner</title>
    <para>
      The default combiner for Boost.Signals2 has changed from the <code>last_value</code>
      combiner used by default in the original Boost.Signals library.
      This is because <code>last_value</code> requires that at least 1 slot be
      connected to the signal when it is invoked (except for the <code>last_value&lt;void&gt;</code> specialization).
      In a multi-threaded environment where signal invocations and slot connections
      and disconnections may be happening concurrently, it is difficult
      to fulfill this requirement.  When using <classname>optional_last_value</classname>,
      there is no requirement for slots to be connected when a signal
      is invoked, since in that case the combiner may simply return an empty
      <classname>boost::optional</classname>.
    </para>
  </section>
  <section>
    <title>Combiner Interface</title>

    <para> The Combiner interface was chosen to mimic a call to an
    algorithm in the C++ standard library. It is felt that by viewing
    slot call results as merely a sequence of values accessed by input
    iterators, the combiner interface would be most natural to a
    proficient C++ programmer. Competing interface design generally
    required the combiners to be constructed to conform to an
    interface that would be customized for (and limited to) the
    Signals2 library. While these interfaces are generally enable more
    straighforward implementation of the signals &amp; slots
    libraries, the combiners are unfortunately not reusable (either in
    other signals &amp; slots libraries or within other generic
    algorithms), and the learning curve is steepened slightly to learn
    the specific combiner interface.</para>

    <para> The Signals2 formulation of combiners is based on the
    combiner using the "pull" mode of communication, instead of the
    more complex "push" mechanism. With a "pull" mechanism, the
    combiner's state can be kept on the stack and in the program
    counter, because whenever new data is required (i.e., calling the
    next slot to retrieve its return value), there is a simple
    interface to retrieve that data immediately and without returning
    from the combiner's code. Contrast this with the "push" mechanism,
    where the combiner must keep all state in class members because
    the combiner's routines will be invoked for each signal
    called. Compare, for example, a combiner that returns the maximum
    element from calling the slots. If the maximum element ever
    exceeds 100, no more slots are to be called.</para>

    <informaltable>
      <tgroup cols="2" align="left">
        <thead>
          <row>
            <entry><para>Pull</para></entry>
            <entry><para>Push</para></entry>
          </row>
        </thead>
        <tbody>
          <row>
            <entry>
<programlisting>
struct pull_max {
  typedef int result_type;

  template&lt;typename InputIterator&gt;
  result_type operator()(InputIterator first,
                         InputIterator last)
  {
    if (first == last)
      throw std::runtime_error("Empty!");

    int max_value = *first++;
    while(first != last &amp;&amp; *first &lt;= 100) {
      if (*first &gt; max_value)
        max_value = *first;
      ++first;
    }

    return max_value;
  }
};
</programlisting>
</entry>
            <entry>
<programlisting>
struct push_max {
  typedef int result_type;

  push_max() : max_value(), got_first(false) {}

  // returns false when we want to stop
  bool operator()(int result) {
    if (result &gt; 100)
      return false;

    if (!got_first) {
      got_first = true;
      max_value = result;
      return true;
    }

    if (result &gt; max_value)
      max_value = result;

    return true;
  }

  int get_value() const
  {
    if (!got_first)
      throw std::runtime_error("Empty!");
    return max_value;
  }

private:
  int  max_value;
  bool got_first;
};
</programlisting>
</entry>
          </row>
        </tbody>
      </tgroup>
    </informaltable>

    <para>There are several points to note in these examples. The
    "pull" version is a reusable function object that is based on an
    input iterator sequence with an integer <code>value_type</code>,
    and is very straightforward in design. The "push" model, on the
    other hand, relies on an interface specific to the caller and is
    not generally reusable. It also requires extra state values to
    determine, for instance, if any elements have been
    received. Though code quality and ease-of-use is generally
    subjective, the "pull" model is clearly shorter and more reusable
    and will often be construed as easier to write and understand,
    even outside the context of a signals &amp; slots library.</para>

    <para> The cost of the "pull" combiner interface is paid in the
    implementation of the Signals2 library itself. To correctly handle
    slot disconnections during calls (e.g., when the dereference
    operator is invoked), one must construct the iterator to skip over
    disconnected slots. Additionally, the iterator must carry with it
    the set of arguments to pass to each slot (although a reference to
    a structure containing those arguments suffices), and must cache
    the result of calling the slot so that multiple dereferences don't
    result in multiple calls. This apparently requires a large degree
    of overhead, though if one considers the entire process of
    invoking slots one sees that the overhead is nearly equivalent to
    that in the "push" model, but we have inverted the control
    structures to make iteration and dereference complex (instead of
    making combiner state-finding complex).</para>
  </section>

  <section>
    <title>Connection Interfaces: +=  operator</title>

    <para> Boost.Signals2 supports a connection syntax with the form
    <code>sig.<methodname>connect</methodname>(slot)</code>, but a
    more terse syntax <code>sig += slot</code> has been suggested (and
    has been used by other signals &amp; slots implementations). There
    are several reasons as to why this syntax has been
    rejected:</para>

    <itemizedlist>
      <listitem>
        <para><emphasis role="bold">It's unnecessary</emphasis>: the
        connection syntax supplied by Boost.Signals2 is no less
        powerful that that supplied by the <code>+=</code>
        operator. The savings in typing (<code>connect()</code>
        vs. <code>+=</code>) is essentially negligible. Furthermore,
        one could argue that calling <code>connect()</code> is more
        readable than an overload of <code>+=</code>.</para>
      </listitem>
      <listitem>
        <para><emphasis role="bold">Ambiguous return type</emphasis>:
        there is an ambiguity concerning the return value of the
        <code>+=</code> operation: should it be a reference to the
        signal itself, to enable <code>sig += slot1 += slot2</code>,
        or should it return a
        <code><classname>connection</classname></code> for the
        newly-created signal/slot connection?</para>
      </listitem>

      <listitem>
        <para><emphasis role="bold">Gateway to operators -=,
        +</emphasis>: when one has added a connection operator
        <code>+=</code>, it seems natural to have a disconnection
        operator <code>-=</code>. However, this presents problems when
        the library allows arbitrary function objects to implicitly
        become slots, because slots are no longer comparable.  <!--
        (see the discussion on this topic in User-level Connection
        Management). --></para>

        <para> The second obvious addition when one has
        <code>operator+=</code> would be to add a <code>+</code>
        operator that supports addition of multiple slots, followed by
        assignment to a signal. However, this would require
        implementing <code>+</code> such that it can accept any two
        function objects, which is technically infeasible.</para>
      </listitem>
    </itemizedlist>
  </section>
  <section>
    <title>Signals2 Mutex Classes</title>
    <para>
      The Boost.Signals2 library provides 2 mutex classes: <classname>boost::signals2::mutex</classname>,
      and <classname>boost::signals2::dummy_mutex</classname>.  The motivation for providing
      <classname>boost::signals2::mutex</classname> is simply that the <classname>boost::mutex</classname>
      class provided by the Boost.Thread library currently requires linking to libboost_thread.
      The <classname>boost::signals2::mutex</classname> class allows Signals2 to remain
      a header-only library.  In the future, <classname>boost::signals2::mutex</classname>
      will probably be turned into a typedef to <classname>std::mutex</classname> when
      compiling in C++0x mode.  You may still choose to use <classname>boost::mutex</classname>
      if you wish, by specifying it as the <code>Mutex</code> template type for your signals.
    </para>
    <para>
      The <classname>boost::signals2::dummy_mutex</classname> class is provided to allow
      performance sensitive single-threaded applications to minimize overhead by avoiding unneeded
      mutex locking.
    </para>
  </section>
  <section>
    <title>Comparison with other Signal/Slot implementations</title>

    <section>
      <title>libsigc++</title>

      <para> <ulink
      url="http://libsigc.sourceforge.net">libsigc++</ulink> is a C++
      signals &amp; slots library that originally started as part of
      an initiative to wrap the C interfaces to <ulink
      url="http://www.gtk.org">GTK</ulink> libraries in C++, and has
      grown to be a separate library maintained by Karl Nelson. There
      are many similarities between libsigc++ and Boost.Signals2, and
      indeed the original Boost.Signals was strongly influenced by
      Karl Nelson and libsigc++. A cursory inspection of each library will find a
      similar syntax for the construction of signals and in the use of
      connections. There
      are some major differences in design that separate these
      libraries:</para>

      <itemizedlist>
        <listitem>
          <para><emphasis role="bold">Slot definitions</emphasis>:
          slots in libsigc++ are created using a set of primitives
          defined by the library. These primitives allow binding of
          objects (as part of the library), explicit adaptation from
          the argument and return types of the signal to the argument
          and return types of the slot (libsigc++ is, by default, more
          strict about types than Boost.Signals2).</para>
        </listitem>

        <listitem>
          <para><emphasis role="bold">Combiner/Marshaller
          interface</emphasis>: the equivalent to Boost.Signals2
          combiners in libsigc++ are the marshallers. Marshallers are
          similar to the "push" interface described in Combiner
          Interface, and a proper treatment of the topic is given
          there.</para>
        </listitem>
      </itemizedlist>
    </section>

    <section>
      <title>.NET delegates</title>

      <para> <ulink url="http://www.microsoft.com">Microsoft</ulink>
      has introduced the .NET Framework and an associated set of
      languages and language extensions, one of which is the
      delegate. Delegates are similar to signals and slots, but they
      are more limited than most C++ signals and slots implementations
      in that they:</para>

      <itemizedlist>
        <listitem>
          <para>Require exact type matches between a delegate and what
          it is calling.</para>
        </listitem>

        <listitem><para>Only return the result of the last target called, with no option for customization.</para></listitem>
        <listitem>
          <para>Must call a method with <code>this</code> already
          bound.</para>
        </listitem>
      </itemizedlist>
    </section>
  </section>
</section>
