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| <div class="section"> |
| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="signals2.rationale"></a>Design Rationale</h2></div></div></div> |
| <div class="toc"><dl> |
| <dt><span class="section"><a href="rationale.html#id2565329">User-level Connection Management</a></span></dt> |
| <dt><span class="section"><a href="rationale.html#id2565501">Automatic Connection Management</a></span></dt> |
| <dt><span class="section"><a href="rationale.html#id2565661"><code class="computeroutput">optional_last_value</code> as the Default Combiner</a></span></dt> |
| <dt><span class="section"><a href="rationale.html#id2565721">Combiner Interface</a></span></dt> |
| <dt><span class="section"><a href="rationale.html#id2565802">Connection Interfaces: += operator</a></span></dt> |
| <dt><span class="section"><a href="rationale.html#id2565973">Signals2 Mutex Classes</a></span></dt> |
| <dt><span class="section"><a href="rationale.html#id2566068">Comparison with other Signal/Slot implementations</a></span></dt> |
| </dl></div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="id2565329"></a>User-level Connection Management</h3></div></div></div> |
| <p> 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 class="computeroutput"><a class="link" href="../boost/signals2/connection.html" title="Class connection">signals2::connection</a></code> object that enables |
| connected/disconnected query, manual disconnection, and an |
| automatic disconnection on destruction mode (<code class="computeroutput"><a class="link" href="../boost/signals2/scoped_connection.html" title="Class scoped_connection">signals2::scoped_connection</a></code>). |
| In addition, two other interfaces are supported by the |
| <code class="computeroutput"><a class="link" href="../boost/signals2/signal.html#id706157-bb">signal::disconnect</a></code> overloaded method:</p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"> |
| <li class="listitem"><p><span class="bold"><strong>Pass slot to |
| disconnect</strong></span>: in this interface model, the |
| disconnection of a slot connected with |
| <code class="computeroutput">sig.<a class="link" href="../boost/signals2/signal.html#id625823-bb">connect</a>(typeof(sig)::slot_type(slot_func))</code> is |
| performed via |
| <code class="computeroutput">sig.<a class="link" href="../boost/signals2/signal.html#id706157-bb">disconnect</a>(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.</p></li> |
| <li class="listitem"> |
| <p><span class="bold"><strong>Pass a token to |
| disconnect</strong></span>: 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:</p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="circle"> |
| <li class="listitem"><p>Connections and disconnections must be paired, so |
| the problem becomes similar to the problems incurred when |
| pairing <code class="computeroutput">new</code> and <code class="computeroutput">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.</p></li> |
| <li class="listitem"><p>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.</p></li> |
| </ul></div> |
| <p> This type of interface is supported in Boost.Signals2 |
| via the slot grouping mechanism, and the overload of |
| <code class="computeroutput"><a class="link" href="../boost/signals2/signal.html#id706157-bb">signal::disconnect</a></code> |
| which takes an argument of the signal's <code class="computeroutput">Group</code> type.</p> |
| </li> |
| </ul></div> |
| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="id2565501"></a>Automatic Connection Management</h3></div></div></div> |
| <p>Automatic connection management in Signals2 |
| depends on the use of <code class="computeroutput">boost::shared_ptr</code> to |
| manage the lifetimes of tracked objects. This is differs from |
| the original Boost.Signals library, which instead relied on derivation |
| from the <code class="computeroutput"><a class="link" href="../boost/signals/trackable.html" title="Class trackable">boost::signals::trackable</a></code> class. |
| The library would be |
| notified of an object's destruction by the |
| <code class="computeroutput"><a class="link" href="../boost/signals/trackable.html" title="Class trackable">boost::signals::trackable</a></code> destructor. |
| </p> |
| <p>Unfortunately, the <code class="computeroutput"><a class="link" href="../boost/signals/trackable.html" title="Class trackable">boost::signals::trackable</a></code> |
| scheme cannot be made thread safe due |
| to destructor ordering. The destructor of an class derived from |
| <code class="computeroutput"><a class="link" href="../boost/signals/trackable.html" title="Class trackable">boost::signals::trackable</a></code> will always be |
| called before the destructor of the base <code class="computeroutput"><a class="link" href="../boost/signals/trackable.html" title="Class trackable">boost::signals::trackable</a></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. |
| </p> |
| <p>We solve this problem by requiring that tracked objects be |
| managed by <code class="computeroutput">shared_ptr</code>. Slots keep a |
| <code class="computeroutput">weak_ptr</code> to every object the slot depends |
| on. Connections to a slot are disconnected when any of its tracked |
| <code class="computeroutput">weak_ptr</code>s expire. Additionally, signals |
| create their own temporary <code class="computeroutput">shared_ptr</code>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. |
| </p> |
| <p>The new connection management scheme has the advantage of being |
| non-intrusive. Objects of any type may be tracked using the |
| <code class="computeroutput">shared_ptr</code>/<code class="computeroutput">weak_ptr</code> scheme. The old |
| <code class="computeroutput"><a class="link" href="../boost/signals/trackable.html" title="Class trackable">boost::signals::trackable</a></code> |
| scheme requires the tracked objects to be derived from the <code class="computeroutput">trackable</code> |
| base class, which is not always practical when interacting |
| with classes from 3rd party libraries. |
| </p> |
| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="id2565661"></a><code class="computeroutput">optional_last_value</code> as the Default Combiner</h3></div></div></div> |
| <p> |
| The default combiner for Boost.Signals2 has changed from the <code class="computeroutput">last_value</code> |
| combiner used by default in the original Boost.Signals library. |
| This is because <code class="computeroutput">last_value</code> requires that at least 1 slot be |
| connected to the signal when it is invoked (except for the <code class="computeroutput">last_value<void></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 <code class="computeroutput"><a class="link" href="../boost/signals2/optional_last_value.html" title="Class template optional_last_value">optional_last_value</a></code>, |
| 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 |
| <code class="computeroutput">boost::optional</code>. |
| </p> |
| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="id2565721"></a>Combiner Interface</h3></div></div></div> |
| <p> 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 & slots |
| libraries, the combiners are unfortunately not reusable (either in |
| other signals & slots libraries or within other generic |
| algorithms), and the learning curve is steepened slightly to learn |
| the specific combiner interface.</p> |
| <p> 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.</p> |
| <div class="informaltable"><table class="table"> |
| <colgroup> |
| <col> |
| <col> |
| </colgroup> |
| <thead><tr> |
| <th align="left"><p>Pull</p></th> |
| <th align="left"><p>Push</p></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 pull_max { |
| typedef int result_type; |
| |
| template<typename InputIterator> |
| result_type operator()(InputIterator first, |
| InputIterator last) |
| { |
| if (first == last) |
| throw std::runtime_error("Empty!"); |
| |
| int max_value = *first++; |
| while(first != last && *first <= 100) { |
| if (*first > max_value) |
| max_value = *first; |
| ++first; |
| } |
| |
| return max_value; |
| } |
| }; |
| </pre> |
| </td> |
| <td align="left"> |
| <pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-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 > 100) |
| return false; |
| |
| if (!got_first) { |
| got_first = true; |
| max_value = result; |
| return true; |
| } |
| |
| if (result > 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; |
| }; |
| </pre> |
| </td> |
| </tr></tbody> |
| </table></div> |
| <p>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 class="computeroutput">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 & slots library.</p> |
| <p> 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).</p> |
| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="id2565802"></a>Connection Interfaces: += operator</h3></div></div></div> |
| <p> Boost.Signals2 supports a connection syntax with the form |
| <code class="computeroutput">sig.<a class="link" href="../boost/signals2/signal.html#id625823-bb">connect</a>(slot)</code>, but a |
| more terse syntax <code class="computeroutput">sig += slot</code> has been suggested (and |
| has been used by other signals & slots implementations). There |
| are several reasons as to why this syntax has been |
| rejected:</p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"> |
| <li class="listitem"><p><span class="bold"><strong>It's unnecessary</strong></span>: the |
| connection syntax supplied by Boost.Signals2 is no less |
| powerful that that supplied by the <code class="computeroutput">+=</code> |
| operator. The savings in typing (<code class="computeroutput">connect()</code> |
| vs. <code class="computeroutput">+=</code>) is essentially negligible. Furthermore, |
| one could argue that calling <code class="computeroutput">connect()</code> is more |
| readable than an overload of <code class="computeroutput">+=</code>.</p></li> |
| <li class="listitem"><p><span class="bold"><strong>Ambiguous return type</strong></span>: |
| there is an ambiguity concerning the return value of the |
| <code class="computeroutput">+=</code> operation: should it be a reference to the |
| signal itself, to enable <code class="computeroutput">sig += slot1 += slot2</code>, |
| or should it return a |
| <code class="computeroutput"><a class="link" href="../boost/signals2/connection.html" title="Class connection">signals2::connection</a></code> for the |
| newly-created signal/slot connection?</p></li> |
| <li class="listitem"> |
| <p><span class="bold"><strong>Gateway to operators -=, |
| +</strong></span>: when one has added a connection operator |
| <code class="computeroutput">+=</code>, it seems natural to have a disconnection |
| operator <code class="computeroutput">-=</code>. However, this presents problems when |
| the library allows arbitrary function objects to implicitly |
| become slots, because slots are no longer comparable. </p> |
| <p> The second obvious addition when one has |
| <code class="computeroutput">operator+=</code> would be to add a <code class="computeroutput">+</code> |
| operator that supports addition of multiple slots, followed by |
| assignment to a signal. However, this would require |
| implementing <code class="computeroutput">+</code> such that it can accept any two |
| function objects, which is technically infeasible.</p> |
| </li> |
| </ul></div> |
| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="id2565973"></a>Signals2 Mutex Classes</h3></div></div></div> |
| <p> |
| The Boost.Signals2 library provides 2 mutex classes: <code class="computeroutput"><a class="link" href="../boost/signals2/mutex.html" title="Class mutex">boost::signals2::mutex</a></code>, |
| and <code class="computeroutput"><a class="link" href="../boost/signals2/dummy_mutex.html" title="Class dummy_mutex">boost::signals2::dummy_mutex</a></code>. The motivation for providing |
| <code class="computeroutput"><a class="link" href="../boost/signals2/mutex.html" title="Class mutex">boost::signals2::mutex</a></code> is simply that the <code class="computeroutput">boost::mutex</code> |
| class provided by the Boost.Thread library currently requires linking to libboost_thread. |
| The <code class="computeroutput"><a class="link" href="../boost/signals2/mutex.html" title="Class mutex">boost::signals2::mutex</a></code> class allows Signals2 to remain |
| a header-only library. You may still choose to use <code class="computeroutput">boost::mutex</code> |
| if you wish, by specifying it as the <code class="computeroutput">Mutex</code> template type for your signals. |
| </p> |
| <p> |
| The <code class="computeroutput"><a class="link" href="../boost/signals2/dummy_mutex.html" title="Class dummy_mutex">boost::signals2::dummy_mutex</a></code> class is provided to allow |
| performance sensitive single-threaded applications to minimize overhead by avoiding unneeded |
| mutex locking. |
| </p> |
| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="id2566068"></a>Comparison with other Signal/Slot implementations</h3></div></div></div> |
| <div class="toc"><dl> |
| <dt><span class="section"><a href="rationale.html#id2566074">libsigc++</a></span></dt> |
| <dt><span class="section"><a href="rationale.html#id2566145">.NET delegates</a></span></dt> |
| </dl></div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h4 class="title"> |
| <a name="id2566074"></a>libsigc++</h4></div></div></div> |
| <p> <a href="http://libsigc.sourceforge.net" target="_top">libsigc++</a> is a C++ |
| signals & slots library that originally started as part of |
| an initiative to wrap the C interfaces to <a href="http://www.gtk.org" target="_top">GTK</a> 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:</p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"> |
| <li class="listitem"><p><span class="bold"><strong>Slot definitions</strong></span>: |
| 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).</p></li> |
| <li class="listitem"><p><span class="bold"><strong>Combiner/Marshaller |
| interface</strong></span>: 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.</p></li> |
| </ul></div> |
| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h4 class="title"> |
| <a name="id2566145"></a>.NET delegates</h4></div></div></div> |
| <p> <a href="http://www.microsoft.com" target="_top">Microsoft</a> |
| 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:</p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"> |
| <li class="listitem"><p>Require exact type matches between a delegate and what |
| it is calling.</p></li> |
| <li class="listitem"><p>Only return the result of the last target called, with no option for customization.</p></li> |
| <li class="listitem"><p>Must call a method with <code class="computeroutput">this</code> already |
| bound.</p></li> |
| </ul></div> |
| </div> |
| </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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