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| <div class="section"> |
| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="id2536784"></a>Design Overview</h2></div></div></div> |
| <div class="toc"><dl> |
| <dt><span class="section"><a href="s05.html#id2536790">Type Erasure</a></span></dt> |
| <dt><span class="section"><a href="s05.html#id2536860"><code class="computeroutput">connection</code> class</a></span></dt> |
| <dt><span class="section"><a href="s05.html#id2536977">Slot Call Iterator</a></span></dt> |
| <dt><span class="section"><a href="s05.html#id2537176"><code class="computeroutput">visit_each</code> function template</a></span></dt> |
| </dl></div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="id2536790"></a>Type Erasure</h3></div></div></div> |
| <p>"Type erasure", where static type information is eliminated |
| by the use of dynamically dispatched interfaces, is used |
| extensively within the Boost.Signals library to reduce the amount |
| of code generated by template instantiation. Each signal must |
| manage a list of slots and their associated connections, along |
| with a <code class="computeroutput">std::map</code> to map from group identifiers to |
| their associated connections. However, instantiating this map for |
| every token type, and perhaps within each translation unit (for |
| some popular template instantiation strategies) increase compile |
| time overhead and space overhead.</p> |
| <p> To combat this so-called "template bloat", we use |
| Boost.Function and Boost.Any to store unknown types and |
| operations. Then, all of the code for handling the list of slots |
| and the mapping from slot identifiers to connections is factored |
| into the class <code class="computeroutput">signal_base</code> |
| that deals exclusively with the <code class="computeroutput">any</code> and |
| <code class="computeroutput"><a class="link" href="../boost/function.html" title="Class template function">function</a></code> objects, hiding the |
| actual implementations using the well-known pimpl idiom. The |
| actual <code class="computeroutput"><a class="link" href="../boost/signalN.html" title="Class template signalN">signalN</a></code> class templates |
| deal only with code that will change depending on the number of |
| arguments or which is inherently template-dependent (such as |
| connection).</p> |
| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="id2536860"></a><code class="computeroutput">connection</code> class</h3></div></div></div> |
| <p> The <code class="computeroutput"><a class="link" href="../boost/signals/connection.html" title="Class connection">connection</a></code> class is |
| central to the behavior of the Boost.Signals library. It is the |
| only entity within the Boost.Signals system that has knowledge of |
| all objects that are associated by a given connection. To be |
| specific, the <code class="computeroutput"><a class="link" href="../boost/signals/connection.html" title="Class connection">connection</a></code> class |
| itself is merely a thin wrapper over a |
| <code class="computeroutput">shared_ptr</code> to a |
| <code class="computeroutput">basic_connection</code> object.</p> |
| <p> <code class="computeroutput"><a class="link" href="../boost/signals/connection.html" title="Class connection">connection</a></code> objects are |
| stored by all participants in the Signals system: each |
| <code class="computeroutput"><a class="link" href="../boost/signals/trackable.html" title="Class trackable">trackable</a></code> object contains a |
| list of <code class="computeroutput"><a class="link" href="../boost/signals/connection.html" title="Class connection">connection</a></code> objects |
| describing all connections it is a part of; similarly, all signals |
| contain a set of pairs that define a slot. The pairs consist of a |
| slot function object (generally a Boost.Function object) and a |
| <code class="computeroutput"><a class="link" href="../boost/signals/connection.html" title="Class connection">connection</a></code> object (that will |
| disconnect on destruction). Finally, the mapping from slot groups |
| to slots is based on the key value in a |
| <code class="computeroutput">std::multimap</code> (the stored data |
| in the <code class="computeroutput">std::multimap</code> is the |
| slot pair).</p> |
| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="id2536977"></a>Slot Call Iterator</h3></div></div></div> |
| <p> The slot call iterator is conceptually a stack of iterator |
| adaptors that modify the behavior of the underlying iterator |
| through the list of slots. The following table describes the type |
| and behavior of each iterator adaptor required. Note that this is |
| only a conceptual model: the implementation collapses all these |
| layers into a single iterator adaptor because several popular |
| compilers failed to compile the implementation of the conceptual |
| model.</p> |
| <div class="informaltable"><table class="table"> |
| <colgroup> |
| <col> |
| <col> |
| </colgroup> |
| <thead><tr> |
| <th align="left">Iterator Adaptor</th> |
| <th align="left">Purpose</th> |
| </tr></thead> |
| <tbody> |
| <tr> |
| <td align="left"><p>Slot List Iterator</p></td> |
| <td align="left"><p>An iterator through the list of slots |
| connected to a signal. The <code class="computeroutput">value_type</code> of this |
| iterator will be |
| <code class="computeroutput">std::pair<any, |
| connection></code>, where the |
| <code class="computeroutput"><a class="link" href="../boost/any.html" title="Class any">any</a></code> contains an |
| instance of the slot function type.</p></td> |
| </tr> |
| <tr> |
| <td align="left"><p>Filter Iterator Adaptor</p></td> |
| <td align="left"><p>This filtering iterator adaptor filters out |
| slots that have been disconnected, so we never see a |
| disconnected slot in later stages.</p></td> |
| </tr> |
| <tr> |
| <td align="left"><p>Projection Iterator Adaptor</p></td> |
| <td align="left"><p>The projection iterator adaptor returns a |
| reference to the first member of the pair that constitutes |
| a connected slot (e.g., just the |
| <code class="computeroutput"><a class="link" href="../boost/any.html" title="Class any">boost::any</a></code> object that |
| holds the slot function).</p></td> |
| </tr> |
| <tr> |
| <td align="left"><p>Transform Iterator Adaptor</p></td> |
| <td align="left"><p>This transform iterator adaptor performs an |
| <code class="computeroutput"><a class="link" href="../boost/any_cast.html" title="Function any_cast">any_cast</a></code> to |
| extract a reference to the slot function with the |
| appropriate slot function type.</p></td> |
| </tr> |
| <tr> |
| <td align="left"><p>Transform Iterator Adaptor</p></td> |
| <td align="left"><p>This transform iterator adaptor calls the |
| function object returned by dereferencing the underlying |
| iterator with the set of arguments given to the signal |
| itself, and returns the result of that slot |
| call.</p></td> |
| </tr> |
| <tr> |
| <td align="left"><p>Input Caching Iterator Adaptor</p></td> |
| <td align="left"><p>This iterator adaptor caches the result of |
| dereferencing the underlying iterator. Therefore, |
| dereferencing this iterator multiple times will only |
| result in the underlying iterator being dereferenced once; |
| thus, a slot can only be called once but its result can be |
| used multiple times.</p></td> |
| </tr> |
| <tr> |
| <td align="left"><p>Slot Call Iterator</p></td> |
| <td align="left"><p>Iterates over calls to each slot.</p></td> |
| </tr> |
| </tbody> |
| </table></div> |
| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="id2537176"></a><code class="computeroutput">visit_each</code> function template</h3></div></div></div> |
| <p> The <code class="computeroutput"><a class="link" href="../boost/visit_each.html" title="Function template visit_each">visit_each</a></code> |
| function template is a mechanism for discovering objects that are |
| stored within another object. Function template |
| <code class="computeroutput"><a class="link" href="../boost/visit_each.html" title="Function template visit_each">visit_each</a></code> takes three |
| arguments: an object to explore, a visitor function object that is |
| invoked with each subobject, and the <code class="computeroutput">int</code> 0. </p> |
| <p> The third parameter is merely a temporary solution to the |
| widespread lack of proper function template partial ordering. The |
| primary <code class="computeroutput"><a class="link" href="../boost/visit_each.html" title="Function template visit_each">visit_each</a></code> |
| function template specifies this third parameter type to be |
| <code class="computeroutput">long</code>, whereas any user specializations must specify |
| their third parameter to be of type <code class="computeroutput">int</code>. Thus, even |
| though a broken compiler cannot tell the ordering between, e.g., a |
| match against a parameter <code class="computeroutput">T</code> and a parameter |
| <code class="computeroutput">A<T></code>, it can determine that the conversion from |
| the integer 0 to <code class="computeroutput">int</code> is better than the conversion to |
| <code class="computeroutput">long</code>. The ordering determined by this conversion thus |
| achieves partial ordering of the function templates in a limited, |
| but successful, way. The following example illustrates the use of |
| this technique:</p> |
| <pre class="programlisting"> |
| template<typename> class A {}; |
| template<typename T> void foo(T, long); |
| template<typename T> void foo(A<T>, int); |
| A<T> at; |
| foo(at, 0); |
| </pre> |
| <p> In this example, we assume that our compiler can not tell |
| that <code class="computeroutput">A<T></code> is a better match than |
| <code class="computeroutput">T</code>, and therefore assume that the function templates |
| cannot be ordered based on that parameter. Then the conversion |
| from 0 to <code class="computeroutput">int</code> is better than the conversion from 0 to |
| <code class="computeroutput">long</code>, and the second function template is |
| chosen. </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: November 25, 2007 at 18:38:02 +0000</small></p></td> |
| <td align="right"><div class="copyright-footer">Copyright © 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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