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4<title>Design Overview</title>
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25<div class="section" lang="en">
26<div class="titlepage"><div><div><h2 class="title" style="clear: both">
27<a name="id1633898"></a>Design Overview</h2></div></div></div>
28<div class="toc"><dl>
29<dt><span class="section"><a href="s05.html#id1633905">Type Erasure</a></span></dt>
30<dt><span class="section"><a href="s05.html#id1633975"><code class="computeroutput">connection</code> class</a></span></dt>
31<dt><span class="section"><a href="s05.html#id1634092">Slot Call Iterator</a></span></dt>
32<dt><span class="section"><a href="s05.html#id1634290"><code class="computeroutput">visit_each</code> function template</a></span></dt>
33</dl></div>
34<div class="section" lang="en">
35<div class="titlepage"><div><div><h3 class="title">
36<a name="id1633905"></a>Type Erasure</h3></div></div></div>
37<p>"Type erasure", where static type information is eliminated
38    by the use of dynamically dispatched interfaces, is used
39    extensively within the Boost.Signals library to reduce the amount
40    of code generated by template instantiation. Each signal must
41    manage a list of slots and their associated connections, along
42    with a <code class="computeroutput">std::map</code> to map from group identifiers to
43    their associated connections. However, instantiating this map for
44    every token type, and perhaps within each translation unit (for
45    some popular template instantiation strategies) increase compile
46    time overhead and space overhead.</p>
47<p> To combat this so-called "template bloat", we use
48    Boost.Function and Boost.Any to store unknown types and
49    operations. Then, all of the code for handling the list of slots
50    and the mapping from slot identifiers to connections is factored
51    into the class <code class="computeroutput">signal_base</code>
52    that deals exclusively with the <code class="computeroutput">any</code> and
53    <code class="computeroutput"><a href="../boost/function.html" title="Class template function">function</a></code> objects, hiding the
54    actual implementations using the well-known pimpl idiom. The
55    actual <code class="computeroutput"><a href="../boost/signalN.html" title="Class template signalN">signalN</a></code> class templates
56    deal only with code that will change depending on the number of
57    arguments or which is inherently template-dependent (such as
58    connection).</p>
59</div>
60<div class="section" lang="en">
61<div class="titlepage"><div><div><h3 class="title">
62<a name="id1633975"></a><code class="computeroutput">connection</code> class</h3></div></div></div>
63<p> The <code class="computeroutput"><a href="../boost/signals/connection.html" title="Class connection">connection</a></code> class is
64    central to the behavior of the Boost.Signals library. It is the
65    only entity within the Boost.Signals system that has knowledge of
66    all objects that are associated by a given connection. To be
67    specific, the <code class="computeroutput"><a href="../boost/signals/connection.html" title="Class connection">connection</a></code> class
68    itself is merely a thin wrapper over a
69    <code class="computeroutput">shared_ptr</code> to a
70    <code class="computeroutput">basic_connection</code> object.</p>
71<p> <code class="computeroutput"><a href="../boost/signals/connection.html" title="Class connection">connection</a></code> objects are
72    stored by all participants in the Signals system: each
73    <code class="computeroutput"><a href="../boost/signals/trackable.html" title="Class trackable">trackable</a></code> object contains a
74    list of <code class="computeroutput"><a href="../boost/signals/connection.html" title="Class connection">connection</a></code> objects
75    describing all connections it is a part of; similarly, all signals
76    contain a set of pairs that define a slot. The pairs consist of a
77    slot function object (generally a Boost.Function object) and a
78    <code class="computeroutput"><a href="../boost/signals/connection.html" title="Class connection">connection</a></code> object (that will
79    disconnect on destruction). Finally, the mapping from slot groups
80    to slots is based on the key value in a
81    <code class="computeroutput">std::multimap</code> (the stored data
82    in the <code class="computeroutput">std::multimap</code> is the
83    slot pair).</p>
84</div>
85<div class="section" lang="en">
86<div class="titlepage"><div><div><h3 class="title">
87<a name="id1634092"></a>Slot Call Iterator</h3></div></div></div>
88<p> The slot call iterator is conceptually a stack of iterator
89    adaptors that modify the behavior of the underlying iterator
90    through the list of slots. The following table describes the type
91    and behavior of each iterator adaptor required. Note that this is
92    only a conceptual model: the implementation collapses all these
93    layers into a single iterator adaptor because several popular
94    compilers failed to compile the implementation of the conceptual
95    model.</p>
96<div class="informaltable"><table class="table">
97<colgroup>
98<col>
99<col>
100</colgroup>
101<thead><tr>
102<th align="left">Iterator Adaptor</th>
103<th align="left">Purpose</th>
104</tr></thead>
105<tbody>
106<tr>
107<td align="left"><p>Slot List Iterator</p></td>
108<td align="left"><p>An iterator through the list of slots
109            connected to a signal. The <code class="computeroutput">value_type</code> of this
110            iterator will be
111            <code class="computeroutput">std::pair&lt;any,
112            connection&gt;</code>, where the
113            <code class="computeroutput"><a href="../boost/any.html" title="Class any">any</a></code> contains an
114            instance of the slot function type.</p></td>
115</tr>
116<tr>
117<td align="left"><p>Filter Iterator Adaptor</p></td>
118<td align="left"><p>This filtering iterator adaptor filters out
119            slots that have been disconnected, so we never see a
120            disconnected slot in later stages.</p></td>
121</tr>
122<tr>
123<td align="left"><p>Projection Iterator Adaptor</p></td>
124<td align="left"><p>The projection iterator adaptor returns a
125            reference to the first member of the pair that constitutes
126            a connected slot (e.g., just the
127            <code class="computeroutput"><a href="../boost/any.html" title="Class any">boost::any</a></code> object that
128            holds the slot function).</p></td>
129</tr>
130<tr>
131<td align="left"><p>Transform Iterator Adaptor</p></td>
132<td align="left"><p>This transform iterator adaptor performs an
133            <code class="computeroutput"><a href="../boost/any_cast.html" title="Function any_cast">any_cast</a></code> to
134            extract a reference to the slot function with the
135            appropriate slot function type.</p></td>
136</tr>
137<tr>
138<td align="left"><p>Transform Iterator Adaptor</p></td>
139<td align="left"><p>This transform iterator adaptor calls the
140            function object returned by dereferencing the underlying
141            iterator with the set of arguments given to the signal
142            itself, and returns the result of that slot
143            call.</p></td>
144</tr>
145<tr>
146<td align="left"><p>Input Caching Iterator Adaptor</p></td>
147<td align="left"><p>This iterator adaptor caches the result of
148            dereferencing the underlying iterator. Therefore,
149            dereferencing this iterator multiple times will only
150            result in the underlying iterator being dereferenced once;
151            thus, a slot can only be called once but its result can be
152            used multiple times.</p></td>
153</tr>
154<tr>
155<td align="left"><p>Slot Call Iterator</p></td>
156<td align="left"><p>Iterates over calls to each slot.</p></td>
157</tr>
158</tbody>
159</table></div>
160</div>
161<div class="section" lang="en">
162<div class="titlepage"><div><div><h3 class="title">
163<a name="id1634290"></a><code class="computeroutput">visit_each</code> function template</h3></div></div></div>
164<p> The <code class="computeroutput"><a href="../boost/visit_each.html" title="Function template visit_each">visit_each</a></code>
165    function template is a mechanism for discovering objects that are
166    stored within another object. Function template
167    <code class="computeroutput"><a href="../boost/visit_each.html" title="Function template visit_each">visit_each</a></code> takes three
168    arguments: an object to explore, a visitor function object that is
169    invoked with each subobject, and the <code class="computeroutput">int</code> 0. </p>
170<p> The third parameter is merely a temporary solution to the
171    widespread lack of proper function template partial ordering. The
172    primary <code class="computeroutput"><a href="../boost/visit_each.html" title="Function template visit_each">visit_each</a></code>
173    function template specifies this third parameter type to be
174    <code class="computeroutput">long</code>, whereas any user specializations must specify
175    their third parameter to be of type <code class="computeroutput">int</code>. Thus, even
176    though a broken compiler cannot tell the ordering between, e.g., a
177    match against a parameter <code class="computeroutput">T</code> and a parameter
178    <code class="computeroutput">A&lt;T&gt;</code>, it can determine that the conversion from
179    the integer 0 to <code class="computeroutput">int</code> is better than the conversion to
180    <code class="computeroutput">long</code>. The ordering determined by this conversion thus
181    achieves partial ordering of the function templates in a limited,
182    but successful, way. The following example illustrates the use of
183    this technique:</p>
184<pre class="programlisting">
185template&lt;typename&gt; class A {};
186template&lt;typename T&gt; void foo(T, long);
187template&lt;typename T&gt; void foo(A&lt;T&gt;, int);
188A&lt;T&gt; at;
189foo(at, 0);
190</pre>
191<p> In this example, we assume that our compiler can not tell
192    that <code class="computeroutput">A&lt;T&gt;</code> is a better match than
193    <code class="computeroutput">T</code>, and therefore assume that the function templates
194    cannot be ordered based on that parameter. Then the conversion
195    from 0 to <code class="computeroutput">int</code> is better than the conversion from 0 to
196    <code class="computeroutput">long</code>, and the second function template is
197    chosen. </p>
198</div>
199</div>
200<table width="100%"><tr>
201<td align="left"><small><p>Last revised: November 03, 2006 at 19:45:40 GMT</p></small></td>
202<td align="right"><small>Copyright © 2001-2004 Douglas Gregor</small></td>
203</tr></table>
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