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26 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
27 <a name="idp426518112"></a>Design Overview</h2></div></div></div>
28 <div class="toc"><dl class="toc">
29 <dt><span class="section"><a href="s05.html#idp426519136">Type Erasure</a></span></dt>
30 <dt><span class="section"><a href="s05.html#idp426526400"><code class="computeroutput">connection</code> class</a></span></dt>
31 <dt><span class="section"><a href="s05.html#idp426539664">Slot Call Iterator</a></span></dt>
32 <dt><span class="section"><a href="s05.html#idp426561472"><code class="computeroutput">visit_each</code> function template</a></span></dt>
35 <div class="titlepage"><div><div><h3 class="title">
36 <a name="idp426519136"></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 class="link" 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 class="link" 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
61 <div class="titlepage"><div><div><h3 class="title">
62 <a name="idp426526400"></a><code class="computeroutput">connection</code> class</h3></div></div></div>
63 <p> The <code class="computeroutput"><a class="link" 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 class="link" 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 class="link" 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 class="link" href="../boost/signals/trackable.html" title="Class trackable">trackable</a></code> object contains a
74 list of <code class="computeroutput"><a class="link" 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 class="link" 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
86 <div class="titlepage"><div><div><h3 class="title">
87 <a name="idp426539664"></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
96 <div class="informaltable"><table class="table">
102 <th align="left">Iterator Adaptor</th>
103 <th align="left">Purpose</th>
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
111 <code class="computeroutput">std::pair<any,
112 connection></code>, where the
113 <code class="computeroutput"><a class="link" href="../boost/any.html" title="Class any">any</a></code> contains an
114 instance of the slot function type.</p></td>
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>
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 class="link" href="../boost/any.html" title="Class any">boost::any</a></code> object that
128 holds the slot function).</p></td>
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 class="link" href="../boost/any_cast_idp41191712.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>
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
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>
155 <td align="left"><p>Slot Call Iterator</p></td>
156 <td align="left"><p>Iterates over calls to each slot.</p></td>
161 <div class="section">
162 <div class="titlepage"><div><div><h3 class="title">
163 <a name="idp426561472"></a><code class="computeroutput">visit_each</code> function template</h3></div></div></div>
164 <p> The <code class="computeroutput"><a class="link" 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 class="link" 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 class="link" 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<T></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
184 <pre class="programlisting">
185 template<typename> class A {};
186 template<typename T> void foo(T, long);
187 template<typename T> void foo(A<T>, int);
191 <p> In this example, we assume that our compiler can not tell
192 that <code class="computeroutput">A<T></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
200 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
201 <td align="left"></td>
202 <td align="right"><div class="copyright-footer">Copyright © 2001-2004 Douglas Gregor<p>Use, modification and distribution is subject to the Boost
203 Software License, Version 1.0. (See accompanying file
204 <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>
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