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<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="idp394324512"></a>Design Overview</h2></div></div></div>
<div class="toc"><dl>
<dt><span class="section"><a href="s05.html#idp394325536">Type Erasure</a></span></dt>
<dt><span class="section"><a href="s05.html#idp394332800"><code class="computeroutput">connection</code> class</a></span></dt>
<dt><span class="section"><a href="s05.html#idp394346064">Slot Call Iterator</a></span></dt>
<dt><span class="section"><a href="s05.html#idp394368352"><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="idp394325536"></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="idp394332800"></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="idp394346064"></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_idp6057536.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="idp394368352"></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>
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<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>
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