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<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<title>Extension</title>
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<div class="section" lang="en">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="fusion.extension"></a><a href="extension.html" title="Extension">Extension</a></h2></div></div></div>
<p>
The Fusion library is designed to be extensible, new sequences types can easily
be added. In fact, the library support for <tt class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span></tt>,
<tt class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">array</span></tt> and <a href="http://www.boost.org/libs/mpl/index.html" target="_top">MPL</a>
sequences is entirely provided using the extension mechanism.
</p>
<p>
The process for adding a new sequence type to Fusion is:
</p>
<div class="orderedlist"><ol type="1">
<li>
Enable the <a href="notes.html#fusion.notes.tag_dispatching"><span class="emphasis"><em>tag dispatching</em></span></a>
mechanism used by Fusion for your sequence type
</li>
<li>
Design an iterator type for the sequence
</li>
<li>
Provide specialized behaviour for the intrinsic operations of the new Fusion
sequence
</li>
</ol></div>
<a name="fusion.extension.our_example"></a><h3>
<a name="id602690"></a>
<a href="extension.html#fusion.extension.our_example">Our example</a>
</h3>
<p>
In order to illustrate enabling a new sequence type for use with Fusion, we
are going to use the type:
</p>
<pre class="programlisting">
<span class="keyword">namespace</span> <span class="identifier">example</span>
<span class="special">{</span>
<span class="keyword">struct</span> <span class="identifier">example_struct</span>
<span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">name</span><span class="special">;</span>
<span class="keyword">int</span> <span class="identifier">age</span><span class="special">;</span>
<span class="identifier">example_struct</span><span class="special">(</span>
<span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&amp;</span> <span class="identifier">n</span><span class="special">,</span>
<span class="keyword">int</span> <span class="identifier">a</span><span class="special">)</span>
<span class="special">:</span> <span class="identifier">name</span><span class="special">(</span><span class="identifier">n</span><span class="special">),</span> <span class="identifier">age</span><span class="special">(</span><span class="identifier">a</span><span class="special">)</span>
<span class="special">{}</span>
<span class="special">};</span>
<span class="special">}</span>
</pre>
<p>
We are going to pretend that this type has been provided by a 3rd party library,
and therefore cannot be modified. We shall work through all the necessary steps
to enable <tt class="computeroutput"><span class="identifier">example_struct</span></tt> to
serve as an <a href="sequences/concepts/associative_sequence.html" title="Associative
Sequence">Associative
Sequence</a> as described in the <a href="quick_start.html" title="Quick Start">Quick
Start</a> guide.
</p>
<a name="fusion.extension.enabling_tag_dispatching"></a><h3>
<a name="id602972"></a>
<a href="extension.html#fusion.extension.enabling_tag_dispatching">Enabling Tag Dispatching</a>
</h3>
<p>
The Fusion extensibility mechanism uses <a href="notes.html#fusion.notes.tag_dispatching"><span class="emphasis"><em>tag
dispatching</em></span></a> to call the correct code for a given sequence
type. In order to exploit the tag dispatching mechanism we must first declare
a new tag type for the mechanism to use. For example:
</p>
<pre class="programlisting">
<span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">fusion</span> <span class="special">{</span>
<span class="keyword">struct</span> <span class="identifier">example_sequence_tag</span><span class="special">;</span> <span class="comment">// Only definition needed
</span><span class="special">}}</span>
</pre>
<p>
Next we need to enable the <tt class="computeroutput"><span class="identifier">traits</span><span class="special">::</span><span class="identifier">tag_of</span></tt>
metafunction to return our newly chosen tag type for operations involving our
sequence. This is done by specializing <tt class="computeroutput"><span class="identifier">traits</span><span class="special">::</span><span class="identifier">tag_of</span></tt>
for our sequence type.
</p>
<pre class="programlisting">
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">support</span><span class="special">/</span><span class="identifier">tag_of_fwd</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">fusion</span> <span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">traits</span> <span class="special">{</span>
<span class="keyword">template</span><span class="special">&lt;&gt;</span>
<span class="keyword">struct</span> <span class="identifier">tag_of</span><span class="special">&lt;</span><span class="identifier">example_struct</span><span class="special">&gt;</span>
<span class="special">{</span>
<span class="keyword">typedef</span> <span class="identifier">example_sequence_tag</span> <span class="identifier">type</span><span class="special">;</span>
<span class="special">};</span>
<span class="special">}}}</span>
</pre>
<p>
<tt class="computeroutput"><span class="identifier">traits</span><span class="special">::</span><span class="identifier">tag_of</span></tt> also has a second template argument,
that can be used in conjuction with <tt class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">enable_if</span></tt>
to provide tag support for whole clases of types. This feature is not necessary
for our sequence, but for an example see the code in:
</p>
<pre class="programlisting">
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">sequence</span><span class="special">/</span><span class="identifier">adapted</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="identifier">tag_of</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
</pre>
<a name="fusion.extension.designing_a_suitable_iterator"></a><h3>
<a name="id603473"></a>
<a href="extension.html#fusion.extension.designing_a_suitable_iterator">Designing a
suitable iterator</a>
</h3>
<p>
We need an iterator to describe positions, and provide access to the data within
our sequence. As it is straightforward to do, we are going to provide a random
access iterator in our example.
</p>
<p>
We will use a simple design, in which the 2 members of <tt class="computeroutput"><span class="identifier">example_struct</span></tt>
are given numbered indices, 0 for <tt class="computeroutput"><span class="identifier">name</span></tt>
and 1 for <tt class="computeroutput"><span class="identifier">age</span></tt> respectively.
</p>
<pre class="programlisting">
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Struct</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">Pos</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">example_struct_iterator</span>
<span class="special">:</span> <span class="identifier">iterator_base</span><span class="special">&lt;</span><span class="identifier">example_struct_iterator</span><span class="special">&lt;</span><span class="identifier">Struct</span><span class="special">,</span> <span class="identifier">Pos</span><span class="special">&gt;</span> <span class="special">&gt;</span>
<span class="special">{</span>
<span class="identifier">BOOST_STATIC_ASSERT</span><span class="special">(</span><span class="identifier">Pos</span> <span class="special">&gt;=</span><span class="number">0</span> <span class="special">&amp;&amp;</span> <span class="identifier">Pos</span> <span class="special">&lt;</span> <span class="number">3</span><span class="special">);</span>
<span class="keyword">typedef</span> <span class="identifier">Struct</span> <span class="identifier">struct_type</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special">&lt;</span><span class="identifier">Pos</span><span class="special">&gt;</span> <span class="identifier">index</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="identifier">random_access_traversal_tag</span> <span class="identifier">category</span><span class="special">;</span>
<span class="identifier">example_struct_iterator</span><span class="special">(</span><span class="identifier">Struct</span><span class="special">&amp;</span> <span class="identifier">str</span><span class="special">)</span>
<span class="special">:</span> <span class="identifier">struct_</span><span class="special">(</span><span class="identifier">str</span><span class="special">)</span> <span class="special">{}</span>
<span class="identifier">Struct</span><span class="special">&amp;</span> <span class="identifier">struct_</span><span class="special">;</span>
<span class="special">};</span>
</pre>
<p>
A quick summary of the details of our iterator:
</p>
<div class="orderedlist"><ol type="1">
<li>
The iterator is parameterized by the type it is iterating over, and the index
of the current element.
</li>
<li>
The typedefs <tt class="computeroutput"><span class="identifier">struct_type</span></tt>
and <tt class="computeroutput"><span class="identifier">index</span></tt> provide convenient
access to information we will need later in the implementation.
</li>
<li>
The typedef <tt class="computeroutput"><span class="identifier">category</span></tt> allows
the <tt class="computeroutput"><span class="identifier">traits</span><span class="special">::</span><a href="support/category_of.html" title="category_of"><tt class="computeroutput"><span class="identifier">category_of</span></tt></a></tt>
metafunction to establish the traversal category of the iterator.
</li>
<li>
The constructor stores a reference to the <tt class="computeroutput"><span class="identifier">example_struct</span></tt>
being iterated over.
</li>
</ol></div>
<p>
We also need to enable <a href="notes.html#fusion.notes.tag_dispatching"><span class="emphasis"><em>tag
dispatching</em></span></a> for our iterator type, with another specialization
of <tt class="computeroutput"><span class="identifier">traits</span><span class="special">::</span><span class="identifier">tag_of</span></tt>:
</p>
<pre class="programlisting">
<span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">fusion</span> <span class="special">{</span>
<span class="keyword">struct</span> <span class="identifier">example_struct_iterator_tag</span><span class="special">;</span>
<span class="keyword">namespace</span> <span class="identifier">traits</span>
<span class="special">{</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Struct</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">Pos</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">tag_of</span><span class="special">&lt;</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">example_struct_iterator</span><span class="special">&lt;</span><span class="identifier">Struct</span><span class="special">,</span> <span class="identifier">Pos</span><span class="special">&gt;</span> <span class="special">&gt;</span>
<span class="special">{</span>
<span class="keyword">typedef</span> <span class="identifier">example_struct_iterator_tag</span> <span class="identifier">type</span><span class="special">;</span>
<span class="special">};</span>
<span class="special">}</span>
<span class="special">}}</span>
</pre>
<p>
In isolation, the iterator implementation is pretty dry. Things should become
clearer as we add features to our implementation.
</p>
<a name="fusion.extension.a_first_couple_of_instructive_features"></a><h3>
<a name="id604307"></a>
<a href="extension.html#fusion.extension.a_first_couple_of_instructive_features">A first
couple of instructive features</a>
</h3>
<p>
To start with, we will get the <a href="iterators/metafunctions/value_of.html" title="value_of"><tt class="computeroutput"><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value_of</span></tt></a> metafunction working. To
do this, we provide a specialization of the <tt class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">extension</span><span class="special">::</span><span class="identifier">value_of_impl</span></tt>
template for our iterator's tag type.
</p>
<pre class="programlisting">
<span class="keyword">template</span><span class="special">&lt;&gt;</span>
<span class="keyword">struct</span> <span class="identifier">value_of_impl</span><span class="special">&lt;</span><span class="identifier">example_struct_iterator_tag</span><span class="special">&gt;</span>
<span class="special">{</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Iterator</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">apply</span><span class="special">;</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Struct</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">apply</span><span class="special">&lt;</span><span class="identifier">example_struct_iterator</span><span class="special">&lt;</span><span class="identifier">Struct</span><span class="special">,</span> <span class="number">0</span><span class="special">&gt;</span> <span class="special">&gt;</span>
<span class="special">{</span>
<span class="keyword">typedef</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">type</span><span class="special">;</span>
<span class="special">};</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Struct</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">apply</span><span class="special">&lt;</span><span class="identifier">example_struct_iterator</span><span class="special">&lt;</span><span class="identifier">Struct</span><span class="special">,</span> <span class="number">1</span><span class="special">&gt;</span> <span class="special">&gt;</span>
<span class="special">{</span>
<span class="keyword">typedef</span> <span class="keyword">int</span> <span class="identifier">type</span><span class="special">;</span>
<span class="special">};</span>
<span class="special">};</span>
</pre>
<p>
The implementation itself is pretty simple, it just uses 2 partial specializations
to provide the type of the 2 different members of <tt class="computeroutput"><span class="identifier">example_struct</span></tt>,
based on the index of the iterator.
</p>
<p>
To understand how <tt class="computeroutput"><span class="identifier">value_of_impl</span></tt>
is used by the library we will look at the implementation of <a href="iterators/metafunctions/value_of.html" title="value_of"><tt class="computeroutput"><span class="identifier">value_of</span></tt></a>:
</p>
<pre class="programlisting">
<span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Iterator</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <a href="iterators/metafunctions/value_of.html" title="value_of"><tt class="computeroutput"><span class="identifier">value_of</span></tt></a>
<span class="special">{</span>
<span class="keyword">typedef</span> <span class="keyword">typename</span>
<span class="identifier">extension</span><span class="special">::</span><span class="identifier">value_of_impl</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Iterator</span><span class="special">::</span><span class="identifier">ftag</span><span class="special">&gt;::</span>
<span class="keyword">template</span> <span class="identifier">apply</span><span class="special">&lt;</span><span class="identifier">Iterator</span><span class="special">&gt;::</span><span class="identifier">type</span>
<span class="identifier">type</span><span class="special">;</span>
<span class="special">};</span>
</pre>
<p>
So <a href="iterators/metafunctions/value_of.html" title="value_of"><tt class="computeroutput"><span class="identifier">value_of</span></tt></a>
uses <a href="notes.html#fusion.notes.tag_dispatching"><span class="emphasis"><em>tag dispatching</em></span></a>
to select an MPL Metafunction Class to provide its functionality. You will
notice this pattern throughout the implementation of Fusion.
</p>
<p>
Ok, lets enable dereferencing of our iterator. In this case we must provide
a suitable specialization of <tt class="computeroutput"><span class="identifier">deref_impl</span></tt>.
</p>
<pre class="programlisting">
<span class="keyword">template</span><span class="special">&lt;&gt;</span>
<span class="keyword">struct</span> <span class="identifier">deref_impl</span><span class="special">&lt;</span><span class="identifier">example_struct_iterator_tag</span><span class="special">&gt;</span>
<span class="special">{</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Iterator</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">apply</span><span class="special">;</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Struct</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">apply</span><span class="special">&lt;</span><span class="identifier">example_struct_iterator</span><span class="special">&lt;</span><span class="identifier">Struct</span><span class="special">,</span> <span class="number">0</span><span class="special">&gt;</span> <span class="special">&gt;</span>
<span class="special">{</span>
<span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">if_</span><span class="special">&lt;</span>
<span class="identifier">is_const</span><span class="special">&lt;</span><span class="identifier">Struct</span><span class="special">&gt;,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="keyword">const</span><span class="special">&amp;,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&amp;&gt;::</span><span class="identifier">type</span> <span class="identifier">type</span><span class="special">;</span>
<span class="keyword">static</span> <span class="identifier">type</span>
<span class="identifier">call</span><span class="special">(</span><span class="identifier">example_struct_iterator</span><span class="special">&lt;</span><span class="identifier">Struct</span><span class="special">,</span> <span class="number">0</span><span class="special">&gt;</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">it</span><span class="special">)</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">it</span><span class="special">.</span><span class="identifier">struct_</span><span class="special">.</span><span class="identifier">name</span><span class="special">;</span>
<span class="special">}</span>
<span class="special">};</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Struct</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">apply</span><span class="special">&lt;</span><span class="identifier">example_struct_iterator</span><span class="special">&lt;</span><span class="identifier">Struct</span><span class="special">,</span> <span class="number">1</span><span class="special">&gt;</span> <span class="special">&gt;</span>
<span class="special">{</span>
<span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">if_</span><span class="special">&lt;</span>
<span class="identifier">is_const</span><span class="special">&lt;</span><span class="identifier">Struct</span><span class="special">&gt;,</span> <span class="keyword">int</span> <span class="keyword">const</span><span class="special">&amp;,</span> <span class="keyword">int</span><span class="special">&amp;&gt;::</span><span class="identifier">type</span> <span class="identifier">type</span><span class="special">;</span>
<span class="keyword">static</span> <span class="identifier">type</span>
<span class="identifier">call</span><span class="special">(</span><span class="identifier">example_struct_iterator</span><span class="special">&lt;</span><span class="identifier">Struct</span><span class="special">,</span> <span class="number">1</span><span class="special">&gt;</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">it</span><span class="special">)</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">it</span><span class="special">.</span><span class="identifier">struct_</span><span class="special">.</span><span class="identifier">age</span><span class="special">;</span>
<span class="special">}</span>
<span class="special">};</span>
<span class="special">};</span>
</pre>
<p>
The use of <tt class="computeroutput"><span class="identifier">deref_impl</span></tt> is very
similar to that of <tt class="computeroutput"><span class="identifier">value_of_impl</span></tt>,
but it also provides some runtime functionality this time via the <tt class="computeroutput"><span class="identifier">call</span></tt> static member function. To see how
<tt class="computeroutput"><span class="identifier">deref_impl</span></tt> is used, lets have
a look at the implementation of <a href="iterators/functions/deref.html" title="deref"><tt class="computeroutput"><span class="identifier">deref</span></tt></a>:
</p>
<pre class="programlisting">
<span class="keyword">namespace</span> <span class="identifier">result_of</span>
<span class="special">{</span>
<span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Iterator</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <a href="iterators/functions/deref.html" title="deref"><tt class="computeroutput"><span class="identifier">deref</span></tt></a>
<span class="special">{</span>
<span class="keyword">typedef</span> <span class="keyword">typename</span>
<span class="identifier">deref_impl</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Iterator</span><span class="special">::</span><span class="identifier">ftag</span><span class="special">&gt;::</span>
<span class="keyword">template</span> <span class="identifier">apply</span><span class="special">&lt;</span><span class="identifier">Iterator</span><span class="special">&gt;::</span><span class="identifier">type</span>
<span class="identifier">type</span><span class="special">;</span>
<span class="special">};</span>
<span class="special">}</span>
<span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Iterator</span><span class="special">&gt;</span>
<span class="keyword">typename</span> <a href="iterators/metafunctions/deref.html" title="deref"><tt class="computeroutput"><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">deref</span></tt></a><span class="special">&lt;</span><span class="identifier">Iterator</span><span class="special">&gt;::</span><span class="identifier">type</span>
<a href="iterators/functions/deref.html" title="deref"><tt class="computeroutput"><span class="identifier">deref</span></tt></a><span class="special">(</span><span class="identifier">Iterator</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">i</span><span class="special">)</span>
<span class="special">{</span>
<span class="keyword">typename</span> <a href="iterators/metafunctions/deref.html" title="deref"><tt class="computeroutput"><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">deref</span></tt></a><span class="special">&lt;</span><span class="identifier">Iterator</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">result</span> <span class="special">=</span>
<span class="identifier">extension</span><span class="special">::</span><span class="identifier">deref_impl</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Iterator</span><span class="special">::</span><span class="identifier">ftag</span><span class="special">&gt;::</span>
<span class="keyword">template</span> <span class="identifier">apply</span><span class="special">&lt;</span><span class="identifier">Iterator</span><span class="special">&gt;::</span><span class="identifier">call</span><span class="special">(</span><span class="identifier">i</span><span class="special">);</span>
<span class="keyword">return</span> <span class="identifier">result</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
So again <a href="iterators/metafunctions/deref.html" title="deref"><tt class="computeroutput"><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">deref</span></tt></a> uses <a href="notes.html#fusion.notes.tag_dispatching"><span class="emphasis"><em>tag
dispatching</em></span></a> in exactly the same way as the <a href="iterators/metafunctions/value_of.html" title="value_of"><tt class="computeroutput"><span class="identifier">value_of</span></tt></a> implementation. The runtime
functionality used by <a href="iterators/functions/deref.html" title="deref"><tt class="computeroutput"><span class="identifier">deref</span></tt></a> is provided by the <tt class="computeroutput"><span class="identifier">call</span></tt> static function of the selected MPL
Metafunction Class.
</p>
<p>
The actual implementation of <tt class="computeroutput"><span class="identifier">deref_impl</span></tt>
is slightly more complex than that of <tt class="computeroutput"><span class="identifier">value_of_impl</span></tt>.
We also need to implement the <tt class="computeroutput"><span class="identifier">call</span></tt>
function, which returns a reference to the appropriate member of the underlying
sequence. We also require a little bit of metaprogramming to return <tt class="computeroutput"><span class="keyword">const</span></tt> references if the underlying sequence
is const.
</p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td colspan="2" align="left" valign="top"><p>
Although there is a fair amount of left to do to produce a fully fledged
Fusion sequence, <a href="iterators/metafunctions/value_of.html" title="value_of"><tt class="computeroutput"><span class="identifier">value_of</span></tt></a> and <a href="iterators/functions/deref.html" title="deref"><tt class="computeroutput"><span class="identifier">deref</span></tt></a> illustrate all the signficant
concepts required. The remainder of the process is very repetitive, simply
requiring implementation of a suitable <tt class="computeroutput"><span class="identifier">xxxx_impl</span></tt>
for each feature <tt class="computeroutput"><span class="identifier">xxxx</span></tt>.
</p></td></tr>
</table></div>
<a name="fusion.extension.implementing_the_remaining_iterator_functionality"></a><h3>
<a name="id606571"></a>
<a href="extension.html#fusion.extension.implementing_the_remaining_iterator_functionality">Implementing
the remaining iterator functionality</a>
</h3>
<p>
Ok, now we have seen the way <a href="iterators/metafunctions/value_of.html" title="value_of"><tt class="computeroutput"><span class="identifier">value_of</span></tt></a> and <a href="iterators/functions/deref.html" title="deref"><tt class="computeroutput"><span class="identifier">deref</span></tt></a> work, everything else will work
in pretty much the same way. Lets start with forward iteration, by providing
a <tt class="computeroutput"><span class="identifier">next_impl</span></tt>:
</p>
<pre class="programlisting">
<span class="keyword">template</span><span class="special">&lt;&gt;</span>
<span class="keyword">struct</span> <span class="identifier">next_impl</span><span class="special">&lt;</span><span class="identifier">example_struct_iterator_tag</span><span class="special">&gt;</span>
<span class="special">{</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Iterator</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">apply</span>
<span class="special">{</span>
<span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">Iterator</span><span class="special">::</span><span class="identifier">struct_type</span> <span class="identifier">struct_type</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">Iterator</span><span class="special">::</span><span class="identifier">index</span> <span class="identifier">index</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="identifier">example_struct_iterator</span><span class="special">&lt;</span><span class="identifier">struct_type</span><span class="special">,</span> <span class="identifier">index</span><span class="special">::</span><span class="identifier">value</span> <span class="special">+</span> <span class="number">1</span><span class="special">&gt;</span> <span class="identifier">type</span><span class="special">;</span>
<span class="keyword">static</span> <span class="identifier">type</span>
<span class="identifier">call</span><span class="special">(</span><span class="identifier">Iterator</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">i</span><span class="special">)</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">type</span><span class="special">(</span><span class="identifier">i</span><span class="special">.</span><span class="identifier">struct_</span><span class="special">);</span>
<span class="special">}</span>
<span class="special">};</span>
<span class="special">};</span>
</pre>
<p>
This should be very familiar from our <tt class="computeroutput"><span class="identifier">deref_impl</span></tt>
implementation, we will be using this approach again and again now. Our design
is simply to increment the <tt class="computeroutput"><span class="identifier">index</span></tt>
counter to move on to the next element. The various other iterator manipulations
we need to perform will all just involve simple calculations with the <tt class="computeroutput"><span class="identifier">index</span></tt> variables.
</p>
<p>
We also need to provide a suitable <tt class="computeroutput"><span class="identifier">equal_to_impl</span></tt>
so that iterators can be correctly compared. A <a href="iterators/concepts/bidirectional_iterator.html" title="Bidirectional
Iterator">Bidirectional
Iterator</a> will also need an implementation of <tt class="computeroutput"><span class="identifier">prior_impl</span></tt>.
For a <a href="iterators/concepts/random_access_iterator.html" title="Random
Access Iterator">Random
Access Iterator</a><EFBFBD><tt class="computeroutput"><span class="identifier">distance_impl</span></tt>
and <tt class="computeroutput"><span class="identifier">advance_impl</span></tt> also need
to be provided in order to satisfy the necessary complexity guarantees. As
our iterator is a <a href="iterators/concepts/random_access_iterator.html" title="Random
Access Iterator">Random
Access Iterator</a> we will have to implement all of these functions.
</p>
<p>
Full implementations of <tt class="computeroutput"><span class="identifier">prior_impl</span></tt>,
<tt class="computeroutput"><span class="identifier">advance_impl</span></tt>, <tt class="computeroutput"><span class="identifier">distance_impl</span></tt> and <tt class="computeroutput"><span class="identifier">equal_to_impl</span></tt>
are provided in the example code.
</p>
<a name="fusion.extension.implementing_the_intrinsic_functions_of_the_sequence"></a><h3>
<a name="id607174"></a>
<a href="extension.html#fusion.extension.implementing_the_intrinsic_functions_of_the_sequence">Implementing
the intrinsic functions of the sequence</a>
</h3>
<p>
In order that Fusion can correctly identify our sequence as a Fusion sequence,
we need to enable <tt class="computeroutput"><span class="identifier">is_sequence</span></tt>
for our sequence type. As usual we just create an <tt class="computeroutput"><span class="identifier">impl</span></tt>
type specialized for our sequence tag:
</p>
<pre class="programlisting">
<span class="keyword">template</span><span class="special">&lt;&gt;</span>
<span class="keyword">struct</span> <span class="identifier">is_sequence_impl</span><span class="special">&lt;</span><span class="identifier">example_sequence_tag</span><span class="special">&gt;</span>
<span class="special">{</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">apply</span> <span class="special">:</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span> <span class="special">{};</span>
<span class="special">};</span>
</pre>
<p>
We've some similar formalities to complete, providing <tt class="computeroutput"><span class="identifier">category_of_impl</span></tt>
so Fusion can correctly identify our sequence type, and <tt class="computeroutput"><span class="identifier">is_view_impl</span></tt>
so Fusion can correctly identify our sequence as not being a <a href="sequences/views.html" title="Views">View</a>
type. Implementations are provide in the example code.
</p>
<p>
Now we've completed some formalities, on to more interesting features. Lets
get <a href="sequences/intrinsics/functions/begin.html" title="begin"><tt class="computeroutput"><span class="identifier">begin</span></tt></a> working so that we can get an
iterator to start accessing the data in our sequence.
</p>
<pre class="programlisting">
<span class="keyword">template</span><span class="special">&lt;&gt;</span>
<span class="keyword">struct</span> <span class="identifier">begin_impl</span><span class="special">&lt;</span><span class="identifier">example_sequence_tag</span><span class="special">&gt;</span>
<span class="special">{</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Sequence</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">apply</span>
<span class="special">{</span>
<span class="keyword">typedef</span> <span class="identifier">example_struct_iterator</span><span class="special">&lt;</span><span class="identifier">Sequence</span><span class="special">,</span> <span class="number">0</span><span class="special">&gt;</span> <span class="identifier">type</span><span class="special">;</span>
<span class="keyword">static</span> <span class="identifier">type</span>
<span class="identifier">call</span><span class="special">(</span><span class="identifier">Sequence</span><span class="special">&amp;</span> <span class="identifier">seq</span><span class="special">)</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">type</span><span class="special">(</span><span class="identifier">seq</span><span class="special">);</span>
<span class="special">}</span>
<span class="special">};</span>
<span class="special">};</span>
</pre>
<p>
The implementation uses the same ideas we have applied throughout, in this
case we are just creating one of the iterators we developed earlier, pointing
to the first element in the sequence. The implementation of <a href="sequences/intrinsics/functions/end.html" title="end"><tt class="computeroutput"><span class="identifier">end</span></tt></a> is very similar, and is provided
in the example code.
</p>
<p>
For our <a href="sequences/concepts/random_access_sequence.html" title="Random
Access Sequence">Random
Access Sequence</a> we will also need to implement <tt class="computeroutput"><span class="identifier">size_impl</span></tt>,
<tt class="computeroutput"><span class="identifier">value_at_impl</span></tt> and <tt class="computeroutput"><span class="identifier">at_impl</span></tt>.
</p>
<a name="fusion.extension.enabling_our_type_as_an_associative_container"></a><h3>
<a name="id607726"></a>
<a href="extension.html#fusion.extension.enabling_our_type_as_an_associative_container">Enabling
our type as an associative container</a>
</h3>
<p>
In order for <tt class="computeroutput"><span class="identifier">example_struct</span></tt>
to serve as an associative container, we need to enable 3 lookup features,
<a href="sequences/intrinsics/functions/at_key.html" title="at_key"><tt class="computeroutput"><span class="identifier">at_key</span></tt></a>, <a href="sequences/intrinsics/metafunctions/value_at_key.html" title="value_at_key"><tt class="computeroutput"><span class="identifier">value_at_key</span></tt></a> and <a href="sequences/intrinsics/functions/has_key.html" title="has_key"><tt class="computeroutput"><span class="identifier">has_key</span></tt></a>. We also need to provide an
implementation of the <tt class="computeroutput"><span class="identifier">is_associative</span></tt>
trait so that our sequence can be correctly identified as an associative container.
</p>
<p>
To implement <tt class="computeroutput"><span class="identifier">at_key_impl</span></tt> we
need to associate the <tt class="computeroutput"><span class="identifier">fields</span><span class="special">::</span><span class="identifier">age</span></tt> and
<tt class="computeroutput"><span class="identifier">fields</span><span class="special">::</span><span class="identifier">age</span></tt> types described in the <a href="quick_start.html" title="Quick Start">Quick
Start</a> guide with the appropriate members of <tt class="computeroutput"><span class="identifier">example_struct</span></tt>.
Our implementation is as follows:
</p>
<pre class="programlisting">
<span class="keyword">template</span><span class="special">&lt;&gt;</span>
<span class="keyword">struct</span> <span class="identifier">at_key_impl</span><span class="special">&lt;</span><span class="identifier">example_sequence_tag</span><span class="special">&gt;</span>
<span class="special">{</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Sequence</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Key</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">apply</span><span class="special">;</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Sequence</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">apply</span><span class="special">&lt;</span><span class="identifier">Sequence</span><span class="special">,</span> <span class="identifier">fields</span><span class="special">::</span><span class="identifier">name</span><span class="special">&gt;</span>
<span class="special">{</span>
<span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">if_</span><span class="special">&lt;</span>
<span class="identifier">is_const</span><span class="special">&lt;</span><span class="identifier">Sequence</span><span class="special">&gt;,</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="keyword">const</span><span class="special">&amp;,</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&amp;&gt;::</span><span class="identifier">type</span> <span class="identifier">type</span><span class="special">;</span>
<span class="keyword">static</span> <span class="identifier">type</span>
<span class="identifier">call</span><span class="special">(</span><span class="identifier">Sequence</span><span class="special">&amp;</span> <span class="identifier">seq</span><span class="special">)</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">seq</span><span class="special">.</span><span class="identifier">name</span><span class="special">;</span>
<span class="special">};</span>
<span class="special">};</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Sequence</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">apply</span><span class="special">&lt;</span><span class="identifier">Sequence</span><span class="special">,</span> <span class="identifier">fields</span><span class="special">::</span><span class="identifier">age</span><span class="special">&gt;</span>
<span class="special">{</span>
<span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">if_</span><span class="special">&lt;</span>
<span class="identifier">is_const</span><span class="special">&lt;</span><span class="identifier">Sequence</span><span class="special">&gt;,</span>
<span class="keyword">int</span> <span class="keyword">const</span><span class="special">&amp;,</span>
<span class="keyword">int</span><span class="special">&amp;&gt;::</span><span class="identifier">type</span> <span class="identifier">type</span><span class="special">;</span>
<span class="keyword">static</span> <span class="identifier">type</span>
<span class="identifier">call</span><span class="special">(</span><span class="identifier">Sequence</span><span class="special">&amp;</span> <span class="identifier">seq</span><span class="special">)</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">seq</span><span class="special">.</span><span class="identifier">age</span><span class="special">;</span>
<span class="special">};</span>
<span class="special">};</span>
<span class="special">};</span>
</pre>
<p>
Its all very similar to the implementations we've seen previously, such as
<tt class="computeroutput"><span class="identifier">deref_impl</span></tt> and <tt class="computeroutput"><span class="identifier">value_of_impl</span></tt>. Instead of identifying the
members by index or position, we are now selecting them using the types <tt class="computeroutput"><span class="identifier">fields</span><span class="special">::</span><span class="identifier">name</span></tt> and <tt class="computeroutput"><span class="identifier">fields</span><span class="special">::</span><span class="identifier">age</span></tt>. The
implementations of <tt class="computeroutput"><span class="identifier">value_at_key_impl</span></tt>
and <tt class="computeroutput"><span class="identifier">has_key_impl</span></tt> are equally
straightforward, and are provided in the example code, along with an implementation
of <tt class="computeroutput"><span class="identifier">is_associative_impl</span></tt>.
</p>
<a name="fusion.extension.summary"></a><h3>
<a name="id608689"></a>
<a href="extension.html#fusion.extension.summary">Summary</a>
</h3>
<p>
We've now worked through the entire process for adding a new random access
sequence and we've also enabled our type to serve as an associative container.
The implementation was slightly longwinded, but followed a simple repeating
pattern.
</p>
<p>
The support for <tt class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span></tt>, <a href="http://www.boost.org/libs/mpl/index.html" target="_top">MPL</a>
sequences, and <tt class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">array</span></tt> all use the same approach, and provide
additional examples of the approach for a variety of types.
</p>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><small>Copyright <20> 2001-2005 Joel de Guzman, Dan Marsden</small></td>
</tr></table>
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