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| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="fusion.extension.ext_full"></a><a class="link" href="ext_full.html" title="The Full Extension Mechanism">The Full Extension Mechanism</a> |
| </h3></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 <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span></code>, |
| <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">array</span></code> 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 class="orderedlist" type="1"> |
| <li class="listitem"> |
| Enable the <a class="link" 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 class="listitem"> |
| Design an iterator type for the sequence |
| </li> |
| <li class="listitem"> |
| Provide specialized behaviour for the intrinsic operations of the new |
| Fusion sequence |
| </li> |
| </ol></div> |
| <a name="fusion.extension.ext_full.our_example"></a><h5> |
| <a name="id1062493"></a> |
| <a class="link" href="ext_full.html#fusion.extension.ext_full.our_example">Our example</a> |
| </h5> |
| <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">&</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 <code class="computeroutput"><span class="identifier">example_struct</span></code> |
| to serve as an <a class="link" href="../sequence/concepts/associative_sequence.html" title="Associative Sequence">Associative |
| Sequence</a> as described in the <a class="link" href="../quick_start.html" title="Quick Start">Quick |
| Start</a> guide. |
| </p> |
| <a name="fusion.extension.ext_full.enabling_tag_dispatching"></a><h5> |
| <a name="id1062697"></a> |
| <a class="link" href="ext_full.html#fusion.extension.ext_full.enabling_tag_dispatching">Enabling |
| Tag Dispatching</a> |
| </h5> |
| <p> |
| The Fusion extensibility mechanism uses <a class="link" 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">example</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 <code class="computeroutput"><span class="identifier">traits</span><span class="special">::</span><span class="identifier">tag_of</span></code> |
| metafunction to return our newly chosen tag type for operations involving |
| our sequence. This is done by specializing <code class="computeroutput"><span class="identifier">traits</span><span class="special">::</span><span class="identifier">tag_of</span></code> |
| for our sequence type. |
| </p> |
| <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</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">></span> |
| <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">tag_of_fwd</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></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"><></span> |
| <span class="keyword">struct</span> <span class="identifier">tag_of</span><span class="special"><</span><span class="identifier">example_struct</span><span class="special">></span> |
| <span class="special">{</span> |
| <span class="keyword">typedef</span> <span class="identifier">example</span><span class="special">::</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> |
| <code class="computeroutput"><span class="identifier">traits</span><span class="special">::</span><span class="identifier">tag_of</span></code> also has a second template argument, |
| that can be used in conjuction with <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">enable_if</span></code> |
| to provide tag support for groups of related 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"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">adapted</span><span class="special">/</span><span class="identifier">array</span><span class="special">/</span><span class="identifier">tag_of</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> |
| <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">tag_of</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> |
| </pre> |
| <a name="fusion.extension.ext_full.designing_a_suitable_iterator"></a><h5> |
| <a name="id1063132"></a> |
| <a class="link" href="ext_full.html#fusion.extension.ext_full.designing_a_suitable_iterator">Designing |
| a suitable iterator</a> |
| </h5> |
| <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 <code class="computeroutput"><span class="identifier">example_struct</span></code> |
| are given numbered indices, 0 for <code class="computeroutput"><span class="identifier">name</span></code> |
| and 1 for <code class="computeroutput"><span class="identifier">age</span></code> respectively. |
| </p> |
| <pre class="programlisting"><span class="keyword">template</span><span class="special"><</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">></span> |
| <span class="keyword">struct</span> <span class="identifier">example_struct_iterator</span> |
| <span class="special">:</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">iterator_base</span><span class="special"><</span><span class="identifier">example_struct_iterator</span><span class="special"><</span><span class="identifier">Struct</span><span class="special">,</span> <span class="identifier">Pos</span><span class="special">></span> <span class="special">></span> |
| <span class="special">{</span> |
| <span class="identifier">BOOST_STATIC_ASSERT</span><span class="special">(</span><span class="identifier">Pos</span> <span class="special">>=</span><span class="number">0</span> <span class="special">&&</span> <span class="identifier">Pos</span> <span class="special"><</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">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special"><</span><span class="identifier">Pos</span><span class="special">></span> <span class="identifier">index</span><span class="special">;</span> |
| <span class="keyword">typedef</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">fusion</span><span class="special">::</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">&</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">&</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 class="orderedlist" type="1"> |
| <li class="listitem"> |
| The iterator is parameterized by the type it is iterating over, and the |
| index of the current element. |
| </li> |
| <li class="listitem"> |
| The typedefs <code class="computeroutput"><span class="identifier">struct_type</span></code> |
| and <code class="computeroutput"><span class="identifier">index</span></code> provide convenient |
| access to information we will need later in the implementation. |
| </li> |
| <li class="listitem"> |
| The typedef <code class="computeroutput"><span class="identifier">category</span></code> |
| allows the <code class="computeroutput"><span class="identifier">traits</span><span class="special">::</span><a class="link" href="../support/category_of.html" title="category_of"><code class="computeroutput"><span class="identifier">category_of</span></code></a></code> |
| metafunction to establish the traversal category of the iterator. |
| </li> |
| <li class="listitem"> |
| The constructor stores a reference to the <code class="computeroutput"><span class="identifier">example_struct</span></code> |
| being iterated over. |
| </li> |
| </ol></div> |
| <p> |
| We also need to enable <a class="link" href="../notes.html#fusion.notes.tag_dispatching"><span class="emphasis"><em>tag |
| dispatching</em></span></a> for our iterator type, with another specialization |
| of <code class="computeroutput"><span class="identifier">traits</span><span class="special">::</span><span class="identifier">tag_of</span></code>. |
| </p> |
| <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.ext_full.a_first_couple_of_instructive_features"></a><h5> |
| <a name="id1063605"></a> |
| <a class="link" href="ext_full.html#fusion.extension.ext_full.a_first_couple_of_instructive_features">A |
| first couple of instructive features</a> |
| </h5> |
| <p> |
| To start with, we will get the <a class="link" href="../iterator/metafunctions/value_of.html" title="value_of"><code class="computeroutput"><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value_of</span></code></a> metafunction working. To |
| do this, we provide a specialization of the <code 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></code> |
| template for our iterator's tag type. |
| </p> |
| <pre class="programlisting"><span class="keyword">template</span><span class="special"><></span> |
| <span class="keyword">struct</span> <span class="identifier">value_of_impl</span><span class="special"><</span><span class="identifier">example</span><span class="special">::</span><span class="identifier">example_struct_iterator_tag</span><span class="special">></span> |
| <span class="special">{</span> |
| <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Iterator</span><span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">apply</span><span class="special">;</span> |
| |
| <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Struct</span><span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">apply</span><span class="special"><</span><span class="identifier">example</span><span class="special">::</span><span class="identifier">example_struct_iterator</span><span class="special"><</span><span class="identifier">Struct</span><span class="special">,</span> <span class="number">0</span><span class="special">></span> <span class="special">></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"><</span><span class="keyword">typename</span> <span class="identifier">Struct</span><span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">apply</span><span class="special"><</span><span class="identifier">example</span><span class="special">::</span><span class="identifier">example_struct_iterator</span><span class="special"><</span><span class="identifier">Struct</span><span class="special">,</span> <span class="number">1</span><span class="special">></span> <span class="special">></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 <code class="computeroutput"><span class="identifier">example_struct</span></code>, |
| based on the index of the iterator. |
| </p> |
| <p> |
| To understand how <code class="computeroutput"><span class="identifier">value_of_impl</span></code> |
| is used by the library we will look at the implementation of <a class="link" href="../iterator/metafunctions/value_of.html" title="value_of"><code class="computeroutput"><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value_of</span></code></a>: |
| </p> |
| <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Iterator</span><span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">value_of</span> |
| <span class="special">:</span> <span class="identifier">extension</span><span class="special">::</span><span class="identifier">value_of_impl</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">detail</span><span class="special">::</span><span class="identifier">tag_of</span><span class="special"><</span><span class="identifier">Iterator</span><span class="special">>::</span><span class="identifier">type</span><span class="special">>::</span> |
| <span class="keyword">template</span> <span class="identifier">apply</span><span class="special"><</span><span class="identifier">Iterator</span><span class="special">></span> |
| <span class="special">{};</span> |
| </pre> |
| <p> |
| So <a class="link" href="../iterator/metafunctions/value_of.html" title="value_of"><code class="computeroutput"><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value_of</span></code></a> |
| uses <a class="link" href="../notes.html#fusion.notes.tag_dispatching"><span class="emphasis"><em>tag dispatching</em></span></a> |
| to select an <a href="http://www.boost.org/libs/mpl/doc/refmanual/metafunction-class.html" target="_top">MPL |
| Metafunction Class</a> 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 <code class="computeroutput"><span class="identifier">deref_impl</span></code>. |
| </p> |
| <pre class="programlisting"><span class="keyword">template</span><span class="special"><></span> |
| <span class="keyword">struct</span> <span class="identifier">deref_impl</span><span class="special"><</span><span class="identifier">example</span><span class="special">::</span><span class="identifier">example_struct_iterator_tag</span><span class="special">></span> |
| <span class="special">{</span> |
| <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Iterator</span><span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">apply</span><span class="special">;</span> |
| |
| <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Struct</span><span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">apply</span><span class="special"><</span><span class="identifier">example</span><span class="special">::</span><span class="identifier">example_struct_iterator</span><span class="special"><</span><span class="identifier">Struct</span><span class="special">,</span> <span class="number">0</span><span class="special">></span> <span class="special">></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"><</span> |
| <span class="identifier">is_const</span><span class="special"><</span><span class="identifier">Struct</span><span class="special">>,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="keyword">const</span><span class="special">&,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&>::</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</span><span class="special">::</span><span class="identifier">example_struct_iterator</span><span class="special"><</span><span class="identifier">Struct</span><span class="special">,</span> <span class="number">0</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</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"><</span><span class="keyword">typename</span> <span class="identifier">Struct</span><span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">apply</span><span class="special"><</span><span class="identifier">example</span><span class="special">::</span><span class="identifier">example_struct_iterator</span><span class="special"><</span><span class="identifier">Struct</span><span class="special">,</span> <span class="number">1</span><span class="special">></span> <span class="special">></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"><</span> |
| <span class="identifier">is_const</span><span class="special"><</span><span class="identifier">Struct</span><span class="special">>,</span> <span class="keyword">int</span> <span class="keyword">const</span><span class="special">&,</span> <span class="keyword">int</span><span class="special">&>::</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</span><span class="special">::</span><span class="identifier">example_struct_iterator</span><span class="special"><</span><span class="identifier">Struct</span><span class="special">,</span> <span class="number">1</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</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> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| The use of <code class="computeroutput"><span class="identifier">deref_impl</span></code> is |
| very similar to that of <code class="computeroutput"><span class="identifier">value_of_impl</span></code>, |
| but it also provides some runtime functionality this time via the <code class="computeroutput"><span class="identifier">call</span></code> static member function. To see how |
| <code class="computeroutput"><span class="identifier">deref_impl</span></code> is used, lets |
| have a look at the implementation of <a class="link" href="../iterator/functions/deref.html" title="deref"><code class="computeroutput"><span class="identifier">deref</span></code></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"><</span><span class="keyword">typename</span> <span class="identifier">Iterator</span><span class="special">></span> |
| <span class="keyword">struct</span> <a class="link" href="../iterator/functions/deref.html" title="deref"><code class="computeroutput"><span class="identifier">deref</span></code></a> |
| <span class="special">:</span> <span class="identifier">extension</span><span class="special">::</span><span class="identifier">deref_impl</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">detail</span><span class="special">::</span><span class="identifier">tag_of</span><span class="special"><</span><span class="identifier">Iterator</span><span class="special">>::</span><span class="identifier">type</span><span class="special">>::</span> |
| <span class="keyword">template</span> <span class="identifier">apply</span><span class="special"><</span><span class="identifier">Iterator</span><span class="special">></span> |
| <span class="special">{};</span> |
| <span class="special">}</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Iterator</span><span class="special">></span> |
| <span class="keyword">typename</span> <span class="identifier">result_of</span><span class="special">::</span><span class="identifier">deref</span><span class="special"><</span><span class="identifier">Iterator</span><span class="special">>::</span><span class="identifier">type</span> |
| <a class="link" href="../iterator/functions/deref.html" title="deref"><code class="computeroutput"><span class="identifier">deref</span></code></a><span class="special">(</span><span class="identifier">Iterator</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">i</span><span class="special">)</span> |
| <span class="special">{</span> |
| <span class="keyword">typedef</span> <span class="identifier">result_of</span><span class="special">::</span><span class="identifier">deref</span><span class="special"><</span><span class="identifier">Iterator</span><span class="special">></span> <span class="identifier">deref_meta</span><span class="special">;</span> |
| <span class="keyword">return</span> <span class="identifier">deref_meta</span><span class="special">::</span><span class="identifier">call</span><span class="special">(</span><span class="identifier">i</span><span class="special">);</span> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| So again <a class="link" href="../iterator/metafunctions/deref.html" title="deref"><code class="computeroutput"><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">deref</span></code></a> uses <a class="link" href="../notes.html#fusion.notes.tag_dispatching"><span class="emphasis"><em>tag |
| dispatching</em></span></a> in exactly the same way as the <a class="link" href="../iterator/metafunctions/value_of.html" title="value_of"><code class="computeroutput"><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value_of</span></code></a> implementation. The runtime |
| functionality used by <a class="link" href="../iterator/functions/deref.html" title="deref"><code class="computeroutput"><span class="identifier">deref</span></code></a> is provided by the <code class="computeroutput"><span class="identifier">call</span></code> static function of the selected <a href="http://www.boost.org/libs/mpl/doc/refmanual/metafunction-class.html" target="_top">MPL |
| Metafunction Class</a>. |
| </p> |
| <p> |
| The actual implementation of <code class="computeroutput"><span class="identifier">deref_impl</span></code> |
| is slightly more complex than that of <code class="computeroutput"><span class="identifier">value_of_impl</span></code>. |
| We also need to implement the <code class="computeroutput"><span class="identifier">call</span></code> |
| function, which returns a reference to the appropriate member of the underlying |
| sequence. We also require a little bit of metaprogramming to return <code class="computeroutput"><span class="keyword">const</span></code> 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="../../../../../../doc/src/images/note.png"></td> |
| <th align="left">Note</th> |
| </tr> |
| <tr><td align="left" valign="top"><p> |
| Although there is a fair amount of left to do to produce a fully fledged |
| Fusion sequence, <a class="link" href="../iterator/metafunctions/value_of.html" title="value_of"><code class="computeroutput"><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value_of</span></code></a> and <a class="link" href="../iterator/functions/deref.html" title="deref"><code class="computeroutput"><span class="identifier">deref</span></code></a> illustrate all the signficant |
| concepts required. The remainder of the process is very repetitive, simply |
| requiring implementation of a suitable <code class="computeroutput"><span class="identifier">xxxx_impl</span></code> |
| for each feature <code class="computeroutput"><span class="identifier">xxxx</span></code>. |
| </p></td></tr> |
| </table></div> |
| <a name="fusion.extension.ext_full.implementing_the_remaining_iterator_functionality"></a><h5> |
| <a name="id1066336"></a> |
| <a class="link" href="ext_full.html#fusion.extension.ext_full.implementing_the_remaining_iterator_functionality">Implementing |
| the remaining iterator functionality</a> |
| </h5> |
| <p> |
| Ok, now we have seen the way <a class="link" href="../iterator/metafunctions/value_of.html" title="value_of"><code class="computeroutput"><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value_of</span></code></a> and <a class="link" href="../iterator/functions/deref.html" title="deref"><code class="computeroutput"><span class="identifier">deref</span></code></a> work, everything else will |
| work in pretty much the same way. Lets start with forward iteration, by providing |
| a <code class="computeroutput"><span class="identifier">next_impl</span></code>: |
| </p> |
| <pre class="programlisting"><span class="keyword">template</span><span class="special"><></span> |
| <span class="keyword">struct</span> <span class="identifier">next_impl</span><span class="special"><</span><span class="identifier">example</span><span class="special">::</span><span class="identifier">example_struct_iterator_tag</span><span class="special">></span> |
| <span class="special">{</span> |
| <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Iterator</span><span class="special">></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</span><span class="special">::</span><span class="identifier">example_struct_iterator</span><span class="special"><</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">></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">&</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 <code class="computeroutput"><span class="identifier">deref_impl</span></code> |
| implementation, we will be using this approach again and again now. Our design |
| is simply to increment the <code class="computeroutput"><span class="identifier">index</span></code> |
| 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 <code class="computeroutput"><span class="identifier">index</span></code> variables. |
| </p> |
| <p> |
| We also need to provide a suitable <code class="computeroutput"><span class="identifier">equal_to_impl</span></code> |
| so that iterators can be correctly compared. A <a class="link" href="../iterator/concepts/bidirectional_iterator.html" title="Bidirectional Iterator">Bidirectional |
| Iterator</a> will also need an implementation of <code class="computeroutput"><span class="identifier">prior_impl</span></code>. |
| For a <a class="link" href="../iterator/concepts/random_access_iterator.html" title="Random Access Iterator">Random |
| Access Iterator</a> <code class="computeroutput"><span class="identifier">distance_impl</span></code> |
| and <code class="computeroutput"><span class="identifier">advance_impl</span></code> also need |
| to be provided in order to satisfy the necessary complexity guarantees. As |
| our iterator is a <a class="link" href="../iterator/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 <code class="computeroutput"><span class="identifier">prior_impl</span></code>, |
| <code class="computeroutput"><span class="identifier">advance_impl</span></code>, <code class="computeroutput"><span class="identifier">distance_impl</span></code> and <code class="computeroutput"><span class="identifier">equal_to_impl</span></code> |
| are provided in the example code. |
| </p> |
| <a name="fusion.extension.ext_full.implementing_the_intrinsic_functions_of_the_sequence"></a><h5> |
| <a name="id1066780"></a> |
| <a class="link" href="ext_full.html#fusion.extension.ext_full.implementing_the_intrinsic_functions_of_the_sequence">Implementing |
| the intrinsic functions of the sequence</a> |
| </h5> |
| <p> |
| In order that Fusion can correctly identify our sequence as a Fusion sequence, |
| we need to enable <code class="computeroutput"><span class="identifier">is_sequence</span></code> |
| for our sequence type. As usual we just create an <code class="computeroutput"><span class="identifier">impl</span></code> |
| type specialized for our sequence tag: |
| </p> |
| <pre class="programlisting"><span class="keyword">template</span><span class="special"><></span> |
| <span class="keyword">struct</span> <span class="identifier">is_sequence_impl</span><span class="special"><</span><span class="identifier">example</span><span class="special">::</span><span class="identifier">example_sequence_tag</span><span class="special">></span> |
| <span class="special">{</span> |
| <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></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 <code class="computeroutput"><span class="identifier">category_of_impl</span></code> |
| so Fusion can correctly identify our sequence type, and <code class="computeroutput"><span class="identifier">is_view_impl</span></code> |
| so Fusion can correctly identify our sequence as not being a <a class="link" href="../view.html" title="View">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 class="link" href="../sequence/intrinsic/functions/begin.html" title="begin"><code class="computeroutput"><span class="identifier">begin</span></code></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"><></span> |
| <span class="keyword">struct</span> <span class="identifier">begin_impl</span><span class="special"><</span><span class="identifier">example</span><span class="special">::</span><span class="identifier">example_sequence_tag</span><span class="special">></span> |
| <span class="special">{</span> |
| <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Sequence</span><span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">apply</span> |
| <span class="special">{</span> |
| <span class="keyword">typedef</span> <span class="identifier">example</span><span class="special">::</span><span class="identifier">example_struct_iterator</span><span class="special"><</span><span class="identifier">Sequence</span><span class="special">,</span> <span class="number">0</span><span class="special">></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">&</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 class="link" href="../sequence/intrinsic/functions/end.html" title="end"><code class="computeroutput"><span class="identifier">end</span></code></a> is very similar, and is provided |
| in the example code. |
| </p> |
| <p> |
| For our <a class="link" href="../sequence/concepts/random_access_sequence.html" title="Random Access Sequence">Random |
| Access Sequence</a> we will also need to implement <code class="computeroutput"><span class="identifier">size_impl</span></code>, |
| <code class="computeroutput"><span class="identifier">value_at_impl</span></code> and <code class="computeroutput"><span class="identifier">at_impl</span></code>. |
| </p> |
| <a name="fusion.extension.ext_full.enabling_our_type_as_an_associative_sequence"></a><h5> |
| <a name="id1067192"></a> |
| <a class="link" href="ext_full.html#fusion.extension.ext_full.enabling_our_type_as_an_associative_sequence">Enabling |
| our type as an associative sequence</a> |
| </h5> |
| <p> |
| In order for <code class="computeroutput"><span class="identifier">example_struct</span></code> |
| to serve as an associative forward sequence, we need to adapt the traversal |
| category of our sequence and our iterator accordingly and enable 3 intrinsic |
| sequence lookup features, <a class="link" href="../sequence/intrinsic/functions/at_key.html" title="at_key"><code class="computeroutput"><span class="identifier">at_key</span></code></a>, __value_at<span class="underline">key</span>_ |
| and <a class="link" href="../sequence/intrinsic/functions/has_key.html" title="has_key"><code class="computeroutput"><span class="identifier">has_key</span></code></a>. We also need to enable |
| 3 iterator lookup features, <a class="link" href="../iterator/metafunctions/key_of.html" title="key_of"><code class="computeroutput"><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">key_of</span></code></a>, <a class="link" href="../iterator/metafunctions/value_of_data.html" title="value_of_data"><code class="computeroutput"><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value_of_data</span></code></a> and <a class="link" href="../iterator/functions/deref_data.html" title="deref_data"><code class="computeroutput"><span class="identifier">deref_data</span></code></a>. |
| </p> |
| <p> |
| To implement <code class="computeroutput"><span class="identifier">at_key_impl</span></code> |
| we need to associate the <code class="computeroutput"><span class="identifier">fields</span><span class="special">::</span><span class="identifier">name</span></code> |
| and <code class="computeroutput"><span class="identifier">fields</span><span class="special">::</span><span class="identifier">age</span></code> types described in the <a class="link" href="../quick_start.html" title="Quick Start">Quick |
| Start</a> guide with the appropriate members of <code class="computeroutput"><span class="identifier">example_struct</span></code>. |
| Our implementation is as follows: |
| </p> |
| <pre class="programlisting"><span class="keyword">template</span><span class="special"><></span> |
| <span class="keyword">struct</span> <span class="identifier">at_key_impl</span><span class="special"><</span><span class="identifier">example</span><span class="special">::</span><span class="identifier">example_sequence_tag</span><span class="special">></span> |
| <span class="special">{</span> |
| <span class="keyword">template</span><span class="special"><</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">></span> |
| <span class="keyword">struct</span> <span class="identifier">apply</span><span class="special">;</span> |
| |
| <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Sequence</span><span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">apply</span><span class="special"><</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">></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"><</span> |
| <span class="identifier">is_const</span><span class="special"><</span><span class="identifier">Sequence</span><span class="special">>,</span> |
| <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="keyword">const</span><span class="special">&,</span> |
| <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&>::</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">&</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"><</span><span class="keyword">typename</span> <span class="identifier">Sequence</span><span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">apply</span><span class="special"><</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">></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"><</span> |
| <span class="identifier">is_const</span><span class="special"><</span><span class="identifier">Sequence</span><span class="special">>,</span> |
| <span class="keyword">int</span> <span class="keyword">const</span><span class="special">&,</span> |
| <span class="keyword">int</span><span class="special">&>::</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">&</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 |
| <code class="computeroutput"><span class="identifier">deref_impl</span></code> and <code class="computeroutput"><span class="identifier">value_of_impl</span></code>. Instead of identifying the |
| members by index or position, we are now selecting them using the types |
| <code class="computeroutput"><span class="identifier">fields</span><span class="special">::</span><span class="identifier">name</span></code> and <code class="computeroutput"><span class="identifier">fields</span><span class="special">::</span><span class="identifier">age</span></code>. |
| The implementations of the other functions are equally straightforward, and |
| are provided in the example code. |
| </p> |
| <a name="fusion.extension.ext_full.summary"></a><h5> |
| <a name="id1067901"></a> |
| <a class="link" href="ext_full.html#fusion.extension.ext_full.summary">Summary</a> |
| </h5> |
| <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 sequence. |
| The implementation was slightly longwinded, but followed a simple repeating |
| pattern. |
| </p> |
| <p> |
| The support for <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span></code>, <a href="http://www.boost.org/libs/mpl/index.html" target="_top">MPL</a> |
| sequences, and <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">array</span></code> 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"><div class="copyright-footer">Copyright © 2001-2007 Joel de Guzman, Dan Marsden, Tobias |
| Schwinger<p> |
| Distributed under the Boost Software License, Version 1.0. (See accompanying |
| file LICENSE_1_0.txt 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> |
| </tr></table> |
| <hr> |
| <div class="spirit-nav"> |
| <a accesskey="p" href="../extension.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../extension.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="sequence_facade.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> |
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