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<html>
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<title>Notes</title>
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<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="fusion.notes"></a><a class="link" href="notes.html" title="Notes">Notes</a>
</h2></div></div></div>
<h4>
<a name="fusion.notes.h0"></a>
<span class="phrase"><a name="fusion.notes.recursive_inlined_functions"></a></span><a class="link" href="notes.html#fusion.notes.recursive_inlined_functions">Recursive
Inlined Functions</a>
</h4>
<p>
An interesting peculiarity of functions like <a class="link" href="sequence/intrinsic/functions/at.html" title="at"><code class="computeroutput"><span class="identifier">at</span></code></a> when applied to a <a class="link" href="sequence/concepts/forward_sequence.html" title="Forward Sequence">Forward
Sequence</a> like <a class="link" href="container/list.html" title="list"><code class="computeroutput"><span class="identifier">list</span></code></a>
is that what could have been linear runtime complexity effectively becomes
constant O(1) due to compiler optimization of C++ inlined functions, however
deeply recursive (up to a certain compiler limit of course). Compile time complexity
remains linear.
</p>
<h4>
<a name="fusion.notes.h1"></a>
<span class="phrase"><a name="fusion.notes.overloaded_functions"></a></span><a class="link" href="notes.html#fusion.notes.overloaded_functions">Overloaded
Functions</a>
</h4>
<p>
Associative sequences use function overloading to implement membership testing
and type associated key lookup. This amounts to constant runtime and amortized
constant compile time complexities. There is an overloaded function, <code class="computeroutput"><span class="identifier">f</span><span class="special">(</span><span class="identifier">k</span><span class="special">)</span></code>, for each key <span class="emphasis"><em>type</em></span> <code class="computeroutput"><span class="identifier">k</span></code>. The compiler chooses the appropriate function
given a key, <code class="computeroutput"><span class="identifier">k</span></code>.
</p>
<h4>
<a name="fusion.notes.h2"></a>
<span class="phrase"><a name="fusion.notes.tag_dispatching"></a></span><a class="link" href="notes.html#fusion.notes.tag_dispatching">Tag
Dispatching</a>
</h4>
<p>
Tag dispatching is a generic programming technique for selecting template specializations.
There are typically 3 components involved in the tag dispatching mechanism:
</p>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
A type for which an appropriate template specialization is required
</li>
<li class="listitem">
A metafunction that associates the type with a tag type
</li>
<li class="listitem">
A template that is specialized for the tag type
</li>
</ol></div>
<p>
For example, the fusion <code class="computeroutput"><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">begin</span></code> metafunction
is implemented as follows:
</p>
<pre class="programlisting"><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">begin</span>
<span class="special">{</span>
<span class="keyword">typedef</span> <span class="keyword">typename</span>
<span class="identifier">result_of</span><span class="special">::</span><span class="identifier">begin_impl</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">traits</span><span class="special">::</span><span class="identifier">tag_of</span><span class="special"><</span><span class="identifier">Sequence</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">Sequence</span><span class="special">>::</span><span class="identifier">type</span>
<span class="identifier">type</span><span class="special">;</span>
<span class="special">};</span>
</pre>
<p>
In the case:
</p>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
<code class="computeroutput"><span class="identifier">Sequence</span></code> is the type for
which a suitable implementation of <code class="computeroutput"><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">begin_impl</span></code>
is required
</li>
<li class="listitem">
<code class="computeroutput"><span class="identifier">traits</span><span class="special">::</span><span class="identifier">tag_of</span></code> is the metafunction that associates
<code class="computeroutput"><span class="identifier">Sequence</span></code> with an appropriate
tag
</li>
<li class="listitem">
<code class="computeroutput"><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">begin_impl</span></code> is the template which is specialized
to provide an implementation for each tag type
</li>
</ol></div>
<h4>
<a name="fusion.notes.h3"></a>
<span class="phrase"><a name="fusion.notes.extensibility"></a></span><a class="link" href="notes.html#fusion.notes.extensibility">Extensibility</a>
</h4>
<p>
Unlike <a href="http://www.boost.org/libs/mpl/index.html" target="_top">MPL</a>, there
is no extensible sequence concept in fusion. This does not mean that Fusion
sequences are not extensible. In fact, all Fusion sequences are inherently
extensible. It is just that the manner of sequence extension in Fusion is different
from both <a href="http://en.wikipedia.org/wiki/Standard_Template_Library" target="_top">STL</a>
and <a href="http://www.boost.org/libs/mpl/index.html" target="_top">MPL</a> on account
of the lazy nature of fusion <a class="link" href="algorithm.html" title="Algorithm">Algorithms</a>.
<a href="http://en.wikipedia.org/wiki/Standard_Template_Library" target="_top">STL</a>
containers extend themselves in place though member functions such as <a class="link" href="algorithm/transformation/functions/push_back.html" title="push_back"><code class="computeroutput"><span class="identifier">push_back</span></code></a> and <a class="link" href="algorithm/transformation/functions/insert.html" title="insert"><code class="computeroutput"><span class="identifier">insert</span></code></a>. <a href="http://www.boost.org/libs/mpl/index.html" target="_top">MPL</a>
sequences, on the other hand, are extended through "intrinsic" functions
that actually return whole sequences. <a href="http://www.boost.org/libs/mpl/index.html" target="_top">MPL</a>
is purely functional and can not have side effects. For example, <a href="http://www.boost.org/libs/mpl/index.html" target="_top">MPL</a>'s
<code class="computeroutput"><span class="identifier">push_back</span></code> does not actually
mutate an <code class="computeroutput"><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span></code>. It can't do that. Instead, it returns
an extended <code class="computeroutput"><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span></code>.
</p>
<p>
Like <a href="http://www.boost.org/libs/mpl/index.html" target="_top">MPL</a>, Fusion
too is purely functional and can not have side effects. With runtime efficiency
in mind, Fusion sequences are extended through generic functions that return
<a class="link" href="view.html" title="View">Views</a>. <a class="link" href="view.html" title="View">Views</a>
are sequences that do not actually contain data, but instead impart an alternative
presentation over the data from one or more underlying sequences. <a class="link" href="view.html" title="View">Views</a>
are proxies. They provide an efficient yet purely functional way to work on
potentially expensive sequence operations. For example, given a <a class="link" href="container/vector.html" title="vector"><code class="computeroutput"><span class="identifier">vector</span></code></a>, Fusion's <a class="link" href="algorithm/transformation/functions/push_back.html" title="push_back"><code class="computeroutput"><span class="identifier">push_back</span></code></a> returns a <a class="link" href="view/joint_view.html" title="joint_view"><code class="computeroutput"><span class="identifier">joint_view</span></code></a>, instead of an actual extended
<a class="link" href="container/vector.html" title="vector"><code class="computeroutput"><span class="identifier">vector</span></code></a>.
A <a class="link" href="view/joint_view.html" title="joint_view"><code class="computeroutput"><span class="identifier">joint_view</span></code></a>
holds a reference to the original sequence plus the appended data --making
it very cheap to pass around.
</p>
<h4>
<a name="fusion.notes.h4"></a>
<span class="phrase"><a name="fusion.notes.element_conversion"></a></span><a class="link" href="notes.html#fusion.notes.element_conversion">Element
Conversion</a>
</h4>
<p>
Functions that take in elemental values to form sequences (e.g. <a class="link" href="container/generation/functions/make_list.html" title="make_list"><code class="computeroutput"><span class="identifier">make_list</span></code></a>) convert their arguments
to something suitable to be stored as a sequence element. In general, the element
types are stored as plain values. Example:
</p>
<pre class="programlisting"><a class="link" href="container/generation/functions/make_list.html" title="make_list"><code class="computeroutput"><span class="identifier">make_list</span></code></a><span class="special">(</span><span class="number">1</span><span class="special">,</span> <span class="char">'x'</span><span class="special">)</span>
</pre>
<p>
returns a <a class="link" href="container/list.html" title="list"><code class="computeroutput"><span class="identifier">list</span></code></a><code class="computeroutput"><span class="special"><</span><span class="keyword">int</span><span class="special">,</span>
<span class="keyword">char</span><span class="special">></span></code>.
</p>
<p>
There are a few exceptions, however.
</p>
<p>
<span class="bold"><strong>Arrays:</strong></span>
</p>
<p>
Array arguments are deduced to reference to const types. For example <a href="#ftn.fusion.notes.f0" class="footnote" name="fusion.notes.f0"><sup class="footnote">[14]</sup></a>:
</p>
<pre class="programlisting"><a class="link" href="container/generation/functions/make_list.html" title="make_list"><code class="computeroutput"><span class="identifier">make_list</span></code></a><span class="special">(</span><span class="string">"Donald"</span><span class="special">,</span> <span class="string">"Daisy"</span><span class="special">)</span>
</pre>
<p>
creates a <a class="link" href="container/list.html" title="list"><code class="computeroutput"><span class="identifier">list</span></code></a>
of type
</p>
<pre class="programlisting"><a class="link" href="container/list.html" title="list"><code class="computeroutput"><span class="identifier">list</span></code></a><span class="special"><</span><span class="keyword">const</span> <span class="keyword">char</span> <span class="special">(&)[</span><span class="number">7</span><span class="special">],</span> <span class="keyword">const</span> <span class="keyword">char</span> <span class="special">(&)[</span><span class="number">6</span><span class="special">]></span>
</pre>
<p>
<span class="bold"><strong>Function pointers:</strong></span>
</p>
<p>
Function pointers are deduced to the plain non-reference type (i.e. to plain
function pointer). Example:
</p>
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">f</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span><span class="special">);</span>
<span class="special">...</span>
<a class="link" href="container/generation/functions/make_list.html" title="make_list"><code class="computeroutput"><span class="identifier">make_list</span></code></a><span class="special">(&</span><span class="identifier">f</span><span class="special">);</span>
</pre>
<p>
creates a <a class="link" href="container/list.html" title="list"><code class="computeroutput"><span class="identifier">list</span></code></a>
of type
</p>
<pre class="programlisting"><a class="link" href="container/list.html" title="list"><code class="computeroutput"><span class="identifier">list</span></code></a><span class="special"><</span><span class="keyword">void</span> <span class="special">(*)(</span><span class="keyword">int</span><span class="special">)></span>
</pre>
<h4>
<a name="fusion.notes.h5"></a>
<span class="phrase"><a name="fusion.notes.reference_wrappers"></a></span><a class="link" href="notes.html#fusion.notes.reference_wrappers">Reference
Wrappers</a>
</h4>
<p>
Fusion's generation functions (e.g. <a class="link" href="container/generation/functions/make_list.html" title="make_list"><code class="computeroutput"><span class="identifier">make_list</span></code></a>) by default stores the element
types as plain non-reference types. Example:
</p>
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">foo</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">A</span><span class="special">&</span> <span class="identifier">a</span><span class="special">,</span> <span class="identifier">B</span><span class="special">&</span> <span class="identifier">b</span><span class="special">)</span> <span class="special">{</span>
<span class="special">...</span>
<a class="link" href="container/generation/functions/make_list.html" title="make_list"><code class="computeroutput"><span class="identifier">make_list</span></code></a><span class="special">(</span><span class="identifier">a</span><span class="special">,</span> <span class="identifier">b</span><span class="special">)</span>
</pre>
<p>
creates a <a class="link" href="container/list.html" title="list"><code class="computeroutput"><span class="identifier">list</span></code></a>
of type
</p>
<pre class="programlisting"><a class="link" href="container/list.html" title="list"><code class="computeroutput"><span class="identifier">list</span></code></a><span class="special"><</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">></span>
</pre>
<p>
Sometimes the plain non-reference type is not desired. You can use <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">ref</span></code>
and <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">cref</span></code> to store references or const references
(respectively) instead. The mechanism does not compromise const correctness
since a const object wrapped with ref results in a tuple element with const
reference type (see the fifth code line below). Examples:
</p>
<p>
For example:
</p>
<pre class="programlisting"><span class="identifier">A</span> <span class="identifier">a</span><span class="special">;</span> <span class="identifier">B</span> <span class="identifier">b</span><span class="special">;</span> <span class="keyword">const</span> <span class="identifier">A</span> <span class="identifier">ca</span> <span class="special">=</span> <span class="identifier">a</span><span class="special">;</span>
<a class="link" href="container/generation/functions/make_list.html" title="make_list"><code class="computeroutput"><span class="identifier">make_list</span></code></a><span class="special">(</span><span class="identifier">cref</span><span class="special">(</span><span class="identifier">a</span><span class="special">),</span> <span class="identifier">b</span><span class="special">);</span> <span class="comment">// creates list<const A&, B></span>
<a class="link" href="container/generation/functions/make_list.html" title="make_list"><code class="computeroutput"><span class="identifier">make_list</span></code></a><span class="special">(</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">a</span><span class="special">),</span> <span class="identifier">b</span><span class="special">);</span> <span class="comment">// creates list<A&, B></span>
<a class="link" href="container/generation/functions/make_list.html" title="make_list"><code class="computeroutput"><span class="identifier">make_list</span></code></a><span class="special">(</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">a</span><span class="special">),</span> <span class="identifier">cref</span><span class="special">(</span><span class="identifier">b</span><span class="special">));</span> <span class="comment">// creates list<A&, const B&></span>
<a class="link" href="container/generation/functions/make_list.html" title="make_list"><code class="computeroutput"><span class="identifier">make_list</span></code></a><span class="special">(</span><span class="identifier">cref</span><span class="special">(</span><span class="identifier">ca</span><span class="special">));</span> <span class="comment">// creates list<const A&></span>
<a class="link" href="container/generation/functions/make_list.html" title="make_list"><code class="computeroutput"><span class="identifier">make_list</span></code></a><span class="special">(</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">ca</span><span class="special">));</span> <span class="comment">// creates list<const A&></span>
</pre>
<p>
See <a href="http://www.boost.org/doc/html/ref.html" target="_top">Boost.Ref</a> for
details.
</p>
<p>
Since C++11, the standard reference wrappers (<code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">ref</span></code> and
<code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">cref</span></code>) work as well.
</p>
<h4>
<a name="fusion.notes.h6"></a>
<span class="phrase"><a name="fusion.notes.adt_attribute_proxy"></a></span><a class="link" href="notes.html#fusion.notes.adt_attribute_proxy">adt_attribute_proxy</a>
</h4>
<p>
To adapt arbitrary data types that do not allow direct access to their members,
but allow indirect access via expressions (such as invocations of get- and
set-methods), fusion's <code class="literal">BOOST_FUSION_ADAPT_<span class="emphasis"><em>xxx</em></span>ADT<span class="emphasis"><em>xxx</em></span></code>-family
(e.g. <a class="link" href="adapted/adapt_adt.html" title="BOOST_FUSION_ADAPT_ADT"><code class="computeroutput"><span class="identifier">BOOST_FUSION_ADAPT_ADT</span></code></a>)
may be used. To bypass the restriction of not having actual lvalues that represent
the elements of the fusion sequence, but rather a sequence of paired expressions
that access the elements, the actual return type of fusion's intrinsic sequence
access functions (<a class="link" href="sequence/intrinsic/functions/at.html" title="at"><code class="computeroutput"><span class="identifier">at</span></code></a>, <a class="link" href="sequence/intrinsic/functions/at_c.html" title="at_c"><code class="computeroutput"><span class="identifier">at_c</span></code></a>, <a class="link" href="sequence/intrinsic/functions/at_key.html" title="at_key"><code class="computeroutput"><span class="identifier">at_key</span></code></a>, <a class="link" href="iterator/functions/deref.html" title="deref"><code class="computeroutput"><span class="identifier">deref</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>) is a proxy type, an instance
of <code class="computeroutput"><span class="identifier">adt_attribute_proxy</span></code>, that
encapsulates these expressions.
</p>
<p>
<code class="computeroutput"><span class="identifier">adt_attribute_proxy</span></code> is defined
in the namespace <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></code> and has three template arguments:
</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">namespace</span> <span class="identifier">extension</span>
<span class="special">{</span>
<span class="keyword">template</span><span class="special"><</span>
<span class="keyword">typename</span> <span class="identifier">Type</span>
<span class="special">,</span> <span class="keyword">int</span> <span class="identifier">Index</span>
<span class="special">,</span> <span class="keyword">bool</span> <span class="identifier">Const</span>
<span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">adt_attribute_proxy</span><span class="special">;</span>
<span class="special">}}}</span>
</pre>
<p>
When adapting a class type, <code class="computeroutput"><span class="identifier">adt_attribute_proxy</span></code>
is specialized for every element of the adapted sequence, with <code class="computeroutput"><span class="identifier">Type</span></code> being the class type that is adapted,
<code class="computeroutput"><span class="identifier">Index</span></code> the 0-based indices of
the elements, and <code class="computeroutput"><span class="identifier">Const</span></code> both
<code class="computeroutput"><span class="keyword">true</span></code> and <code class="computeroutput"><span class="keyword">false</span></code>.
The return type of fusion's intrinsic sequence access functions for the <span class="emphasis"><em>N</em></span>th
element of an adapted class type <code class="computeroutput"><span class="identifier">type_name</span></code>
is <code class="literal">adt_attribute_proxy<type_name, <span class="emphasis"><em>N</em></span>, <span class="emphasis"><em>Const</em></span>></code>,
with <code class="literal"><span class="emphasis"><em>Const</em></span></code> being <code class="computeroutput"><span class="keyword">true</span></code>
for constant instances of <code class="computeroutput"><span class="identifier">type_name</span></code>
and <code class="computeroutput"><span class="keyword">false</span></code> for non-constant ones.
</p>
<div class="variablelist">
<p class="title"><b>Notation</b></p>
<dl class="variablelist">
<dt><span class="term"><code class="computeroutput"><span class="identifier">type_name</span></code></span></dt>
<dd><p>
The type to be adapted, with M attributes
</p></dd>
<dt><span class="term"><code class="computeroutput"><span class="identifier">inst</span></code></span></dt>
<dd><p>
Object of type <code class="computeroutput"><span class="identifier">type_name</span></code>
</p></dd>
<dt><span class="term"><code class="computeroutput"><span class="identifier">const_inst</span></code></span></dt>
<dd><p>
Object of type <code class="computeroutput"><span class="identifier">type_name</span> <span class="keyword">const</span></code>
</p></dd>
<dt><span class="term"><code class="literal">(attribute_type<span class="emphasis"><em>N</em></span>, attribute_const_type<span class="emphasis"><em>N</em></span>,
get_expr<span class="emphasis"><em>N</em></span>, set_expr<span class="emphasis"><em>N</em></span>)</code></span></dt>
<dd><p>
Attribute descriptor of the <span class="emphasis"><em>N</em></span>th attribute of <code class="computeroutput"><span class="identifier">type_name</span></code> as passed to the adaption
macro, 0≤<span class="emphasis"><em>N</em></span><M
</p></dd>
<dt><span class="term"><code class="literal">proxy_type<span class="emphasis"><em>N</em></span></code></span></dt>
<dd><p>
<code class="literal">adt_attribute_proxy<type_name, <span class="emphasis"><em>N</em></span>, <code class="computeroutput"><span class="keyword">false</span></code>></code> with <span class="emphasis"><em>N</em></span>
being an integral constant, 0≤<span class="emphasis"><em>N</em></span><M
</p></dd>
<dt><span class="term"><code class="literal">const_proxy_type<span class="emphasis"><em>N</em></span></code></span></dt>
<dd><p>
<code class="literal">adt_attribute_proxy<type_name, <span class="emphasis"><em>N</em></span>, <code class="computeroutput"><span class="keyword">true</span></code>></code> with <span class="emphasis"><em>N</em></span>
being an integral constant, 0≤<span class="emphasis"><em>N</em></span><M
</p></dd>
<dt><span class="term"><code class="literal">proxy<span class="emphasis"><em>N</em></span></code></span></dt>
<dd><p>
Object of type <code class="literal">proxy_type<span class="emphasis"><em>N</em></span></code>
</p></dd>
<dt><span class="term"><code class="literal">const_proxy<span class="emphasis"><em>N</em></span></code></span></dt>
<dd><p>
Object of type <code class="literal">const_proxy_type<span class="emphasis"><em>N</em></span></code>
</p></dd>
</dl>
</div>
<p>
<span class="bold"><strong>Expression Semantics</strong></span>
</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Expression
</p>
</th>
<th>
<p>
Semantics
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
<code class="literal">proxy_type<span class="emphasis"><em>N</em></span>(inst)</code>
</p>
</td>
<td>
<p>
Creates an instance of <code class="literal">proxy_type<span class="emphasis"><em>N</em></span></code>
with underlying object <code class="computeroutput"><span class="identifier">inst</span></code>
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="literal">const_proxy_type<span class="emphasis"><em>N</em></span>(const_inst)</code>
</p>
</td>
<td>
<p>
Creates an instance of <code class="literal">const_proxy_type<span class="emphasis"><em>N</em></span></code>
with underlying object <code class="computeroutput"><span class="identifier">const_inst</span></code>
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="literal">proxy_type<span class="emphasis"><em>N</em></span>::type</code>
</p>
</td>
<td>
<p>
Another name for <code class="literal">attribute_type<span class="emphasis"><em>N</em></span></code>
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="literal">const_proxy_type<span class="emphasis"><em>N</em></span>::type</code>
</p>
</td>
<td>
<p>
Another name for <code class="literal">const_attribute_type<span class="emphasis"><em>N</em></span></code>
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="literal">proxy<span class="emphasis"><em>N</em></span>=t</code>
</p>
</td>
<td>
<p>
Invokes <code class="literal">set_expr<span class="emphasis"><em>N</em></span></code>, with
<code class="computeroutput"><span class="identifier">t</span></code> being an arbitrary
object. <code class="literal">set_expr<span class="emphasis"><em>N</em></span></code> may access
the variables named <code class="computeroutput"><span class="identifier">obj</span></code>
of type <code class="computeroutput"><span class="identifier">type_name</span><span class="special">&</span></code>, which represent the corresponding
instance of <code class="computeroutput"><span class="identifier">type_name</span></code>,
and <code class="computeroutput"><span class="identifier">val</span></code> of an arbitrary
const-qualified reference template type parameter <code class="computeroutput"><span class="identifier">Val</span></code>,
which represents <code class="computeroutput"><span class="identifier">t</span></code>.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="literal">proxy<span class="emphasis"><em>N</em></span>.get()</code>
</p>
</td>
<td>
<p>
Invokes <code class="literal">get_expr<span class="emphasis"><em>N</em></span></code> and forwards
its return value. <code class="literal">get_expr<span class="emphasis"><em>N</em></span></code>
may access the variable named <code class="computeroutput"><span class="identifier">obj</span></code>
of type <code class="computeroutput"><span class="identifier">type_name</span><span class="special">&</span></code> which represents the underlying
instance of <code class="computeroutput"><span class="identifier">type_name</span></code>.
<code class="literal">attribute_type<span class="emphasis"><em>N</em></span></code> may specify
the type that <code class="literal">get_expr<span class="emphasis"><em>N</em></span></code> denotes
to.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="literal">const_proxy<span class="emphasis"><em>N</em></span>.get()</code>
</p>
</td>
<td>
<p>
Invokes <code class="literal">get_expr<span class="emphasis"><em>N</em></span></code> and forwards
its return value. <code class="literal">get_expr<span class="emphasis"><em>N</em></span></code>
may access the variable named <code class="computeroutput"><span class="identifier">obj</span></code>
of type <code class="computeroutput"><span class="identifier">type_name</span> <span class="keyword">const</span><span class="special">&</span></code>
which represents the underlying instance of <code class="computeroutput"><span class="identifier">type_name</span></code>.
<code class="literal">attribute_const_type<span class="emphasis"><em>N</em></span></code> may
specify the type that <code class="literal">get_expr<span class="emphasis"><em>N</em></span></code>
denotes to.
</p>
</td>
</tr>
</tbody>
</table></div>
<p>
Additionally, <code class="literal">proxy_type<span class="emphasis"><em>N</em></span></code> and <code class="literal">const_proxy_type<span class="emphasis"><em>N</em></span></code>
are copy constructible, copy assignable and implicitly convertible to <code class="literal">proxy_type<span class="emphasis"><em>N</em></span>::type</code>
or <code class="literal">const_proxy_type<span class="emphasis"><em>N</em></span>::type</code>.
</p>
<div class="tip"><table border="0" summary="Tip">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="../../../../../doc/src/images/tip.png"></td>
<th align="left">Tip</th>
</tr>
<tr><td align="left" valign="top"><p>
To avoid the pitfalls of the proxy type, an arbitrary class type may also
be adapted directly using fusion's <a class="link" href="extension.html" title="Extension">intrinsic
extension mechanism</a>.
</p></td></tr>
</table></div>
<div class="footnotes">
<br><hr style="width:100; text-align:left;margin-left: 0">
<div id="ftn.fusion.notes.f0" class="footnote"><p><a href="#fusion.notes.f0" class="para"><sup class="para">[14] </sup></a>
Note that the type of a string literal is an array of const characters, not
<code class="computeroutput"><span class="keyword">const</span> <span class="keyword">char</span><span class="special">*</span></code>. To get <a class="link" href="container/generation/functions/make_list.html" title="make_list"><code class="computeroutput"><span class="identifier">make_list</span></code></a> to create a <a class="link" href="container/list.html" title="list"><code class="computeroutput"><span class="identifier">list</span></code></a> with an element of a non-const
array type one must use the <code class="computeroutput"><span class="identifier">ref</span></code>
wrapper (see <a class="link" href="notes.html#fusion.notes.reference_wrappers"><code class="computeroutput"><span class="identifier">Reference</span> <span class="identifier">Wrappers</span></code></a>).
</p></div>
</div>
</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 © 2001-2006, 2011, 2012 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>
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