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<title>Chapter1.Boost.Optional</title>
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<div class="titlepage"><div>
<div><h2 class="title">
<a name="optional"></a>Chapter1.Boost.Optional</h2></div>
<div><div class="author"><h3 class="author">
<span class="firstname">Fernando Luis</span> <span class="surname">Cacciola Carballal</span>
</h3></div></div>
<div><p class="copyright">Copyright 2003 -2007 Fernando Luis Cacciola Carballal</p></div>
<div><div class="legalnotice">
<a name="id2625900"></a><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>)
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<div class="toc">
<p><b>Table of Contents</b></p>
<dl>
<dt><span class="section"><a href="index.html#optional.motivation">Motivation</a></span></dt>
<dt><span class="section"><a href="boost_optional/development.html">Development</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="boost_optional/development.html#boost_optional.development.the_models">The models</a></span></dt>
<dt><span class="section"><a href="boost_optional/development.html#boost_optional.development.the_semantics">The semantics</a></span></dt>
<dt><span class="section"><a href="boost_optional/development.html#boost_optional.development.the_interface">The Interface</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="boost_optional/synopsis.html">Synopsis</a></span></dt>
<dt><span class="section"><a href="boost_optional/detailed_semantics.html">Detailed Semantics</a></span></dt>
<dt><span class="section"><a href="boost_optional/examples.html">Examples</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="boost_optional/examples.html#boost_optional.examples.optional_return_values">Optional
return values</a></span></dt>
<dt><span class="section"><a href="boost_optional/examples.html#boost_optional.examples.optional_local_variables">Optional
local variables</a></span></dt>
<dt><span class="section"><a href="boost_optional/examples.html#boost_optional.examples.optional_data_members">Optional
data members</a></span></dt>
<dt><span class="section"><a href="boost_optional/examples.html#boost_optional.examples.bypassing_expensive_unnecessary_default_construction">Bypassing
expensive unnecessary default construction</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="boost_optional/optional_references.html">Optional references</a></span></dt>
<dt><span class="section"><a href="boost_optional/rebinding_semantics_for_assignment_of_optional_references.html">Rebinding
semantics for assignment of optional references</a></span></dt>
<dt><span class="section"><a href="boost_optional/in_place_factories.html">In-Place Factories</a></span></dt>
<dt><span class="section"><a href="boost_optional/a_note_about_optional_bool_.html">A note about
optional<bool></a></span></dt>
<dt><span class="section"><a href="boost_optional/exception_safety_guarantees.html">Exception Safety
Guarantees</a></span></dt>
<dt><span class="section"><a href="boost_optional/type_requirements.html">Type requirements</a></span></dt>
<dt><span class="section"><a href="boost_optional/implementation_notes.html">Implementation Notes</a></span></dt>
<dt><span class="section"><a href="boost_optional/dependencies_and_portability.html">Dependencies
and Portability</a></span></dt>
<dt><span class="section"><a href="boost_optional/acknowledgments.html">Acknowledgments</a></span></dt>
</dl>
</div>
<div class="section" lang="en">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="optional.motivation"></a><a class="link" href="index.html#optional.motivation" title="Motivation">Motivation</a>
</h2></div></div></div>
<p>
Consider these functions which should return a value but which might not have
a value to return:
</p>
<div class="itemizedlist"><ul type="disc">
<li>
(A) <code class="computeroutput"><span class="keyword">double</span> <span class="identifier">sqrt</span><span class="special">(</span><span class="keyword">double</span> <span class="identifier">n</span>
<span class="special">);</span></code>
</li>
<li>
(B) <code class="computeroutput"><span class="keyword">char</span> <span class="identifier">get_async_input</span><span class="special">();</span></code>
</li>
<li>
(C) <code class="computeroutput"><span class="identifier">point</span> <span class="identifier">polygon</span><span class="special">::</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span></code>
</li>
</ul></div>
<p>
There are different approaches to the issue of not having a value to return.
</p>
<p>
A typical approach is to consider the existence of a valid return value as
a postcondition, so that if the function cannot compute the value to return,
it has either undefined behavior (and can use assert in a debug build) or uses
a runtime check and throws an exception if the postcondition is violated. This
is a reasonable choice for example, for function (A), because the lack of a
proper return value is directly related to an invalid parameter (out of domain
argument), so it is appropriate to require the callee to supply only parameters
in a valid domain for execution to continue normally.
</p>
<p>
However, function (B), because of its asynchronous nature, does not fail just
because it can't find a value to return; so it is incorrect to consider such
a situation an error and assert or throw an exception. This function must return,
and somehow, must tell the callee that it is not returning a meaningful value.
</p>
<p>
A similar situation occurs with function (C): it is conceptually an error to
ask a <span class="emphasis"><em>null-area</em></span> polygon to return a point inside itself,
but in many applications, it is just impractical for performance reasons to
treat this as an error (because detecting that the polygon has no area might
be too expensive to be required to be tested previously), and either an arbitrary
point (typically at infinity) is returned, or some efficient way to tell the
callee that there is no such point is used.
</p>
<p>
There are various mechanisms to let functions communicate that the returned
value is not valid. One such mechanism, which is quite common since it has
zero or negligible overhead, is to use a special value which is reserved to
communicate this. Classical examples of such special values are <code class="computeroutput"><span class="identifier">EOF</span></code>, <code class="computeroutput"><span class="identifier">string</span><span class="special">::</span><span class="identifier">npos</span></code>, points
at infinity, etc...
</p>
<p>
When those values exist, i.e. the return type can hold all meaningful values
<span class="emphasis"><em>plus</em></span> the <span class="emphasis"><em>signal</em></span> value, this mechanism
is quite appropriate and well known. Unfortunately, there are cases when such
values do not exist. In these cases, the usual alternative is either to use
a wider type, such as <code class="computeroutput"><span class="keyword">int</span></code> in place
of <code class="computeroutput"><span class="keyword">char</span></code>; or a compound type, such
as <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span></code>.
</p>
<p>
Returning a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span></code>, thus attaching a boolean flag to the result
which indicates if the result is meaningful, has the advantage that can be
turned into a consistent idiom since the first element of the pair can be whatever
the function would conceptually return. For example, the last two functions
could have the following interface:
</p>
<pre class="programlisting">
<span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="keyword">char</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">get_async_input</span><span class="special">();</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">polygon</span><span class="special">::</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span>
</pre>
<p>
These functions use a consistent interface for dealing with possibly inexistent
results:
</p>
<pre class="programlisting">
<span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">p</span> <span class="special">=</span> <span class="identifier">poly</span><span class="special">.</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span>
<span class="keyword">if</span> <span class="special">(</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">second</span> <span class="special">)</span>
<span class="identifier">flood_fill</span><span class="special">(</span><span class="identifier">p</span><span class="special">.</span><span class="identifier">first</span><span class="special">);</span>
</pre>
<p>
However, not only is this quite a burden syntactically, it is also error prone
since the user can easily use the function result (first element of the pair)
without ever checking if it has a valid value.
</p>
<p>
Clearly, we need a better idiom.
</p>
</div>
<p>
</p>
<a name="optional_refassign"></a><p>
</p>
<a name="optional_in_place_factories"></a><p>
</p>
<a name="optional_implementation_notes"></a><p>
</p>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"><p><small>Last revised: March 15, 2008 at 13:58:42 GMT</small></p></td>
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