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| <div class="chapter" title="Chapter 1. Boost.Optional"> |
| <div class="titlepage"><div> |
| <div><h2 class="title"> |
| <a name="optional"></a>Chapter 1. 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" title="Legal Notice"> |
| <a name="id553943"></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" title="Motivation"> |
| <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 class="itemizedlist" type="disc"> |
| <li class="listitem"> |
| (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 class="listitem"> |
| (B) <code class="computeroutput"><span class="keyword">char</span> <span class="identifier">get_async_input</span><span class="special">();</span></code> |
| </li> |
| <li class="listitem"> |
| (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> |
| </div> |
| <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> |
| <td align="left"><p><small>Last revised: November 20, 2009 at 10:24:28 GMT</small></p></td> |
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