1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Tutorial</title> 5<link rel="stylesheet" href="../../../../../doc/src/boostbook.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="../index.html" title="Boost.Optional"> 8<link rel="up" href="../index.html" title="Boost.Optional"> 9<link rel="prev" href="../boost_optional/quick_start/storage_in_containers.html" title="Storage in containers"> 10<link rel="next" href="../boost_optional/tutorial/design_overview.html" title="Design Overview"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr> 14<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../boost.png"></td> 15<td align="center"><a href="../../../../../index.html">Home</a></td> 16<td align="center"><a href="../../../../../libs/libraries.htm">Libraries</a></td> 17<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 18<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 19<td align="center"><a href="../../../../../more/index.htm">More</a></td> 20</tr></table> 21<hr> 22<div class="spirit-nav"> 23<a accesskey="p" href="../boost_optional/quick_start/storage_in_containers.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.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="../boost_optional/tutorial/design_overview.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> 24</div> 25<div class="section"> 26<div class="titlepage"><div><div><h2 class="title" style="clear: both"> 27<a name="optional.tutorial"></a><a class="link" href="tutorial.html" title="Tutorial">Tutorial</a> 28</h2></div></div></div> 29<div class="toc"><dl class="toc"> 30<dt><span class="section"><a href="tutorial.html#boost_optional.tutorial.motivation">Motivation</a></span></dt> 31<dt><span class="section"><a href="../boost_optional/tutorial/design_overview.html">Design Overview</a></span></dt> 32<dt><span class="section"><a href="../boost_optional/tutorial/when_to_use_optional.html">When to 33 use Optional</a></span></dt> 34<dt><span class="section"><a href="../boost_optional/tutorial/relational_operators.html">Relational 35 operators</a></span></dt> 36<dt><span class="section"><a href="../boost_optional/tutorial/io_operators.html">IO operators</a></span></dt> 37<dt><span class="section"><a href="../boost_optional/tutorial/optional_references.html">Optional 38 references</a></span></dt> 39<dt><span class="section"><a href="../boost_optional/tutorial/in_place_factories.html">In-Place 40 Factories</a></span></dt> 41<dt><span class="section"><a href="../boost_optional/tutorial/gotchas.html">Gotchas</a></span></dt> 42<dt><span class="section"><a href="../boost_optional/tutorial/exception_safety_guarantees.html">Exception 43 Safety Guarantees</a></span></dt> 44<dt><span class="section"><a href="../boost_optional/tutorial/type_requirements.html">Type requirements</a></span></dt> 45<dt><span class="section"><a href="../boost_optional/tutorial/performance_considerations.html">Performance 46 considerations</a></span></dt> 47</dl></div> 48<div class="section"> 49<div class="titlepage"><div><div><h3 class="title"> 50<a name="boost_optional.tutorial.motivation"></a><a class="link" href="tutorial.html#boost_optional.tutorial.motivation" title="Motivation">Motivation</a> 51</h3></div></div></div> 52<p> 53 Consider these functions which should return a value but which might not 54 have a value to return: 55 </p> 56<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 57<li class="listitem"> 58 (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> 59 </li> 60<li class="listitem"> 61 (B) <code class="computeroutput"><span class="keyword">char</span> <span class="identifier">get_async_input</span><span class="special">();</span></code> 62 </li> 63<li class="listitem"> 64 (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> 65 </li> 66</ul></div> 67<p> 68 There are different approaches to the issue of not having a value to return. 69 </p> 70<p> 71 A typical approach is to consider the existence of a valid return value as 72 a postcondition, so that if the function cannot compute the value to return, 73 it has either undefined behavior (and can use assert in a debug build) or 74 uses a runtime check and throws an exception if the postcondition is violated. 75 This is a reasonable choice for example, for function (A), because the lack 76 of a proper return value is directly related to an invalid parameter (out 77 of domain argument), so it is appropriate to require the callee to supply 78 only parameters in a valid domain for execution to continue normally. 79 </p> 80<p> 81 However, function (B), because of its asynchronous nature, does not fail 82 just because it can't find a value to return; so it is incorrect to consider 83 such a situation an error and assert or throw an exception. This function 84 must return, and somehow, must tell the callee that it is not returning a 85 meaningful value. 86 </p> 87<p> 88 A similar situation occurs with function (C): it is conceptually an error 89 to ask a <span class="emphasis"><em>null-area</em></span> polygon to return a point inside 90 itself, but in many applications, it is just impractical for performance 91 reasons to treat this as an error (because detecting that the polygon has 92 no area might be too expensive to be required to be tested previously), and 93 either an arbitrary point (typically at infinity) is returned, or some efficient 94 way to tell the callee that there is no such point is used. 95 </p> 96<p> 97 There are various mechanisms to let functions communicate that the returned 98 value is not valid. One such mechanism, which is quite common since it has 99 zero or negligible overhead, is to use a special value which is reserved 100 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>, 101 points at infinity, etc... 102 </p> 103<p> 104 When those values exist, i.e. the return type can hold all meaningful values 105 <span class="emphasis"><em>plus</em></span> the <span class="emphasis"><em>signal</em></span> value, this mechanism 106 is quite appropriate and well known. Unfortunately, there are cases when 107 such values do not exist. In these cases, the usual alternative is either 108 to use a wider type, such as <code class="computeroutput"><span class="keyword">int</span></code> 109 in place of <code class="computeroutput"><span class="keyword">char</span></code>; or a compound 110 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>. 111 </p> 112<p> 113 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 114 result which indicates if the result is meaningful, has the advantage that 115 can be turned into a consistent idiom since the first element of the pair 116 can be whatever the function would conceptually return. For example, the 117 last two functions could have the following interface: 118 </p> 119<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> 120<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> 121</pre> 122<p> 123 These functions use a consistent interface for dealing with possibly nonexistent 124 results: 125 </p> 126<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> 127<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> 128 <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> 129</pre> 130<p> 131 However, not only is this quite a burden syntactically, it is also error 132 prone since the user can easily use the function result (first element of 133 the pair) without ever checking if it has a valid value. 134 </p> 135<p> 136 Clearly, we need a better idiom. 137 </p> 138</div> 139</div> 140<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 141<td align="left"></td> 142<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2018 Andrzej Krzemieński<p> 143 Distributed under the Boost Software License, Version 1.0. (See accompanying 144 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>) 145 </p> 146</div></td> 147</tr></table> 148<hr> 149<div class="spirit-nav"> 150<a accesskey="p" href="../boost_optional/quick_start/storage_in_containers.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.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="../boost_optional/tutorial/design_overview.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> 151</div> 152</body> 153</html> 154
