1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> 4<title>Motivation</title> 5<link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.78.1"> 7<link rel="home" href="../../index.html" title="Chapter 1. Boost.Optional"> 8<link rel="up" href="../../optional/tutorial.html" title="Tutorial"> 9<link rel="prev" href="../../optional/tutorial.html" title="Tutorial"> 10<link rel="next" href="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="../../optional/tutorial.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../optional/tutorial.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="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><h3 class="title"> 27<a name="boost_optional.tutorial.motivation"></a><a class="link" href="motivation.html" title="Motivation">Motivation</a> 28</h3></div></div></div> 29<p> 30 Consider these functions which should return a value but which might not 31 have a value to return: 32 </p> 33<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 34<li class="listitem"> 35 (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> 36 </li> 37<li class="listitem"> 38 (B) <code class="computeroutput"><span class="keyword">char</span> <span class="identifier">get_async_input</span><span class="special">();</span></code> 39 </li> 40<li class="listitem"> 41 (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> 42 </li> 43</ul></div> 44<p> 45 There are different approaches to the issue of not having a value to return. 46 </p> 47<p> 48 A typical approach is to consider the existence of a valid return value as 49 a postcondition, so that if the function cannot compute the value to return, 50 it has either undefined behavior (and can use assert in a debug build) or 51 uses a runtime check and throws an exception if the postcondition is violated. 52 This is a reasonable choice for example, for function (A), because the lack 53 of a proper return value is directly related to an invalid parameter (out 54 of domain argument), so it is appropriate to require the callee to supply 55 only parameters in a valid domain for execution to continue normally. 56 </p> 57<p> 58 However, function (B), because of its asynchronous nature, does not fail 59 just because it can't find a value to return; so it is incorrect to consider 60 such a situation an error and assert or throw an exception. This function 61 must return, and somehow, must tell the callee that it is not returning a 62 meaningful value. 63 </p> 64<p> 65 A similar situation occurs with function (C): it is conceptually an error 66 to ask a <span class="emphasis"><em>null-area</em></span> polygon to return a point inside 67 itself, but in many applications, it is just impractical for performance 68 reasons to treat this as an error (because detecting that the polygon has 69 no area might be too expensive to be required to be tested previously), and 70 either an arbitrary point (typically at infinity) is returned, or some efficient 71 way to tell the callee that there is no such point is used. 72 </p> 73<p> 74 There are various mechanisms to let functions communicate that the returned 75 value is not valid. One such mechanism, which is quite common since it has 76 zero or negligible overhead, is to use a special value which is reserved 77 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>, 78 points at infinity, etc... 79 </p> 80<p> 81 When those values exist, i.e. the return type can hold all meaningful values 82 <span class="emphasis"><em>plus</em></span> the <span class="emphasis"><em>signal</em></span> value, this mechanism 83 is quite appropriate and well known. Unfortunately, there are cases when 84 such values do not exist. In these cases, the usual alternative is either 85 to use a wider type, such as <code class="computeroutput"><span class="keyword">int</span></code> 86 in place of <code class="computeroutput"><span class="keyword">char</span></code>; or a compound 87 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>. 88 </p> 89<p> 90 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 91 result which indicates if the result is meaningful, has the advantage that 92 can be turned into a consistent idiom since the first element of the pair 93 can be whatever the function would conceptually return. For example, the 94 last two functions could have the following interface: 95 </p> 96<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> 97<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> 98</pre> 99<p> 100 These functions use a consistent interface for dealing with possibly nonexistent 101 results: 102 </p> 103<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> 104<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> 105 <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> 106</pre> 107<p> 108 However, not only is this quite a burden syntactically, it is also error 109 prone since the user can easily use the function result (first element of 110 the pair) without ever checking if it has a valid value. 111 </p> 112<p> 113 Clearly, we need a better idiom. 114 </p> 115</div> 116<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 117<td align="left"></td> 118<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 Andrzej Krzemieński<p> 119 Distributed under the Boost Software License, Version 1.0. (See accompanying 120 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>) 121 </p> 122</div></td> 123</tr></table> 124<hr> 125<div class="spirit-nav"> 126<a accesskey="p" href="../../optional/tutorial.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../optional/tutorial.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="design_overview.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> 127</div> 128</body> 129</html> 130
