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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> | |
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