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25 | <div class="section"> | |
26 | <div class="titlepage"><div><div><h2 class="title" style="clear: both"> | |
27 | <a name="boost_optional.quick_start"></a><a class="link" href="quick_start.html" title="Quick Start">Quick Start</a> | |
28 | </h2></div></div></div> | |
29 | <div class="toc"><dl class="toc"> | |
30 | <dt><span class="section"><a href="quick_start.html#boost_optional.quick_start.optional_return_values">Optional | |
31 | return values</a></span></dt> | |
32 | <dt><span class="section"><a href="quick_start/optional_automatic_variables.html">Optional | |
33 | automatic variables</a></span></dt> | |
34 | <dt><span class="section"><a href="quick_start/optional_data_members.html">Optional | |
35 | data members</a></span></dt> | |
36 | <dt><span class="section"><a href="quick_start/bypassing_unnecessary_default_construction.html">Bypassing | |
37 | unnecessary default construction</a></span></dt> | |
38 | <dt><span class="section"><a href="quick_start/storage_in_containers.html">Storage | |
39 | in containers</a></span></dt> | |
40 | </dl></div> | |
41 | <div class="section"> | |
42 | <div class="titlepage"><div><div><h3 class="title"> | |
43 | <a name="boost_optional.quick_start.optional_return_values"></a><a class="link" href="quick_start.html#boost_optional.quick_start.optional_return_values" title="Optional return values">Optional | |
44 | return values</a> | |
45 | </h3></div></div></div> | |
46 | <p> | |
47 | Let's write and use a converter function that converts a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code> | |
48 | to an <code class="computeroutput"><span class="keyword">int</span></code>. It is possible that | |
49 | for a given string (e.g. <code class="computeroutput"><span class="string">"cat"</span></code>) | |
50 | there exists no value of type <code class="computeroutput"><span class="keyword">int</span></code> | |
51 | capable of representing the conversion result. We do not consider such situation | |
52 | an error. We expect that the converter can be used only to check if the conversion | |
53 | is possible. A natural signature for this function can be: | |
54 | </p> | |
55 | <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">optional</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> | |
56 | <span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">convert</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&</span> <span class="identifier">text</span><span class="special">);</span> | |
57 | </pre> | |
58 | <p> | |
59 | All necessary functionality can be included with one header <code class="computeroutput"><span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">optional</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code>. | |
60 | The above function signature means that the function can either return a | |
61 | value of type <code class="computeroutput"><span class="keyword">int</span></code> or a flag | |
62 | indicating that no value of <code class="computeroutput"><span class="keyword">int</span></code> | |
63 | is available. This does not indicate an error. It is like one additional | |
64 | value of <code class="computeroutput"><span class="keyword">int</span></code>. This is how we | |
65 | can use our function: | |
66 | </p> | |
67 | <pre class="programlisting"><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&</span> <span class="identifier">text</span> <span class="special">=</span> <span class="comment">/*... */</span><span class="special">;</span> | |
68 | <span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">oi</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">text</span><span class="special">);</span> <span class="comment">// move-construct</span> | |
69 | <span class="keyword">if</span> <span class="special">(</span><span class="identifier">oi</span><span class="special">)</span> <span class="comment">// contextual conversion to bool</span> | |
70 | <span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="special">*</span><span class="identifier">oi</span><span class="special">;</span> <span class="comment">// operator*</span> | |
71 | </pre> | |
72 | <p> | |
73 | In order to test if <code class="computeroutput"><span class="identifier">optional</span></code> | |
74 | contains a value, we use the contextual conversion to type <code class="computeroutput"><span class="keyword">bool</span></code>. Because of this we can combine the initialization | |
75 | of the optional object and the test into one instruction: | |
76 | </p> | |
77 | <pre class="programlisting"><span class="keyword">if</span> <span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">oi</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">text</span><span class="special">))</span> | |
78 | <span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="special">*</span><span class="identifier">oi</span><span class="special">;</span> | |
79 | </pre> | |
80 | <p> | |
81 | We extract the contained value with <code class="computeroutput"><span class="keyword">operator</span><span class="special">*</span></code> (and with <code class="computeroutput"><span class="keyword">operator</span><span class="special">-></span></code> where it makes sense). An attempt to | |
82 | extract the contained value of an uninitialized optional object is an <span class="emphasis"><em>undefined | |
83 | behaviour</em></span> (UB). This implementation guards the call with <code class="computeroutput"><span class="identifier">BOOST_ASSERT</span></code>. Therefore you should be sure | |
84 | that the contained value is there before extracting. For instance, the following | |
85 | code is reasonably UB-safe: | |
86 | </p> | |
87 | <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="special">*</span><span class="identifier">convert</span><span class="special">(</span><span class="string">"100"</span><span class="special">);</span> | |
88 | </pre> | |
89 | <p> | |
90 | This is because we know that string value <code class="computeroutput"><span class="string">"100"</span></code> | |
91 | converts to a valid value of <code class="computeroutput"><span class="keyword">int</span></code>. | |
92 | If you do not like this potential UB, you can use an alternative way of extracting | |
93 | the contained value: | |
94 | </p> | |
95 | <pre class="programlisting"><span class="keyword">try</span> <span class="special">{</span> | |
96 | <span class="keyword">int</span> <span class="identifier">j</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">text</span><span class="special">).</span><span class="identifier">value</span><span class="special">();</span> | |
97 | <span class="special">}</span> | |
98 | <span class="keyword">catch</span> <span class="special">(</span><span class="keyword">const</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">bad_optional_access</span><span class="special">&)</span> <span class="special">{</span> | |
99 | <span class="comment">// deal with it</span> | |
100 | <span class="special">}</span> | |
101 | </pre> | |
102 | <p> | |
103 | This version throws an exception upon an attempt to access a non-existent | |
104 | contained value. If your way of dealing with the missing value is to use | |
105 | some default, like <code class="computeroutput"><span class="number">0</span></code>, there exists | |
106 | a yet another alternative: | |
107 | </p> | |
108 | <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">k</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">text</span><span class="special">).</span><span class="identifier">value_or</span><span class="special">(</span><span class="number">0</span><span class="special">);</span> | |
109 | </pre> | |
110 | <p> | |
111 | This uses the <code class="computeroutput"><span class="identifier">atoi</span></code>-like approach | |
112 | to conversions: if <code class="computeroutput"><span class="identifier">text</span></code> does | |
113 | not represent an integral number just return <code class="computeroutput"><span class="number">0</span></code>. | |
114 | Finally, you can provide a callback to be called when trying to access the | |
115 | contained value fails: | |
116 | </p> | |
117 | <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">fallback_to_default</span><span class="special">()</span> | |
118 | <span class="special">{</span> | |
119 | <span class="identifier">cerr</span> <span class="special"><<</span> <span class="string">"could not convert; using -1 instead"</span> <span class="special"><<</span> <span class="identifier">endl</span><span class="special">;</span> | |
120 | <span class="keyword">return</span> <span class="special">-</span><span class="number">1</span><span class="special">;</span> | |
121 | <span class="special">}</span> | |
122 | ||
123 | <span class="keyword">int</span> <span class="identifier">l</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">text</span><span class="special">).</span><span class="identifier">value_or_eval</span><span class="special">(</span><span class="identifier">fallback_to_default</span><span class="special">);</span> | |
124 | </pre> | |
125 | <p> | |
126 | This will call the provided callback and return whatever the callback returns. | |
127 | The callback can have side effects: they will only be observed when the optional | |
128 | object does not contain a value. | |
129 | </p> | |
130 | <p> | |
131 | Now, let's consider how function <code class="computeroutput"><span class="identifier">convert</span></code> | |
132 | can be implemented. | |
133 | </p> | |
134 | <pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">convert</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&</span> <span class="identifier">text</span><span class="special">)</span> | |
135 | <span class="special">{</span> | |
136 | <span class="identifier">std</span><span class="special">::</span><span class="identifier">stringstream</span> <span class="identifier">s</span><span class="special">(</span><span class="identifier">text</span><span class="special">);</span> | |
137 | <span class="keyword">int</span> <span class="identifier">i</span><span class="special">;</span> | |
138 | <span class="keyword">if</span> <span class="special">((</span><span class="identifier">s</span> <span class="special">>></span> <span class="identifier">i</span><span class="special">)</span> <span class="special">&&</span> <span class="identifier">s</span><span class="special">.</span><span class="identifier">get</span><span class="special">()</span> <span class="special">==</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">char_traits</span><span class="special"><</span><span class="keyword">char</span><span class="special">>::</span><span class="identifier">eof</span><span class="special">())</span> | |
139 | <span class="keyword">return</span> <span class="identifier">i</span><span class="special">;</span> | |
140 | <span class="keyword">else</span> | |
141 | <span class="keyword">return</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span><span class="special">;</span> | |
142 | <span class="special">}</span> | |
143 | </pre> | |
144 | <p> | |
145 | Observe the two return statements. <code class="computeroutput"><span class="keyword">return</span> | |
146 | <span class="identifier">i</span></code> uses the converting constructor | |
147 | that can create <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> | |
148 | from <code class="computeroutput"><span class="identifier">T</span></code>. Thus constructed | |
149 | optional object is initialized and its value is a copy of <code class="computeroutput"><span class="identifier">i</span></code>. | |
150 | The other return statement uses another converting constructor from a special | |
151 | tag <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code>. It is used to indicate that we want | |
152 | to create an uninitialized optional object. | |
153 | </p> | |
154 | </div> | |
155 | </div> | |
156 | <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> | |
157 | <td align="left"></td> | |
158 | <td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2016 Andrzej Krzemieński<p> | |
159 | Distributed under the Boost Software License, Version 1.0. (See accompanying | |
160 | 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>) | |
161 | </p> | |
162 | </div></td> | |
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