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25 | <div class="section"> | |
26 | <div class="titlepage"><div><div><h3 class="title"> | |
27 | <a name="boost_optional.quick_start.optional_return_values"></a><a class="link" href="optional_return_values.html" title="Optional return values">Optional | |
28 | return values</a> | |
29 | </h3></div></div></div> | |
30 | <p> | |
31 | Let's write and use a converter function that converts an a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code> | |
32 | to an <code class="computeroutput"><span class="keyword">int</span></code>. It is possible that | |
33 | for a given string (e.g. <code class="computeroutput"><span class="string">"cat"</span></code>) | |
34 | there exist no value of type <code class="computeroutput"><span class="keyword">int</span></code> | |
35 | capable of representing the conversion result. We do not consider such situation | |
36 | an error. We expect that the converter can be used only to check if the conversion | |
37 | is possible. A natural signature for this function can be: | |
38 | </p> | |
39 | <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> | |
40 | <span class="identifier">boost</span><span class="special">::</span><span class="identifier">optionl</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> | |
41 | </pre> | |
42 | <p> | |
43 | 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>. | |
44 | The above function signature means that the function can either return a | |
45 | value of type <code class="computeroutput"><span class="keyword">int</span></code> or a flag | |
46 | indicating that no value of <code class="computeroutput"><span class="keyword">int</span></code> | |
47 | is available. This does not indicate an error. It is like one additional | |
48 | value of <code class="computeroutput"><span class="keyword">int</span></code>. This is how we | |
49 | can use our function: | |
50 | </p> | |
51 | <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> | |
52 | <span class="identifier">boost</span><span class="special">::</span><span class="identifier">optionl</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> | |
53 | <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> | |
54 | <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> | |
55 | </pre> | |
56 | <p> | |
57 | In order to test if <code class="computeroutput"><span class="identifier">optional</span></code> | |
58 | 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 | |
59 | of the optional object and the test into one instruction: | |
60 | </p> | |
61 | <pre class="programlisting"><span class="keyword">if</span> <span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optionl</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> | |
62 | <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> | |
63 | </pre> | |
64 | <p> | |
65 | 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 | |
66 | extract the contained value of an uninitialized optional object is an <span class="emphasis"><em>undefined | |
67 | 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 | |
68 | that the contained value is there before extracting. For instance, the following | |
69 | code is reasonably UB-safe: | |
70 | </p> | |
71 | <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> | |
72 | </pre> | |
73 | <p> | |
74 | This is because we know that string value <code class="computeroutput"><span class="string">"100"</span></code> | |
75 | converts to a valid value of <code class="computeroutput"><span class="keyword">int</span></code>. | |
76 | If you do not like this potential UB, you can use an alternative way of extracting | |
77 | the contained value: | |
78 | </p> | |
79 | <pre class="programlisting"><span class="keyword">try</span> <span class="special">{</span> | |
80 | <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> | |
81 | <span class="special">}</span> | |
82 | <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> | |
83 | <span class="comment">// deal with it</span> | |
84 | <span class="special">}</span> | |
85 | </pre> | |
86 | <p> | |
87 | This version throws an exception upon an attempt to access a non-existent | |
88 | contained value. If your way of dealing with the missing value is to use | |
89 | some default, like <code class="computeroutput"><span class="number">0</span></code>, there exists | |
90 | a yet another alternative: | |
91 | </p> | |
92 | <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> | |
93 | </pre> | |
94 | <p> | |
95 | This uses the <code class="computeroutput"><span class="identifier">atoi</span></code>-like approach | |
96 | to conversions: if <code class="computeroutput"><span class="identifier">text</span></code> does | |
97 | not represent an integral number just return <code class="computeroutput"><span class="number">0</span></code>. | |
98 | Now, let's consider how function <code class="computeroutput"><span class="identifier">convert</span></code> | |
99 | can be implemented. | |
100 | </p> | |
101 | <pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optionl</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> | |
102 | <span class="special">{</span> | |
103 | <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> | |
104 | <span class="keyword">int</span> <span class="identifier">i</span><span class="special">;</span> | |
105 | <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> | |
106 | <span class="keyword">return</span> <span class="identifier">i</span><span class="special">;</span> | |
107 | <span class="keyword">else</span> | |
108 | <span class="keyword">return</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span><span class="special">;</span> | |
109 | <span class="special">}</span> | |
110 | </pre> | |
111 | <p> | |
112 | Observe the two return statements. <code class="computeroutput"><span class="keyword">return</span> | |
113 | <span class="identifier">i</span></code> uses the converting constructor | |
114 | 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> | |
115 | from <code class="computeroutput"><span class="identifier">T</span></code>. Thus constructed | |
116 | optional object is initialized and its value is a copy of <code class="computeroutput"><span class="identifier">i</span></code>. | |
117 | The other return statement uses another converting constructor from a special | |
118 | 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 | |
119 | to create an uninitialized optional object. | |
120 | </p> | |
121 | </div> | |
122 | <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> | |
123 | <td align="left"></td> | |
124 | <td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 Andrzej Krzemieński<p> | |
125 | Distributed under the Boost Software License, Version 1.0. (See accompanying | |
126 | 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>) | |
127 | </p> | |
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