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26<div class="titlepage"><div><div><h2 class="title" style="clear: both">
27<a name="boost_optional.examples"></a><a class="link" href="examples.html" title="Examples">Examples</a>
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29<div class="toc"><dl class="toc">
30<dt><span class="section"><a href="examples.html#boost_optional.examples.optional_return_values">Optional
31 return values</a></span></dt>
32<dt><span class="section"><a href="examples.html#boost_optional.examples.optional_local_variables">Optional
33 local variables</a></span></dt>
34<dt><span class="section"><a href="examples.html#boost_optional.examples.optional_data_members">Optional
35 data members</a></span></dt>
36<dt><span class="section"><a href="examples.html#boost_optional.examples.bypassing_expensive_unnecessary_default_construction">Bypassing
37 expensive unnecessary default construction</a></span></dt>
38</dl></div>
39<div class="section">
40<div class="titlepage"><div><div><h3 class="title">
41<a name="boost_optional.examples.optional_return_values"></a><a class="link" href="examples.html#boost_optional.examples.optional_return_values" title="Optional return values">Optional
42 return values</a>
43</h3></div></div></div>
44<pre class="programlisting"><span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">char</span><span class="special">&gt;</span> <span class="identifier">get_async_input</span><span class="special">()</span>
45<span class="special">{</span>
46 <span class="keyword">if</span> <span class="special">(</span> <span class="special">!</span><span class="identifier">queue</span><span class="special">.</span><span class="identifier">empty</span><span class="special">()</span> <span class="special">)</span>
47 <span class="keyword">return</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">char</span><span class="special">&gt;(</span><span class="identifier">queue</span><span class="special">.</span><span class="identifier">top</span><span class="special">());</span>
48 <span class="keyword">else</span> <span class="keyword">return</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">char</span><span class="special">&gt;();</span> <span class="comment">// uninitialized</span>
49<span class="special">}</span>
50
51<span class="keyword">void</span> <span class="identifier">receive_async_message</span><span class="special">()</span>
52<span class="special">{</span>
53 <span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">char</span><span class="special">&gt;</span> <span class="identifier">rcv</span> <span class="special">;</span>
54 <span class="comment">// The safe boolean conversion from 'rcv' is used here.</span>
55 <span class="keyword">while</span> <span class="special">(</span> <span class="special">(</span><span class="identifier">rcv</span> <span class="special">=</span> <span class="identifier">get_async_input</span><span class="special">())</span> <span class="special">&amp;&amp;</span> <span class="special">!</span><span class="identifier">timeout</span><span class="special">()</span> <span class="special">)</span>
56 <span class="identifier">output</span><span class="special">(*</span><span class="identifier">rcv</span><span class="special">);</span>
57<span class="special">}</span>
58</pre>
59</div>
60<div class="section">
61<div class="titlepage"><div><div><h3 class="title">
62<a name="boost_optional.examples.optional_local_variables"></a><a class="link" href="examples.html#boost_optional.examples.optional_local_variables" title="Optional local variables">Optional
63 local variables</a>
64</h3></div></div></div>
65<pre class="programlisting"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">string</span><span class="special">&gt;</span> <span class="identifier">name</span> <span class="special">;</span>
66<span class="keyword">if</span> <span class="special">(</span> <span class="identifier">database</span><span class="special">.</span><span class="identifier">open</span><span class="special">()</span> <span class="special">)</span>
67<span class="special">{</span>
68 <span class="identifier">name</span> <span class="special">=</span> <span class="identifier">database</span><span class="special">.</span><span class="identifier">lookup</span><span class="special">(</span><span class="identifier">employer_name</span><span class="special">)</span> <span class="special">;</span>
69<span class="special">}</span>
70<span class="keyword">else</span>
71<span class="special">{</span>
72 <span class="keyword">if</span> <span class="special">(</span> <span class="identifier">can_ask_user</span> <span class="special">)</span>
73 <span class="identifier">name</span> <span class="special">=</span> <span class="identifier">user</span><span class="special">.</span><span class="identifier">ask</span><span class="special">(</span><span class="identifier">employer_name</span><span class="special">)</span> <span class="special">;</span>
74<span class="special">}</span>
75
76<span class="keyword">if</span> <span class="special">(</span> <span class="identifier">name</span> <span class="special">)</span>
77 <span class="identifier">print</span><span class="special">(*</span><span class="identifier">name</span><span class="special">);</span>
78<span class="keyword">else</span> <span class="identifier">print</span><span class="special">(</span><span class="string">"employer's name not found!"</span><span class="special">);</span>
79</pre>
80</div>
81<div class="section">
82<div class="titlepage"><div><div><h3 class="title">
83<a name="boost_optional.examples.optional_data_members"></a><a class="link" href="examples.html#boost_optional.examples.optional_data_members" title="Optional data members">Optional
84 data members</a>
85</h3></div></div></div>
86<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">figure</span>
87<span class="special">{</span>
88 <span class="keyword">public</span><span class="special">:</span>
89
90 <span class="identifier">figure</span><span class="special">()</span>
91 <span class="special">{</span>
92 <span class="comment">// data member 'm_clipping_rect' is uninitialized at this point.</span>
93 <span class="special">}</span>
94
95 <span class="keyword">void</span> <span class="identifier">clip_in_rect</span> <span class="special">(</span> <span class="identifier">rect</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">rect</span> <span class="special">)</span>
96 <span class="special">{</span>
97 <span class="special">....</span>
98 <span class="identifier">m_clipping_rect</span> <span class="special">=</span> <span class="identifier">rect</span> <span class="special">;</span> <span class="comment">// initialized here.</span>
99 <span class="special">}</span>
100
101 <span class="keyword">void</span> <span class="identifier">draw</span> <span class="special">(</span> <span class="identifier">canvas</span><span class="special">&amp;</span> <span class="identifier">cvs</span> <span class="special">)</span>
102 <span class="special">{</span>
103 <span class="keyword">if</span> <span class="special">(</span> <span class="identifier">m_clipping_rect</span> <span class="special">)</span>
104 <span class="identifier">do_clipping</span><span class="special">(*</span><span class="identifier">m_clipping_rect</span><span class="special">);</span>
105
106 <span class="identifier">cvs</span><span class="special">.</span><span class="identifier">drawXXX</span><span class="special">(..);</span>
107 <span class="special">}</span>
108
109 <span class="comment">// this can return NULL.</span>
110 <span class="identifier">rect</span> <span class="keyword">const</span><span class="special">*</span> <span class="identifier">get_clipping_rect</span><span class="special">()</span> <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">get_pointer</span><span class="special">(</span><span class="identifier">m_clipping_rect</span><span class="special">);</span> <span class="special">}</span>
111
112 <span class="keyword">private</span> <span class="special">:</span>
113
114 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">rect</span><span class="special">&gt;</span> <span class="identifier">m_clipping_rect</span> <span class="special">;</span>
115
116<span class="special">};</span>
117</pre>
118</div>
119<div class="section">
120<div class="titlepage"><div><div><h3 class="title">
121<a name="boost_optional.examples.bypassing_expensive_unnecessary_default_construction"></a><a class="link" href="examples.html#boost_optional.examples.bypassing_expensive_unnecessary_default_construction" title="Bypassing expensive unnecessary default construction">Bypassing
122 expensive unnecessary default construction</a>
123</h3></div></div></div>
124<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">ExpensiveCtor</span> <span class="special">{</span> <span class="special">...</span> <span class="special">}</span> <span class="special">;</span>
125<span class="keyword">class</span> <span class="identifier">Fred</span>
126<span class="special">{</span>
127 <span class="identifier">Fred</span><span class="special">()</span> <span class="special">:</span> <span class="identifier">mLargeVector</span><span class="special">(</span><span class="number">10000</span><span class="special">)</span> <span class="special">{}</span>
128
129 <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">ExpensiveCtor</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="identifier">mLargeVector</span> <span class="special">;</span>
130<span class="special">}</span> <span class="special">;</span>
131</pre>
132</div>
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134<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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136<td align="right"><div class="copyright-footer">Copyright &#169; 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright &#169; 2014 Andrzej Krzemie&#324;ski<p>
137 Distributed under the Boost Software License, Version 1.0. (See accompanying
138 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>)
139 </p>
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