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<div class="section" title="Examples">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="boost_optional.examples"></a><a class="link" href="examples.html" title="Examples">Examples</a>
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<div class="toc"><dl>
<dt><span class="section"><a href="examples.html#boost_optional.examples.optional_return_values">Optional
return values</a></span></dt>
<dt><span class="section"><a href="examples.html#boost_optional.examples.optional_local_variables">Optional
local variables</a></span></dt>
<dt><span class="section"><a href="examples.html#boost_optional.examples.optional_data_members">Optional
data members</a></span></dt>
<dt><span class="section"><a href="examples.html#boost_optional.examples.bypassing_expensive_unnecessary_default_construction">Bypassing
expensive unnecessary default construction</a></span></dt>
</dl></div>
<div class="section" title="Optional return values">
<div class="titlepage"><div><div><h3 class="title">
<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
return values</a>
</h3></div></div></div>
<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>
<span class="special">{</span>
<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>
<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>
<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><span class="special">}</span>
<span class="keyword">void</span> <span class="identifier">receive_async_message</span><span class="special">()</span>
<span class="special">{</span>
<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>
<span class="comment">// The safe boolean conversion from 'rcv' is used here.
</span> <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>
<span class="identifier">output</span><span class="special">(*</span><span class="identifier">rcv</span><span class="special">);</span>
<span class="special">}</span>
</pre>
</div>
<div class="section" title="Optional local variables">
<div class="titlepage"><div><div><h3 class="title">
<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
local variables</a>
</h3></div></div></div>
<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>
<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>
<span class="special">{</span>
<span class="identifier">name</span><span class="special">.</span><span class="identifier">reset</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> <span class="special">;</span>
<span class="special">}</span>
<span class="keyword">else</span>
<span class="special">{</span>
<span class="keyword">if</span> <span class="special">(</span> <span class="identifier">can_ask_user</span> <span class="special">)</span>
<span class="identifier">name</span><span class="special">.</span><span class="identifier">reset</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> <span class="special">;</span>
<span class="special">}</span>
<span class="keyword">if</span> <span class="special">(</span> <span class="identifier">name</span> <span class="special">)</span>
<span class="identifier">print</span><span class="special">(*</span><span class="identifier">name</span><span class="special">);</span>
<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>
</pre>
</div>
<div class="section" title="Optional data members">
<div class="titlepage"><div><div><h3 class="title">
<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
data members</a>
</h3></div></div></div>
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">figure</span>
<span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
<span class="identifier">figure</span><span class="special">()</span>
<span class="special">{</span>
<span class="comment">// data member 'm_clipping_rect' is uninitialized at this point.
</span> <span class="special">}</span>
<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>
<span class="special">{</span>
<span class="special">....</span>
<span class="identifier">m_clipping_rect</span><span class="special">.</span><span class="identifier">reset</span> <span class="special">(</span> <span class="identifier">rect</span> <span class="special">)</span> <span class="special">;</span> <span class="comment">// initialized here.
</span> <span class="special">}</span>
<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>
<span class="special">{</span>
<span class="keyword">if</span> <span class="special">(</span> <span class="identifier">m_clipping_rect</span> <span class="special">)</span>
<span class="identifier">do_clipping</span><span class="special">(*</span><span class="identifier">m_clipping_rect</span><span class="special">);</span>
<span class="identifier">cvs</span><span class="special">.</span><span class="identifier">drawXXX</span><span class="special">(..);</span>
<span class="special">}</span>
<span class="comment">// this can return NULL.
</span> <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>
<span class="keyword">private</span> <span class="special">:</span>
<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>
<span class="special">};</span>
</pre>
</div>
<div class="section" title="Bypassing expensive unnecessary default construction">
<div class="titlepage"><div><div><h3 class="title">
<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
expensive unnecessary default construction</a>
</h3></div></div></div>
<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>
<span class="keyword">class</span> <span class="identifier">Fred</span>
<span class="special">{</span>
<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>
<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>
<span class="special">}</span> <span class="special">;</span>
</pre>
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