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- <a name="boost_optional.quick_start"></a><a class="link" href="quick_start.html" title="Quick Start">Quick Start</a>
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- <dt><span class="section"><a href="quick_start.html#boost_optional.quick_start.optional_return_values">Optional
- return values</a></span></dt>
- <dt><span class="section"><a href="quick_start/optional_automatic_variables.html">Optional
- automatic variables</a></span></dt>
- <dt><span class="section"><a href="quick_start/optional_data_members.html">Optional
- data members</a></span></dt>
- <dt><span class="section"><a href="quick_start/bypassing_unnecessary_default_construction.html">Bypassing
- unnecessary default construction</a></span></dt>
- <dt><span class="section"><a href="quick_start/storage_in_containers.html">Storage
- in containers</a></span></dt>
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- <div class="section">
- <div class="titlepage"><div><div><h3 class="title">
- <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
- return values</a>
- </h3></div></div></div>
- <p>
- 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>
- to an <code class="computeroutput"><span class="keyword">int</span></code>. It is possible that
- for a given string (e.g. <code class="computeroutput"><span class="string">"cat"</span></code>)
- there exists no value of type <code class="computeroutput"><span class="keyword">int</span></code>
- capable of representing the conversion result. We do not consider such situation
- an error. We expect that the converter can be used only to check if the conversion
- is possible. A natural signature for this function can be:
- </p>
- <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>
- <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>
- </pre>
- <p>
- 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>.
- The above function signature means that the function can either return a
- value of type <code class="computeroutput"><span class="keyword">int</span></code> or a flag
- indicating that no value of <code class="computeroutput"><span class="keyword">int</span></code>
- is available. This does not indicate an error. It is like one additional
- value of <code class="computeroutput"><span class="keyword">int</span></code>. This is how we
- can use our function:
- </p>
- <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>
- <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>
- <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>
- <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>
- </pre>
- <p>
- In order to test if <code class="computeroutput"><span class="identifier">optional</span></code>
- 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
- of the optional object and the test into one instruction:
- </p>
- <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>
- <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>
- </pre>
- <p>
- 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
- extract the contained value of an uninitialized optional object is an <span class="emphasis"><em>undefined
- 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
- that the contained value is there before extracting. For instance, the following
- code is reasonably UB-safe:
- </p>
- <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>
- </pre>
- <p>
- This is because we know that string value <code class="computeroutput"><span class="string">"100"</span></code>
- converts to a valid value of <code class="computeroutput"><span class="keyword">int</span></code>.
- If you do not like this potential UB, you can use an alternative way of extracting
- the contained value:
- </p>
- <pre class="programlisting"><span class="keyword">try</span> <span class="special">{</span>
- <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>
- <span class="special">}</span>
- <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>
- <span class="comment">// deal with it</span>
- <span class="special">}</span>
- </pre>
- <p>
- This version throws an exception upon an attempt to access a non-existent
- contained value. If your way of dealing with the missing value is to use
- some default, like <code class="computeroutput"><span class="number">0</span></code>, there exists
- a yet another alternative:
- </p>
- <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>
- </pre>
- <p>
- This uses the <code class="computeroutput"><span class="identifier">atoi</span></code>-like approach
- to conversions: if <code class="computeroutput"><span class="identifier">text</span></code> does
- not represent an integral number just return <code class="computeroutput"><span class="number">0</span></code>.
- Finally, you can provide a callback to be called when trying to access the
- contained value fails:
- </p>
- <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">fallback_to_default</span><span class="special">()</span>
- <span class="special">{</span>
- <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>
- <span class="keyword">return</span> <span class="special">-</span><span class="number">1</span><span class="special">;</span>
- <span class="special">}</span>
- <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>
- </pre>
- <p>
- This will call the provided callback and return whatever the callback returns.
- The callback can have side effects: they will only be observed when the optional
- object does not contain a value.
- </p>
- <p>
- Now, let's consider how function <code class="computeroutput"><span class="identifier">convert</span></code>
- can be implemented.
- </p>
- <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>
- <span class="special">{</span>
- <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>
- <span class="keyword">int</span> <span class="identifier">i</span><span class="special">;</span>
- <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>
- <span class="keyword">return</span> <span class="identifier">i</span><span class="special">;</span>
- <span class="keyword">else</span>
- <span class="keyword">return</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span><span class="special">;</span>
- <span class="special">}</span>
- </pre>
- <p>
- Observe the two return statements. <code class="computeroutput"><span class="keyword">return</span>
- <span class="identifier">i</span></code> uses the converting constructor
- 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>
- from <code class="computeroutput"><span class="identifier">T</span></code>. Thus constructed
- optional object is initialized and its value is a copy of <code class="computeroutput"><span class="identifier">i</span></code>.
- The other return statement uses another converting constructor from a special
- 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
- to create an uninitialized optional object.
- </p>
- </div>
- </div>
- <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
- <td align="left"></td>
- <td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2018 Andrzej Krzemieński<p>
- Distributed under the Boost Software License, Version 1.0. (See accompanying
- 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>)
- </p>
- </div></td>
- </tr></table>
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