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| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="boost_optional.quick_start"></a><a class="link" href="quick_start.html" title="Quick Start">Quick Start</a> |
| </h2></div></div></div> |
| <div class="toc"><dl class="toc"> |
| <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> |
| </dl></div> |
| <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, 2015 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> |
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