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Fixed code, updated docs, added emplace()
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Andrzej Krzemienski
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="boost_optional.tutorial"></a><a class="link" href="tutorial.html" title="Tutorial">Tutorial</a>
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</h2></div></div></div>
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<div class="toc"><dl class="toc">
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<dt><span class="section"><a href="tutorial.html#boost_optional.tutorial.optional_return_values">Optional
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return values</a></span></dt>
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<dt><span class="section"><a href="tutorial.html#boost_optional.tutorial.optional_data_members">Optional
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data members</a></span></dt>
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<dt><span class="section"><a href="tutorial.html#boost_optional.tutorial.bypassing_unnecessary_default_construction">Bypassing
|
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unnecessary default construction</a></span></dt>
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</dl></div>
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="boost_optional.tutorial.optional_return_values"></a><a class="link" href="tutorial.html#boost_optional.tutorial.optional_return_values" title="Optional return values">Optional
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return values</a>
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</h3></div></div></div>
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<p>
|
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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>
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to an <code class="computeroutput"><span class="keyword">int</span></code>. It is possible that
|
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for a given string (e.g. <code class="computeroutput"><span class="string">"cat"</span></code>)
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there exist no value of type <code class="computeroutput"><span class="keyword">int</span></code>
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capable of representing the conversion result. We do not consider such situation
|
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an error. We expect that the converter can be used only to check if the conversion
|
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is possible. A natural signature for this function can be:
|
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</p>
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<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>
|
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<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>
|
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</pre>
|
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<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>.
|
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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>
|
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<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">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>
|
||||
<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>
|
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<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>
|
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<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>
|
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</pre>
|
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<p>
|
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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
|
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extract the contained value of an uninitialized optional object is an <span class="emphasis"><em>undefined
|
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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>
|
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<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>
|
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<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>.
|
||||
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">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>
|
||||
<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 class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.tutorial.optional_data_members"></a><a class="link" href="tutorial.html#boost_optional.tutorial.optional_data_members" title="Optional data members">Optional
|
||||
data members</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
Suppose we want to implement a <span class="emphasis"><em>lazy load</em></span> optimization.
|
||||
This is because we do not want to perform an expensive initialization of
|
||||
our <code class="computeroutput"><span class="identifier">Resource</span></code> until (if at
|
||||
all) it is really used. We can do it this way:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">Widget</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">Resource</span><span class="special">></span> <span class="identifier">resource_</span><span class="special">;</span>
|
||||
|
||||
<span class="keyword">public</span><span class="special">:</span>
|
||||
<span class="identifier">Widget</span><span class="special">()</span> <span class="special">{}</span>
|
||||
|
||||
<span class="identifier">Resource</span><span class="special">&</span> <span class="identifier">getResource</span><span class="special">()</span> <span class="comment">// not thread-safe</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">resource_</span> <span class="special">==</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span><span class="special">)</span>
|
||||
<span class="identifier">resource_</span><span class="special">.</span><span class="identifier">emplace</span><span class="special">(</span><span class="string">"resource"</span><span class="special">,</span> <span class="string">"arguments"</span><span class="special">);</span>
|
||||
|
||||
<span class="keyword">return</span> <span class="special">*</span><span class="identifier">resource_</span><span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
<span class="special">};</span>
|
||||
</pre>
|
||||
<p>
|
||||
<code class="computeroutput"><span class="identifier">optional</span></code>'s default constructor
|
||||
creates an uninitialized optional. No call to <code class="computeroutput"><span class="identifier">Resource</span></code>'s
|
||||
default constructor is attempted. <code class="computeroutput"><span class="identifier">Resource</span></code>
|
||||
doesn't have to be <a href="http://www.sgi.com/tech/stl/DefaultConstructible.html" target="_top">Default
|
||||
Constructible</a>. In function <code class="computeroutput"><span class="identifier">getResource</span></code>
|
||||
we first check if <code class="computeroutput"><span class="identifier">resource_</span></code>
|
||||
is initialized. This time we do not use the contextual conversion to <code class="computeroutput"><span class="keyword">bool</span></code>, but a comparison with <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code>. These two ways are equivalent. Function
|
||||
<code class="computeroutput"><span class="identifier">emplace</span></code> initializes the optional
|
||||
in-place by perfect-forwarding the arguments to the constructor of <code class="computeroutput"><span class="identifier">Resource</span></code>. No copy- or move-construction
|
||||
is involved here. <code class="computeroutput"><span class="identifier">Resource</span></code>
|
||||
doesn't even have to be <code class="computeroutput"><span class="identifier">MoveConstructible</span></code>.
|
||||
</p>
|
||||
<div class="note"><table border="0" summary="Note">
|
||||
<tr>
|
||||
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../doc/src/images/note.png"></td>
|
||||
<th align="left">Note</th>
|
||||
</tr>
|
||||
<tr><td align="left" valign="top"><p>
|
||||
Function <code class="computeroutput"><span class="identifier">emplace</span></code> is only
|
||||
available on compilers that support rvalue references and variadic templates.
|
||||
If your compiler does not support these features and you still need to
|
||||
avoid any move-constructions, use <a class="link" href="in_place_factories.html" title="In-Place Factories">In-Place
|
||||
Factories</a>.
|
||||
</p></td></tr>
|
||||
</table></div>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.tutorial.bypassing_unnecessary_default_construction"></a><a class="link" href="tutorial.html#boost_optional.tutorial.bypassing_unnecessary_default_construction" title="Bypassing unnecessary default construction">Bypassing
|
||||
unnecessary default construction</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
Suppose we have class <code class="computeroutput"><span class="identifier">Date</span></code>,
|
||||
which does not have a default constructor: there is no good candidate for
|
||||
a default date. We have a function that returns two dates in form of a <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">tuple</span></code>:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">tuple</span><span class="special"><</span><span class="identifier">Date</span><span class="special">,</span> <span class="identifier">Date</span><span class="special">></span> <span class="identifier">getPeriod</span><span class="special">();</span>
|
||||
</pre>
|
||||
<p>
|
||||
In other place we want to use the result of <code class="computeroutput"><span class="identifier">getPeriod</span></code>,
|
||||
but want the two dates to be named: <code class="computeroutput"><span class="identifier">begin</span></code>
|
||||
and <code class="computeroutput"><span class="identifier">end</span></code>. We want to implement
|
||||
something like 'multiple return values':
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">Date</span> <span class="identifier">begin</span><span class="special">,</span> <span class="identifier">end</span><span class="special">;</span> <span class="comment">// Error: no default ctor!</span>
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">tie</span><span class="special">(</span><span class="identifier">begin</span><span class="special">,</span> <span class="identifier">end</span><span class="special">)</span> <span class="special">=</span> <span class="identifier">getPeriod</span><span class="special">();</span>
|
||||
</pre>
|
||||
<p>
|
||||
The second line works already, this is the capability of Boost.Tuple library,
|
||||
but the first line won't work. We could set some initial invented dates,
|
||||
but it is confusing and may be an unacceptable cost, given that these values
|
||||
will be overwritten in the next line anyway. This is where <code class="computeroutput"><span class="identifier">optional</span></code> can help:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="identifier">Date</span><span class="special">></span> <span class="identifier">begin</span><span class="special">,</span> <span class="identifier">end</span><span class="special">;</span>
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">tie</span><span class="special">(</span><span class="identifier">begin</span><span class="special">,</span> <span class="identifier">end</span><span class="special">)</span> <span class="special">=</span> <span class="identifier">getPeriod</span><span class="special">();</span>
|
||||
</pre>
|
||||
<p>
|
||||
It works because inside <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">tie</span></code> a
|
||||
move-assignment from <code class="computeroutput"><span class="identifier">T</span></code> is
|
||||
invoked on <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>,
|
||||
which internally calls a move-constructor of <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
</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 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>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
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Reference in New Issue
Block a user