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<title>Acknowledgments</title>
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<a name="boost_optional.dependencies_and_portability"></a><a class="link" href="dependencies_and_portability.html" title="Dependencies and Portability">Dependencies
and Portability</a>
</h2></div></div></div>
<div class="toc"><dl class="toc"><dt><span class="section"><a href="dependencies_and_portability/optional_reference_binding.html">Optional
Reference Binding</a></span></dt></dl></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="dependencies_and_portability.html#boost_optional.dependencies_and_portability.dependencies">Dependencies</a></span></dt>
<dt><span class="section"><a href="dependencies_and_portability/optional_reference_binding.html">Optional
Reference Binding</a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_optional.dependencies_and_portability.dependencies"></a><a class="link" href="dependencies_and_portability.html#boost_optional.dependencies_and_portability.dependencies" title="Dependencies">Dependencies</a>
</h3></div></div></div>
<p>
The implementation uses <code class="computeroutput"><span class="identifier">type_traits</span><span class="special">/</span><span class="identifier">alignment_of</span><span class="special">.</span><span class="identifier">hpp</span></code> and
<code class="computeroutput"><span class="identifier">type_traits</span><span class="special">/</span><span class="identifier">type_with_alignment</span><span class="special">.</span><span class="identifier">hpp</span></code>
</p>
The implementation uses the following other Boost modules:
</p>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
assert
</li>
<li class="listitem">
config
</li>
<li class="listitem">
core
</li>
<li class="listitem">
detail
</li>
<li class="listitem">
move
</li>
<li class="listitem">
mpl
</li>
<li class="listitem">
static_assert
</li>
<li class="listitem">
throw_exception
</li>
<li class="listitem">
type_traits
</li>
<li class="listitem">
utility
</li>
</ol></div>
</div>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<title>Quick Start</title>
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<hr>
<div class="spirit-nav">
<a accesskey="p" href="../index.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="../optional/tutorial.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
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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>
<h4>
<a name="boost_optional.quick_start.h0"></a>
<span class="phrase"><a name="boost_optional.quick_start.optional_return_values"></a></span><a class="link" href="quick_start.html#boost_optional.quick_start.optional_return_values">Optional
<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>
</h4>
</h3></div></div></div>
<p>
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>
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 exist 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>
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>
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 exist 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">&lt;</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">&gt;</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optionl</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</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">&amp;</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">&lt;</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">&gt;</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>
All necessary functionality can be included with one header <code class="computeroutput"><span class="special">&lt;</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">&gt;</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">&amp;</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">&lt;</span><span class="keyword">int</span><span class="special">&gt;</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>
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">optionl</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</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">-&gt;</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>
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">-&gt;</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>
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>
@ -87,20 +100,20 @@
<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>
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>
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">&lt;</span><span class="keyword">int</span><span class="special">&gt;</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">&amp;</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>
@ -112,168 +125,16 @@
<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">&lt;</span><span class="identifier">T</span><span class="special">&gt;</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>
<h4>
<a name="boost_optional.quick_start.h1"></a>
<span class="phrase"><a name="boost_optional.quick_start.optional_automatic_variables"></a></span><a class="link" href="quick_start.html#boost_optional.quick_start.optional_automatic_variables">Optional
automatic variables</a>
</h4>
<p>
We could write function <code class="computeroutput"><span class="identifier">convert</span></code>
in a slightly different manner, so that it has a single <code class="computeroutput"><span class="keyword">return</span></code>-statement:
</p>
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optionl</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</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">&amp;</span> <span class="identifier">text</span><span class="special">)</span>
<span class="special">{</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optionl</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">ans</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">&gt;&gt;</span> <span class="identifier">i</span><span class="special">)</span> <span class="special">&amp;&amp;</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">&lt;</span><span class="keyword">char</span><span class="special">&gt;::</span><span class="identifier">eof</span><span class="special">())</span>
<span class="identifier">ans</span> <span class="special">=</span> <span class="identifier">i</span><span class="special">;</span>
<span class="keyword">return</span> <span class="identifier">ans</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
The default constructor of <code class="computeroutput"><span class="identifier">optional</span></code>
creates an unitialized optional object. Unlike with <code class="computeroutput"><span class="keyword">int</span></code>s
you cannot have an <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span></code>
in an indeterminate state. Its state is always well defined. Instruction <code class="computeroutput"><span class="identifier">ans</span> <span class="special">=</span> <span class="identifier">i</span></code>
initializes the optional object. It uses the assignment from <code class="computeroutput"><span class="keyword">int</span></code>. In general, for <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>,
when an assignment from <code class="computeroutput"><span class="identifier">T</span></code> is
invoked, it can do two things. If the optional object is not initialized our
case here), it initializes it with <code class="computeroutput"><span class="identifier">T</span></code>'s
copy constructor. If the optional object is already initialized, it assigns
the new value to it using <code class="computeroutput"><span class="identifier">T</span></code>'s
copy assignment.
</p>
<h4>
<a name="boost_optional.quick_start.h2"></a>
<span class="phrase"><a name="boost_optional.quick_start.optional_data_members"></a></span><a class="link" href="quick_start.html#boost_optional.quick_start.optional_data_members">Optional
data members</a>
</h4>
<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">&lt;</span><span class="identifier">Resource</span><span class="special">&gt;</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">&amp;</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="tutorial/in_place_factories.html" title="In-Place Factories">In-Place
Factories</a>.
</p></td></tr>
</table></div>
<h4>
<a name="boost_optional.quick_start.h3"></a>
<span class="phrase"><a name="boost_optional.quick_start.bypassing_unnecessary_default_construction"></a></span><a class="link" href="quick_start.html#boost_optional.quick_start.bypassing_unnecessary_default_construction">Bypassing
unnecessary default construction</a>
</h4>
<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">&lt;</span><span class="identifier">Date</span><span class="special">,</span> <span class="identifier">Date</span><span class="special">&gt;</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 invented initial 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">&lt;</span><span class="identifier">Date</span><span class="special">&gt;</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">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>, which
internally calls a move-constructor of <code class="computeroutput"><span class="identifier">T</span></code>.
</p>
<h4>
<a name="boost_optional.quick_start.h4"></a>
<span class="phrase"><a name="boost_optional.quick_start.storage_in_containers"></a></span><a class="link" href="quick_start.html#boost_optional.quick_start.storage_in_containers">Storage
in containers</a>
</h4>
<p>
Suppose you want to ask users to choose some number (an <code class="computeroutput"><span class="keyword">int</span></code>).
One of the valid responses is to choose nothing, which is represented by an
uninitialized <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span></code>. You
want to make a histogram showing how many times each choice was made. You can
use an <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span></code>:
</p>
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special">&lt;</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;,</span> <span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">choices</span><span class="special">;</span>
<span class="keyword">for</span> <span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special">&lt;</span> <span class="identifier">LIMIT</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span> <span class="special">{</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">choice</span> <span class="special">=</span> <span class="identifier">readChoice</span><span class="special">();</span>
<span class="special">++</span><span class="identifier">choices</span><span class="special">[</span><span class="identifier">choice</span><span class="special">];</span>
<span class="special">}</span>
</pre>
<p>
This works because <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
is <code class="computeroutput"><span class="identifier">LessThanComparable</span></code> whenever
<code class="computeroutput"><span class="identifier">T</span></code> is <code class="computeroutput"><span class="identifier">LessThanComparable</span></code>.
In this case the state of being uninitialized is treated as a yet another value
of <code class="computeroutput"><span class="identifier">T</span></code>, which is compared less
than any value of <code class="computeroutput"><span class="identifier">T</span></code>. So the
set of values that type <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
can assume is {<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code>, -2147483648, -2147483647, ..., -1,
0, 1, ..., 2147483647} (assuming a 32-bit <code class="computeroutput"><span class="keyword">int</span></code>).
</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">&lt;</span><span class="identifier">T</span><span class="special">&gt;</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>
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<a name="boost_optional.quick_start.storage_in_containers"></a><a class="link" href="storage_in_containers.html" title="Storage in containers">Storage
in containers</a>
</h3></div></div></div>
<p>
Suppose you want to ask users to choose some number (an <code class="computeroutput"><span class="keyword">int</span></code>).
One of the valid responses is to choose nothing, which is represented by
an uninitialized <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span></code>.
You want to make a histogram showing how many times each choice was made.
You can use an <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span></code>:
</p>
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special">&lt;</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;,</span> <span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">choices</span><span class="special">;</span>
<span class="keyword">for</span> <span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special">&lt;</span> <span class="identifier">LIMIT</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span> <span class="special">{</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">choice</span> <span class="special">=</span> <span class="identifier">readChoice</span><span class="special">();</span>
<span class="special">++</span><span class="identifier">choices</span><span class="special">[</span><span class="identifier">choice</span><span class="special">];</span>
<span class="special">}</span>
</pre>
<p>
This works because <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
is <code class="computeroutput"><span class="identifier">LessThanComparable</span></code> whenever
<code class="computeroutput"><span class="identifier">T</span></code> is <code class="computeroutput"><span class="identifier">LessThanComparable</span></code>.
In this case the state of being uninitialized is treated as a yet another
value of <code class="computeroutput"><span class="identifier">T</span></code>, which is compared
less than any value of <code class="computeroutput"><span class="identifier">T</span></code>.
So the set of values that type <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
can assume is {<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code>, -2147483648, -2147483647, ..., -1,
0, 1, ..., 2147483647} (assuming a 32-bit <code class="computeroutput"><span class="keyword">int</span></code>).
</p>
</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 &#169; 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright &#169; 2014 Andrzej Krzemie&#324;ski<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>)
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<title>A note about optional&lt;bool&gt;</title>
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<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_optional.tutorial.design_overview"></a><a class="link" href="design_overview.html" title="Design Overview">Design Overview</a>
</h3></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="design_overview.html#boost_optional.tutorial.design_overview.the_models">The
models</a></span></dt>
<dt><span class="section"><a href="design_overview.html#boost_optional.tutorial.design_overview.the_semantics">The
semantics</a></span></dt>
<dt><span class="section"><a href="design_overview.html#boost_optional.tutorial.design_overview.the_interface">The
Interface</a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_optional.tutorial.design_overview.the_models"></a><a class="link" href="design_overview.html#boost_optional.tutorial.design_overview.the_models" title="The models">The
@ -175,237 +167,6 @@
</li>
</ul></div>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_optional.tutorial.design_overview.the_semantics"></a><a class="link" href="design_overview.html#boost_optional.tutorial.design_overview.the_semantics" title="The semantics">The
semantics</a>
</h4></div></div></div>
<p>
Objects of type <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code> are intended to be used in places where
objects of type <code class="computeroutput"><span class="identifier">T</span></code> would
but which might be uninitialized. Hence, <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>'s purpose is to formalize the additional
possibly uninitialized state. From the perspective of this role, <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
can have the same operational semantics of <code class="computeroutput"><span class="identifier">T</span></code>
plus the additional semantics corresponding to this special state. As such,
<code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
could be thought of as a <span class="emphasis"><em>supertype</em></span> of <code class="computeroutput"><span class="identifier">T</span></code>. Of course, we can't do that in C++,
so we need to compose the desired semantics using a different mechanism.
Doing it the other way around, that is, making <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code> a <span class="emphasis"><em>subtype</em></span> of
<code class="computeroutput"><span class="identifier">T</span></code> is not only conceptually
wrong but also impractical: it is not allowed to derive from a non-class
type, such as a built-in type.
</p>
<p>
We can draw from the purpose of <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code> the required basic semantics:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
<span class="bold"><strong>Default Construction:</strong></span> To introduce
a formally uninitialized wrapped object.
</li>
<li class="listitem">
<span class="bold"><strong>Direct Value Construction via copy:</strong></span>
To introduce a formally initialized wrapped object whose value is obtained
as a copy of some object.
</li>
<li class="listitem">
<span class="bold"><strong>Deep Copy Construction:</strong></span> To obtain
a new yet equivalent wrapped object.
</li>
<li class="listitem">
<span class="bold"><strong>Direct Value Assignment (upon initialized):</strong></span>
To assign a value to the wrapped object.
</li>
<li class="listitem">
<span class="bold"><strong>Direct Value Assignment (upon uninitialized):</strong></span>
To initialize the wrapped object with a value obtained as a copy of
some object.
</li>
<li class="listitem">
<span class="bold"><strong>Assignment (upon initialized):</strong></span> To
assign to the wrapped object the value of another wrapped object.
</li>
<li class="listitem">
<span class="bold"><strong>Assignment (upon uninitialized):</strong></span> To
initialize the wrapped object with value of another wrapped object.
</li>
<li class="listitem">
<span class="bold"><strong>Deep Relational Operations (when supported by
the type T):</strong></span> To compare wrapped object values taking into
account the presence of uninitialized states.
</li>
<li class="listitem">
<span class="bold"><strong>Value access:</strong></span> To unwrap the wrapped
object.
</li>
<li class="listitem">
<span class="bold"><strong>Initialization state query:</strong></span> To determine
if the object is formally initialized or not.
</li>
<li class="listitem">
<span class="bold"><strong>Swap:</strong></span> To exchange wrapped objects.
(with whatever exception safety guarantees are provided by <code class="computeroutput"><span class="identifier">T</span></code>'s swap).
</li>
<li class="listitem">
<span class="bold"><strong>De-initialization:</strong></span> To release the
wrapped object (if any) and leave the wrapper in the uninitialized
state.
</li>
</ul></div>
<p>
Additional operations are useful, such as converting constructors and converting
assignments, in-place construction and assignment, and safe value access
via a pointer to the wrapped object or null.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_optional.tutorial.design_overview.the_interface"></a><a class="link" href="design_overview.html#boost_optional.tutorial.design_overview.the_interface" title="The Interface">The
Interface</a>
</h4></div></div></div>
<p>
Since the purpose of optional is to allow us to use objects with a formal
uninitialized additional state, the interface could try to follow the interface
of the underlying <code class="computeroutput"><span class="identifier">T</span></code> type
as much as possible. In order to choose the proper degree of adoption of
the native <code class="computeroutput"><span class="identifier">T</span></code> interface,
the following must be noted: Even if all the operations supported by an
instance of type <code class="computeroutput"><span class="identifier">T</span></code> are
defined for the entire range of values for such a type, an <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
extends such a set of values with a new value for which most (otherwise
valid) operations are not defined in terms of <code class="computeroutput"><span class="identifier">T</span></code>.
</p>
<p>
Furthermore, since <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code> itself is merely a <code class="computeroutput"><span class="identifier">T</span></code>
wrapper (modeling a <code class="computeroutput"><span class="identifier">T</span></code> supertype),
any attempt to define such operations upon uninitialized optionals will
be totally artificial w.r.t. <code class="computeroutput"><span class="identifier">T</span></code>.
</p>
<p>
This library chooses an interface which follows from <code class="computeroutput"><span class="identifier">T</span></code>'s
interface only for those operations which are well defined (w.r.t the type
<code class="computeroutput"><span class="identifier">T</span></code>) even if any of the operands
are uninitialized. These operations include: construction, copy-construction,
assignment, swap and relational operations.
</p>
<p>
For the value access operations, which are undefined (w.r.t the type <code class="computeroutput"><span class="identifier">T</span></code>) when the operand is uninitialized,
a different interface is chosen (which will be explained next).
</p>
<p>
Also, the presence of the possibly uninitialized state requires additional
operations not provided by <code class="computeroutput"><span class="identifier">T</span></code>
itself which are supported by a special interface.
</p>
<h6>
<a name="boost_optional.tutorial.design_overview.the_interface.h0"></a>
<span class="phrase"><a name="boost_optional.tutorial.design_overview.the_interface.lexically_hinted_value_access_in_the_presence_of_possibly_untitialized_optional_objects__the_operators___and___gt_"></a></span><a class="link" href="design_overview.html#boost_optional.tutorial.design_overview.the_interface.lexically_hinted_value_access_in_the_presence_of_possibly_untitialized_optional_objects__the_operators___and___gt_">Lexically-hinted
Value Access in the presence of possibly untitialized optional objects:
The operators * and -&gt;</a>
</h6>
<p>
A relevant feature of a pointer is that it can have a <span class="bold"><strong>null
pointer value</strong></span>. This is a <span class="emphasis"><em>special</em></span> value
which is used to indicate that the pointer is not referring to any object
at all. In other words, null pointer values convey the notion of nonexistent
objects.
</p>
<p>
This meaning of the null pointer value allowed pointers to became a <span class="emphasis"><em>de
facto</em></span> standard for handling optional objects because all you
have to do to refer to a value which you don't really have is to use a
null pointer value of the appropriate type. Pointers have been used for
decades&#8212;from the days of C APIs to modern C++ libraries&#8212;to <span class="emphasis"><em>refer</em></span>
to optional (that is, possibly nonexistent) objects; particularly as optional
arguments to a function, but also quite often as optional data members.
</p>
<p>
The possible presence of a null pointer value makes the operations that
access the pointee's value possibly undefined, therefore, expressions which
use dereference and access operators, such as: <code class="computeroutput"><span class="special">(</span>
<span class="special">*</span><span class="identifier">p</span>
<span class="special">=</span> <span class="number">2</span> <span class="special">)</span></code> and <code class="computeroutput"><span class="special">(</span>
<span class="identifier">p</span><span class="special">-&gt;</span><span class="identifier">foo</span><span class="special">()</span> <span class="special">)</span></code>, implicitly convey the notion of optionality,
and this information is tied to the <span class="emphasis"><em>syntax</em></span> of the
expressions. That is, the presence of operators <code class="computeroutput"><span class="special">*</span></code>
and <code class="computeroutput"><span class="special">-&gt;</span></code> tell by themselves
&#8212;without any additional context&#8212; that the expression will be undefined
unless the implied pointee actually exist.
</p>
<p>
Such a <span class="emphasis"><em>de facto</em></span> idiom for referring to optional objects
can be formalized in the form of a concept: the <a href="../../../../../utility/OptionalPointee.html" target="_top">OptionalPointee</a>
concept. This concept captures the syntactic usage of operators <code class="computeroutput"><span class="special">*</span></code>, <code class="computeroutput"><span class="special">-&gt;</span></code>
and contextual conversion to <code class="computeroutput"><span class="keyword">bool</span></code>
to convey the notion of optionality.
</p>
<p>
However, pointers are good to <span class="underline">refer</span>
to optional objects, but not particularly good to handle the optional objects
in all other respects, such as initializing or moving/copying them. The
problem resides in the shallow-copy of pointer semantics: if you need to
effectively move or copy the object, pointers alone are not enough. The
problem is that copies of pointers do not imply copies of pointees. For
example, as was discussed in the motivation, pointers alone cannot be used
to return optional objects from a function because the object must move
outside from the function and into the caller's context.
</p>
<p>
A solution to the shallow-copy problem that is often used is to resort
to dynamic allocation and use a smart pointer to automatically handle the
details of this. For example, if a function is to optionally return an
object <code class="computeroutput"><span class="identifier">X</span></code>, it can use <code class="computeroutput"><span class="identifier">shared_ptr</span><span class="special">&lt;</span><span class="identifier">X</span><span class="special">&gt;</span></code>
as the return value. However, this requires dynamic allocation of <code class="computeroutput"><span class="identifier">X</span></code>. If <code class="computeroutput"><span class="identifier">X</span></code>
is a built-in or small POD, this technique is very poor in terms of required
resources. Optional objects are essentially values so it is very convenient
to be able to use automatic storage and deep-copy semantics to manipulate
optional values just as we do with ordinary values. Pointers do not have
this semantics, so are inappropriate for the initialization and transport
of optional values, yet are quite convenient for handling the access to
the possible undefined value because of the idiomatic aid present in the
<a href="../../../../../utility/OptionalPointee.html" target="_top">OptionalPointee</a>
concept incarnated by pointers.
</p>
<h6>
<a name="boost_optional.tutorial.design_overview.the_interface.h1"></a>
<span class="phrase"><a name="boost_optional.tutorial.design_overview.the_interface.optional_lt_t_gt__as_a_model_of_optionalpointee"></a></span><a class="link" href="design_overview.html#boost_optional.tutorial.design_overview.the_interface.optional_lt_t_gt__as_a_model_of_optionalpointee">Optional&lt;T&gt;
as a model of OptionalPointee</a>
</h6>
<p>
For value access operations <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;&gt;</span></code> uses operators <code class="computeroutput"><span class="special">*</span></code>
and <code class="computeroutput"><span class="special">-&gt;</span></code> to lexically warn
about the possibly uninitialized state appealing to the familiar pointer
semantics w.r.t. to null pointers.
</p>
<div class="warning"><table border="0" summary="Warning">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../../../../doc/src/images/warning.png"></td>
<th align="left">Warning</th>
</tr>
<tr><td align="left" valign="top"><p>
However, it is particularly important to note that <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;&gt;</span></code> objects are not pointers. <span class="underline"><code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;&gt;</span></code> is not, and does not model, a
pointer</span>.
</p></td></tr>
</table></div>
<p>
For instance, <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;&gt;</span></code> does not have shallow-copy so does
not alias: two different optionals never refer to the <span class="emphasis"><em>same</em></span>
value unless <code class="computeroutput"><span class="identifier">T</span></code> itself is
a reference (but may have <span class="emphasis"><em>equivalent</em></span> values). The
difference between an <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code> and a pointer must be kept in mind,
particularly because the semantics of relational operators are different:
since <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
is a value-wrapper, relational operators are deep: they compare optional
values; but relational operators for pointers are shallow: they do not
compare pointee values. As a result, you might be able to replace <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
by <code class="computeroutput"><span class="identifier">T</span><span class="special">*</span></code>
on some situations but not always. Specifically, on generic code written
for both, you cannot use relational operators directly, and must use the
template functions <a href="../../../../../utility/OptionalPointee.html#equal" target="_top"><code class="computeroutput"><span class="identifier">equal_pointees</span><span class="special">()</span></code></a>
and <a href="../../../../../utility/OptionalPointee.html#less" target="_top"><code class="computeroutput"><span class="identifier">less_pointees</span><span class="special">()</span></code></a>
instead.
</p>
</div>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
@ -417,7 +178,7 @@
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<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_optional.tutorial.design_overview.the_interface"></a><a class="link" href="the_interface.html" title="The Interface">The
Interface</a>
</h4></div></div></div>
<p>
Since the purpose of optional is to allow us to use objects with a formal
uninitialized additional state, the interface could try to follow the interface
of the underlying <code class="computeroutput"><span class="identifier">T</span></code> type
as much as possible. In order to choose the proper degree of adoption of
the native <code class="computeroutput"><span class="identifier">T</span></code> interface,
the following must be noted: Even if all the operations supported by an
instance of type <code class="computeroutput"><span class="identifier">T</span></code> are
defined for the entire range of values for such a type, an <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
extends such a set of values with a new value for which most (otherwise
valid) operations are not defined in terms of <code class="computeroutput"><span class="identifier">T</span></code>.
</p>
<p>
Furthermore, since <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code> itself is merely a <code class="computeroutput"><span class="identifier">T</span></code>
wrapper (modeling a <code class="computeroutput"><span class="identifier">T</span></code> supertype),
any attempt to define such operations upon uninitialized optionals will
be totally artificial w.r.t. <code class="computeroutput"><span class="identifier">T</span></code>.
</p>
<p>
This library chooses an interface which follows from <code class="computeroutput"><span class="identifier">T</span></code>'s
interface only for those operations which are well defined (w.r.t the type
<code class="computeroutput"><span class="identifier">T</span></code>) even if any of the operands
are uninitialized. These operations include: construction, copy-construction,
assignment, swap and relational operations.
</p>
<p>
For the value access operations, which are undefined (w.r.t the type <code class="computeroutput"><span class="identifier">T</span></code>) when the operand is uninitialized,
a different interface is chosen (which will be explained next).
</p>
<p>
Also, the presence of the possibly uninitialized state requires additional
operations not provided by <code class="computeroutput"><span class="identifier">T</span></code>
itself which are supported by a special interface.
</p>
<h6>
<a name="boost_optional.tutorial.design_overview.the_interface.h0"></a>
<span class="phrase"><a name="boost_optional.tutorial.design_overview.the_interface.lexically_hinted_value_access_in_the_presence_of_possibly_untitialized_optional_objects__the_operators___and___gt_"></a></span><a class="link" href="the_interface.html#boost_optional.tutorial.design_overview.the_interface.lexically_hinted_value_access_in_the_presence_of_possibly_untitialized_optional_objects__the_operators___and___gt_">Lexically-hinted
Value Access in the presence of possibly untitialized optional objects:
The operators * and -&gt;</a>
</h6>
<p>
A relevant feature of a pointer is that it can have a <span class="bold"><strong>null
pointer value</strong></span>. This is a <span class="emphasis"><em>special</em></span> value
which is used to indicate that the pointer is not referring to any object
at all. In other words, null pointer values convey the notion of nonexistent
objects.
</p>
<p>
This meaning of the null pointer value allowed pointers to became a <span class="emphasis"><em>de
facto</em></span> standard for handling optional objects because all you
have to do to refer to a value which you don't really have is to use a
null pointer value of the appropriate type. Pointers have been used for
decades&#8212;from the days of C APIs to modern C++ libraries&#8212;to <span class="emphasis"><em>refer</em></span>
to optional (that is, possibly nonexistent) objects; particularly as optional
arguments to a function, but also quite often as optional data members.
</p>
<p>
The possible presence of a null pointer value makes the operations that
access the pointee's value possibly undefined, therefore, expressions which
use dereference and access operators, such as: <code class="computeroutput"><span class="special">(</span>
<span class="special">*</span><span class="identifier">p</span>
<span class="special">=</span> <span class="number">2</span> <span class="special">)</span></code> and <code class="computeroutput"><span class="special">(</span>
<span class="identifier">p</span><span class="special">-&gt;</span><span class="identifier">foo</span><span class="special">()</span> <span class="special">)</span></code>, implicitly convey the notion of optionality,
and this information is tied to the <span class="emphasis"><em>syntax</em></span> of the
expressions. That is, the presence of operators <code class="computeroutput"><span class="special">*</span></code>
and <code class="computeroutput"><span class="special">-&gt;</span></code> tell by themselves
&#8212;without any additional context&#8212; that the expression will be undefined
unless the implied pointee actually exist.
</p>
<p>
Such a <span class="emphasis"><em>de facto</em></span> idiom for referring to optional objects
can be formalized in the form of a concept: the <a href="../../../../../../utility/OptionalPointee.html" target="_top">OptionalPointee</a>
concept. This concept captures the syntactic usage of operators <code class="computeroutput"><span class="special">*</span></code>, <code class="computeroutput"><span class="special">-&gt;</span></code>
and contextual conversion to <code class="computeroutput"><span class="keyword">bool</span></code>
to convey the notion of optionality.
</p>
<p>
However, pointers are good to <span class="underline">refer</span>
to optional objects, but not particularly good to handle the optional objects
in all other respects, such as initializing or moving/copying them. The
problem resides in the shallow-copy of pointer semantics: if you need to
effectively move or copy the object, pointers alone are not enough. The
problem is that copies of pointers do not imply copies of pointees. For
example, as was discussed in the motivation, pointers alone cannot be used
to return optional objects from a function because the object must move
outside from the function and into the caller's context.
</p>
<p>
A solution to the shallow-copy problem that is often used is to resort
to dynamic allocation and use a smart pointer to automatically handle the
details of this. For example, if a function is to optionally return an
object <code class="computeroutput"><span class="identifier">X</span></code>, it can use <code class="computeroutput"><span class="identifier">shared_ptr</span><span class="special">&lt;</span><span class="identifier">X</span><span class="special">&gt;</span></code>
as the return value. However, this requires dynamic allocation of <code class="computeroutput"><span class="identifier">X</span></code>. If <code class="computeroutput"><span class="identifier">X</span></code>
is a built-in or small POD, this technique is very poor in terms of required
resources. Optional objects are essentially values so it is very convenient
to be able to use automatic storage and deep-copy semantics to manipulate
optional values just as we do with ordinary values. Pointers do not have
this semantics, so are inappropriate for the initialization and transport
of optional values, yet are quite convenient for handling the access to
the possible undefined value because of the idiomatic aid present in the
<a href="../../../../../../utility/OptionalPointee.html" target="_top">OptionalPointee</a>
concept incarnated by pointers.
</p>
<h6>
<a name="boost_optional.tutorial.design_overview.the_interface.h1"></a>
<span class="phrase"><a name="boost_optional.tutorial.design_overview.the_interface.optional_lt_t_gt__as_a_model_of_optionalpointee"></a></span><a class="link" href="the_interface.html#boost_optional.tutorial.design_overview.the_interface.optional_lt_t_gt__as_a_model_of_optionalpointee">Optional&lt;T&gt;
as a model of OptionalPointee</a>
</h6>
<p>
For value access operations <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;&gt;</span></code> uses operators <code class="computeroutput"><span class="special">*</span></code>
and <code class="computeroutput"><span class="special">-&gt;</span></code> to lexically warn
about the possibly uninitialized state appealing to the familiar pointer
semantics w.r.t. to null pointers.
</p>
<div class="warning"><table border="0" summary="Warning">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../../../../../doc/src/images/warning.png"></td>
<th align="left">Warning</th>
</tr>
<tr><td align="left" valign="top"><p>
However, it is particularly important to note that <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;&gt;</span></code> objects are not pointers. <span class="underline"><code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;&gt;</span></code> is not, and does not model, a
pointer</span>.
</p></td></tr>
</table></div>
<p>
For instance, <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;&gt;</span></code> does not have shallow-copy so does
not alias: two different optionals never refer to the <span class="emphasis"><em>same</em></span>
value unless <code class="computeroutput"><span class="identifier">T</span></code> itself is
a reference (but may have <span class="emphasis"><em>equivalent</em></span> values). The
difference between an <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code> and a pointer must be kept in mind,
particularly because the semantics of relational operators are different:
since <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
is a value-wrapper, relational operators are deep: they compare optional
values; but relational operators for pointers are shallow: they do not
compare pointee values. As a result, you might be able to replace <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
by <code class="computeroutput"><span class="identifier">T</span><span class="special">*</span></code>
on some situations but not always. Specifically, on generic code written
for both, you cannot use relational operators directly, and must use the
template functions <a href="../../../../../../utility/OptionalPointee.html#equal" target="_top"><code class="computeroutput"><span class="identifier">equal_pointees</span><span class="special">()</span></code></a>
and <a href="../../../../../../utility/OptionalPointee.html#less" target="_top"><code class="computeroutput"><span class="identifier">less_pointees</span><span class="special">()</span></code></a>
instead.
</p>
</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 &#169; 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright &#169; 2014 Andrzej Krzemie&#324;ski<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>
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<title>The semantics</title>
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<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_optional.tutorial.design_overview.the_semantics"></a><a class="link" href="the_semantics.html" title="The semantics">The
semantics</a>
</h4></div></div></div>
<p>
Objects of type <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code> are intended to be used in places where
objects of type <code class="computeroutput"><span class="identifier">T</span></code> would
but which might be uninitialized. Hence, <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>'s purpose is to formalize the additional
possibly uninitialized state. From the perspective of this role, <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
can have the same operational semantics of <code class="computeroutput"><span class="identifier">T</span></code>
plus the additional semantics corresponding to this special state. As such,
<code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
could be thought of as a <span class="emphasis"><em>supertype</em></span> of <code class="computeroutput"><span class="identifier">T</span></code>. Of course, we can't do that in C++,
so we need to compose the desired semantics using a different mechanism.
Doing it the other way around, that is, making <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code> a <span class="emphasis"><em>subtype</em></span> of
<code class="computeroutput"><span class="identifier">T</span></code> is not only conceptually
wrong but also impractical: it is not allowed to derive from a non-class
type, such as a built-in type.
</p>
<p>
We can draw from the purpose of <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code> the required basic semantics:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
<span class="bold"><strong>Default Construction:</strong></span> To introduce
a formally uninitialized wrapped object.
</li>
<li class="listitem">
<span class="bold"><strong>Direct Value Construction via copy:</strong></span>
To introduce a formally initialized wrapped object whose value is obtained
as a copy of some object.
</li>
<li class="listitem">
<span class="bold"><strong>Deep Copy Construction:</strong></span> To obtain
a new yet equivalent wrapped object.
</li>
<li class="listitem">
<span class="bold"><strong>Direct Value Assignment (upon initialized):</strong></span>
To assign a value to the wrapped object.
</li>
<li class="listitem">
<span class="bold"><strong>Direct Value Assignment (upon uninitialized):</strong></span>
To initialize the wrapped object with a value obtained as a copy of
some object.
</li>
<li class="listitem">
<span class="bold"><strong>Assignment (upon initialized):</strong></span> To
assign to the wrapped object the value of another wrapped object.
</li>
<li class="listitem">
<span class="bold"><strong>Assignment (upon uninitialized):</strong></span> To
initialize the wrapped object with value of another wrapped object.
</li>
<li class="listitem">
<span class="bold"><strong>Deep Relational Operations (when supported by
the type T):</strong></span> To compare wrapped object values taking into
account the presence of uninitialized states.
</li>
<li class="listitem">
<span class="bold"><strong>Value access:</strong></span> To unwrap the wrapped
object.
</li>
<li class="listitem">
<span class="bold"><strong>Initialization state query:</strong></span> To determine
if the object is formally initialized or not.
</li>
<li class="listitem">
<span class="bold"><strong>Swap:</strong></span> To exchange wrapped objects.
(with whatever exception safety guarantees are provided by <code class="computeroutput"><span class="identifier">T</span></code>'s swap).
</li>
<li class="listitem">
<span class="bold"><strong>De-initialization:</strong></span> To release the
wrapped object (if any) and leave the wrapper in the uninitialized
state.
</li>
</ul></div>
<p>
Additional operations are useful, such as converting constructors and converting
assignments, in-place construction and assignment, and safe value access
via a pointer to the wrapped object or null.
</p>
</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 &#169; 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright &#169; 2014 Andrzej Krzemie&#324;ski<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>
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