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Fixed code, updated docs, added emplace()

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Andrzej Krzemienski
2014-06-03 17:36:18 +02:00
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38
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@ -1080,6 +1080,42 @@
</pre>
</li>
</ul></div>
<p>
<span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
</p>
<a name="reference_optional_emplace"></a><div class="blockquote"><blockquote class="blockquote"><p>
<code class="computeroutput"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span><span class="special">...</span> <span class="identifier">Args</span><span class="special">&gt;</span> <span class="keyword">void</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span></code>
<span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">&gt;::</span><span class="identifier">emplace</span><span class="special">(</span> <span class="identifier">Args</span><span class="special">...&amp;&amp;</span>
<span class="identifier">args</span> <span class="special">);</span></code>
</p></blockquote></div>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
<span class="bold"><strong>Requires:</strong></span> The compiler supports rvalue
references and variadic templates.
</li>
<li class="listitem">
<span class="bold"><strong>Effect:</strong></span> If <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is initialized calls <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span> <span class="special">=</span>
<span class="identifier">none</span></code>. Then initializes in-place
the contained value as if direct-initializing an object of type <code class="computeroutput"><span class="identifier">T</span></code> with <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">forward</span><span class="special">&lt;</span><span class="identifier">Args</span><span class="special">&gt;(</span><span class="identifier">args</span><span class="special">)...</span></code>.
</li>
<li class="listitem">
<span class="bold"><strong>Postconditions: </strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is <span class="underline">initialized</span>.
</li>
<li class="listitem">
<span class="bold"><strong>Throws:</strong></span> Whatever the selected <code class="computeroutput"><span class="identifier">T</span></code>'s constructor throws.
</li>
<li class="listitem">
<span class="bold"><strong>Notes:</strong></span> <code class="computeroutput"><span class="identifier">T</span></code>
need not be <code class="computeroutput"><span class="identifier">MoveConstructible</span></code>
or <code class="computeroutput"><span class="identifier">MoveAssignable</span></code>.
</li>
<li class="listitem">
<span class="bold"><strong>Exception Safety:</strong></span> If an exception is thrown
during the initialization of <code class="computeroutput"><span class="identifier">T</span></code>,
<code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
is <span class="emphasis"><em>uninitialized</em></span>.
</li>
</ul></div>
<p>
<span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
</p>
@ -1232,7 +1268,7 @@
<span class="identifier">assert</span> <span class="special">(</span> <span class="keyword">false</span> <span class="special">);</span>
<span class="special">}</span>
<span class="keyword">catch</span><span class="special">(</span><span class="identifier">bad_optional_access</span><span class="special">&amp;)</span> <span class="special">{</span>
<span class="identifier">asert</span> <span class="special">(</span> <span class="keyword">true</span> <span class="special">);</span>
<span class="identifier">assert</span> <span class="special">(</span> <span class="keyword">true</span> <span class="special">);</span>
<span class="special">}</span>
</pre>
<span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>

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@ -6,7 +6,7 @@
<meta name="generator" content="DocBook XSL Stylesheets V1.78.1">
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@ -20,7 +20,7 @@
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@ -39,12 +39,12 @@
In C++, we can <span class="emphasis"><em>declare</em></span> an object (a variable) of type
<code class="computeroutput"><span class="identifier">T</span></code>, and we can give this variable
an <span class="emphasis"><em>initial value</em></span> (through an <span class="emphasis"><em>initializer</em></span>.
(c.f. 8.5)). When a declaration includes a non-empty initializer (an initial
(cf. 8.5)). When a declaration includes a non-empty initializer (an initial
value is given), it is said that the object has been initialized. If the
declaration uses an empty initializer (no initial value is given), and neither
default nor value initialization applies, it is said that the object is
<span class="bold"><strong>uninitialized</strong></span>. Its actual value exist but
has an <span class="emphasis"><em>indeterminate initial value</em></span> (c.f. 8.5.9). <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code> intends
has an <span class="emphasis"><em>indeterminate initial value</em></span> (cf. 8.5/11). <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code> intends
to formalize the notion of initialization (or lack of it) allowing a program
to test whether an object has been initialized and stating that access to
the value of an uninitialized object is undefined behavior. That is, when
@ -67,13 +67,12 @@
special value: <code class="computeroutput"><span class="identifier">EOF</span></code>, <code class="computeroutput"><span class="identifier">npos</span></code>, -1, etc... This is equivalent to
adding the special value to the set of possible values of a given type. This
super set of <code class="computeroutput"><span class="identifier">T</span></code> plus some
<span class="emphasis"><em>nil_t</em></span>&#8212;were <code class="computeroutput"><span class="identifier">nil_t</span></code>
is some stateless POD-can be modeled in modern languages as a <span class="bold"><strong>discriminated
union</strong></span> of T and nil_t. Discriminated unions are often called <span class="emphasis"><em>variants</em></span>.
A variant has a <span class="emphasis"><em>current type</em></span>, which in our case is either
<code class="computeroutput"><span class="identifier">T</span></code> or <code class="computeroutput"><span class="identifier">nil_t</span></code>.
Using the <a href="../../../../variant/index.html" target="_top">Boost.Variant</a>
library, this model can be implemented in terms of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">variant</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span><span class="identifier">nil_t</span><span class="special">&gt;</span></code>.
<span class="emphasis"><em>nil_t</em></span>&#8212;where <code class="computeroutput"><span class="identifier">nil_t</span></code>
is some stateless POD&#8212;can be modeled in modern languages as a <span class="bold"><strong>discriminated union</strong></span> of T and nil_t. Discriminated
unions are often called <span class="emphasis"><em>variants</em></span>. A variant has a <span class="emphasis"><em>current
type</em></span>, which in our case is either <code class="computeroutput"><span class="identifier">T</span></code>
or <code class="computeroutput"><span class="identifier">nil_t</span></code>. Using the <a href="../../../../variant/index.html" target="_top">Boost.Variant</a> library, this model
can be implemented in terms of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">variant</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span><span class="identifier">nil_t</span><span class="special">&gt;</span></code>.
There is precedent for a discriminated union as a model for an optional value:
the <a href="http://www.haskell.org/" target="_top">Haskell</a> <span class="bold"><strong>Maybe</strong></span>
built-in type constructor. Thus, a discriminated union <code class="computeroutput"><span class="identifier">T</span><span class="special">+</span><span class="identifier">nil_t</span></code>
@ -329,8 +328,8 @@
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 conversion to <code class="computeroutput"><span class="keyword">bool</span></code> to convey
the notion of optionality.
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>
@ -407,7 +406,7 @@
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<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="boost_optional.discussion"></a><a class="link" href="discussion.html" title="Discussion">Discussion</a>
</h2></div></div></div>
<p>
Consider these functions which should return a value but which might not have
a value to return:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
(A) <code class="computeroutput"><span class="keyword">double</span> <span class="identifier">sqrt</span><span class="special">(</span><span class="keyword">double</span> <span class="identifier">n</span> <span class="special">);</span></code>
</li>
<li class="listitem">
(B) <code class="computeroutput"><span class="keyword">char</span> <span class="identifier">get_async_input</span><span class="special">();</span></code>
</li>
<li class="listitem">
(C) <code class="computeroutput"><span class="identifier">point</span> <span class="identifier">polygon</span><span class="special">::</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span></code>
</li>
</ul></div>
<p>
There are different approaches to the issue of not having a value to return.
</p>
<p>
A typical approach is to consider the existence of a valid return value as
a postcondition, so that if the function cannot compute the value to return,
it has either undefined behavior (and can use assert in a debug build) or uses
a runtime check and throws an exception if the postcondition is violated. This
is a reasonable choice for example, for function (A), because the lack of a
proper return value is directly related to an invalid parameter (out of domain
argument), so it is appropriate to require the callee to supply only parameters
in a valid domain for execution to continue normally.
</p>
<p>
However, function (B), because of its asynchronous nature, does not fail just
because it can't find a value to return; so it is incorrect to consider such
a situation an error and assert or throw an exception. This function must return,
and somehow, must tell the callee that it is not returning a meaningful value.
</p>
<p>
A similar situation occurs with function (C): it is conceptually an error to
ask a <span class="emphasis"><em>null-area</em></span> polygon to return a point inside itself,
but in many applications, it is just impractical for performance reasons to
treat this as an error (because detecting that the polygon has no area might
be too expensive to be required to be tested previously), and either an arbitrary
point (typically at infinity) is returned, or some efficient way to tell the
callee that there is no such point is used.
</p>
<p>
There are various mechanisms to let functions communicate that the returned
value is not valid. One such mechanism, which is quite common since it has
zero or negligible overhead, is to use a special value which is reserved to
communicate this. Classical examples of such special values are <code class="computeroutput"><span class="identifier">EOF</span></code>, <code class="computeroutput"><span class="identifier">string</span><span class="special">::</span><span class="identifier">npos</span></code>, points
at infinity, etc...
</p>
<p>
When those values exist, i.e. the return type can hold all meaningful values
<span class="emphasis"><em>plus</em></span> the <span class="emphasis"><em>signal</em></span> value, this mechanism
is quite appropriate and well known. Unfortunately, there are cases when such
values do not exist. In these cases, the usual alternative is either to use
a wider type, such as <code class="computeroutput"><span class="keyword">int</span></code> in place
of <code class="computeroutput"><span class="keyword">char</span></code>; or a compound type, such
as <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special">&lt;</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">&gt;</span></code>.
</p>
<p>
Returning a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span><span class="keyword">bool</span><span class="special">&gt;</span></code>, thus attaching a boolean flag to the result
which indicates if the result is meaningful, has the advantage that can be
turned into a consistent idiom since the first element of the pair can be whatever
the function would conceptually return. For example, the last two functions
could have the following interface:
</p>
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special">&lt;</span><span class="keyword">char</span><span class="special">,</span><span class="keyword">bool</span><span class="special">&gt;</span> <span class="identifier">get_async_input</span><span class="special">();</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special">&lt;</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">&gt;</span> <span class="identifier">polygon</span><span class="special">::</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span>
</pre>
<p>
These functions use a consistent interface for dealing with possibly nonexistent
results:
</p>
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special">&lt;</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">&gt;</span> <span class="identifier">p</span> <span class="special">=</span> <span class="identifier">poly</span><span class="special">.</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span>
<span class="keyword">if</span> <span class="special">(</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">second</span> <span class="special">)</span>
<span class="identifier">flood_fill</span><span class="special">(</span><span class="identifier">p</span><span class="special">.</span><span class="identifier">first</span><span class="special">);</span>
</pre>
<p>
However, not only is this quite a burden syntactically, it is also error prone
since the user can easily use the function result (first element of the pair)
without ever checking if it has a valid value.
</p>
<p>
Clearly, we need a better idiom.
</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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@ -118,6 +118,13 @@
This also applies to move assignments/constructors. However, move operations
are made no-throw more often.
</p>
<p>
Operation <code class="computeroutput"><span class="identifier">emplace</span></code> provides
basic exception safety guarantee. If it throws, the optional object becomes
uninitialized regardless of its initial state, and its previous contained value
(if any) is destroyed. It doesn't call any assignment or move/copy constructor
on <code class="computeroutput"><span class="identifier">T</span></code>.
</p>
<h4>
<a name="boost_optional.exception_safety_guarantees.h0"></a>
<span class="phrase"><a name="boost_optional.exception_safety_guarantees.swap"></a></span><a class="link" href="exception_safety_guarantees.html#boost_optional.exception_safety_guarantees.swap">Swap</a>

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@ -74,6 +74,8 @@
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">&gt;</span> <span class="identifier">optional</span><span class="special">&amp;</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">U</span><span class="special">&gt;&amp;&amp;</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_move_equal_other_optional"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span><span class="special">...</span> <span class="identifier">Args</span><span class="special">&gt;</span> <span class="keyword">void</span> <span class="identifier">emplace</span> <span class="special">(</span> <span class="identifier">Args</span><span class="special">...&amp;&amp;</span> <span class="identifier">args</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_emplace"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">InPlaceFactory</span><span class="special">&gt;</span> <span class="identifier">optional</span><span class="special">&amp;</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">InPlaceFactory</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">f</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_equal_factory"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">TypedInPlaceFactory</span><span class="special">&gt;</span> <span class="identifier">optional</span><span class="special">&amp;</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">TypedInPlaceFactory</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">f</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_equal_factory"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>

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<a name="boost_optional.tutorial"></a><a class="link" href="tutorial.html" title="Tutorial">Tutorial</a>
</h2></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="tutorial.html#boost_optional.tutorial.optional_return_values">Optional
return values</a></span></dt>
<dt><span class="section"><a href="tutorial.html#boost_optional.tutorial.optional_data_members">Optional
data members</a></span></dt>
<dt><span class="section"><a href="tutorial.html#boost_optional.tutorial.bypassing_unnecessary_default_construction">Bypassing
unnecessary default construction</a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<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
return values</a>
</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>
<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>
<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>
<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>
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="special">*</span><span class="identifier">convert</span><span class="special">(</span><span class="string">"100"</span><span class="special">);</span>
</pre>
<p>
This is because we know that string value <code class="computeroutput"><span class="string">"100"</span></code>
converts to a valid value of <code class="computeroutput"><span class="keyword">int</span></code>.
If you do not like this potential UB, you can use an alternative way of extracting
the contained value:
</p>
<pre class="programlisting"><span class="keyword">try</span> <span class="special">{</span>
<span class="keyword">int</span> <span class="identifier">j</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">text</span><span class="special">).</span><span class="identifier">value</span><span class="special">();</span>
<span class="special">}</span>
<span class="keyword">catch</span> <span class="special">(</span><span class="keyword">const</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">bad_optional_access</span><span class="special">&amp;)</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">&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>
<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="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">&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>
<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">&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="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">&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 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">&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>
</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 &#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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@ -27,9 +27,17 @@
<a name="boost_optional.type_requirements"></a><a class="link" href="type_requirements.html" title="Type requirements">Type requirements</a>
</h2></div></div></div>
<p>
In general, <code class="computeroutput"><span class="identifier">T</span></code> must be <code class="computeroutput"><span class="identifier">MoveConstructible</span></code> and have a no-throw destructor.
The <code class="computeroutput"><span class="identifier">MoveConstructible</span></code> requirement
is not needed if <span class="bold"><strong>InPlaceFactories</strong></span> are used.
At the very minimum 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>
to work with a minimum interface it is required that <code class="computeroutput"><span class="identifier">T</span></code>
has a publicly accessible no-throw destructor. In that case you need to initialize
the optional object with function <code class="computeroutput"><span class="identifier">emplace</span><span class="special">()</span></code> or use <span class="bold"><strong>InPlaceFactories</strong></span>.
Additionally, if <code class="computeroutput"><span class="identifier">T</span></code> is <code class="computeroutput"><span class="identifier">Moveable</span></code>, <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 also <code class="computeroutput"><span class="identifier">Moveable</span></code> and can be
easily initialized from an rvalue of type <code class="computeroutput"><span class="identifier">T</span></code>
and be passed by value. Additionally if <code class="computeroutput"><span class="identifier">T</span></code>
is <code class="computeroutput"><span class="identifier">Copyable</span></code>, <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
also <code class="computeroutput"><span class="identifier">Copyable</span></code> and can be easily
initialized from an lvalue of type <code class="computeroutput"><span class="identifier">T</span></code>.
</p>
<p>
<code class="computeroutput"><span class="identifier">T</span></code> <span class="underline">is

73
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@ -5,7 +5,7 @@
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<div class="chapter">
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@ -38,7 +38,20 @@
<p><b>Table of Contents</b></p>
<dl class="toc">
<dt><span class="section"><a href="index.html#optional.introduction">Introduction</a></span></dt>
<dt><span class="section"><a href="boost_optional/motivation.html">Motivation</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="index.html#optional.introduction.problem">Problem</a></span></dt>
<dt><span class="section"><a href="index.html#optional.introduction.solution">Solution</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="boost_optional/tutorial.html">Tutorial</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="boost_optional/tutorial.html#boost_optional.tutorial.optional_return_values">Optional
return values</a></span></dt>
<dt><span class="section"><a href="boost_optional/tutorial.html#boost_optional.tutorial.optional_data_members">Optional
data members</a></span></dt>
<dt><span class="section"><a href="boost_optional/tutorial.html#boost_optional.tutorial.bypassing_unnecessary_default_construction">Bypassing
unnecessary default construction</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="boost_optional/discussion.html">Discussion</a></span></dt>
<dt><span class="section"><a href="boost_optional/development.html">Development</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="boost_optional/development.html#boost_optional.development.the_models">The models</a></span></dt>
@ -78,24 +91,60 @@
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="optional.introduction"></a><a class="link" href="index.html#optional.introduction" title="Introduction">Introduction</a>
</h2></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="index.html#optional.introduction.problem">Problem</a></span></dt>
<dt><span class="section"><a href="index.html#optional.introduction.solution">Solution</a></span></dt>
</dl></div>
<p>
This library can be used to represent 'optional' (or 'nullable') objects that
can be safely passed by value:
Class template <code class="computeroutput"><span class="identifier">optional</span></code> is
a wrapper for representing 'optional' (or 'nullable') objects who may not (yet)
contain a valid value. Optional objects offer full value semantics; they are
good for passing by value and usage inside STL containers. This is a header-only
library.
</p>
<pre class="programlisting"><span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">readInt</span><span class="special">();</span> <span class="comment">// this function may return either an int or a not-an-int</span>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="optional.introduction.problem"></a><a class="link" href="index.html#optional.introduction.problem" title="Problem">Problem</a>
</h3></div></div></div>
<p>
Suppose we want to read a parameter form a config file which represents some
integral value, let's call it <code class="computeroutput"><span class="string">"MaxValue"</span></code>.
It is possible that this parameter is not specified; such situation is no
error. It is valid to not specify the parameter and in that case the program
is supposed to behave slightly different. Also suppose that any possible
value of type <code class="computeroutput"><span class="keyword">int</span></code> is a valid
value for <code class="computeroutput"><span class="string">"MaxValue"</span></code>,
so we cannot jut use <code class="computeroutput"><span class="special">-</span><span class="number">1</span></code>
to represent the absence of the parameter in the config file.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="optional.introduction.solution"></a><a class="link" href="index.html#optional.introduction.solution" title="Solution">Solution</a>
</h3></div></div></div>
<p>
This is how you solve it with <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span></code>:
</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="keyword">if</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">oi</span> <span class="special">=</span> <span class="identifier">readInt</span><span class="special">())</span> <span class="comment">// did I get a real int</span>
<span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"my int is: "</span> <span class="special">&lt;&lt;</span> <span class="special">*</span><span class="identifier">oi</span><span class="special">;</span> <span class="comment">// use my int</span>
<span class="keyword">else</span>
<span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"I have no int"</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">getConfigParam</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">name</span><span class="special">);</span> <span class="comment">// return either an int or a `not-an-int`</span>
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">oi</span> <span class="special">=</span> <span class="identifier">getConfigParam</span><span class="special">(</span><span class="string">"MaxValue"</span><span class="special">))</span> <span class="comment">// did I get a real int?</span>
<span class="identifier">runWithMax</span><span class="special">(*</span><span class="identifier">oi</span><span class="special">);</span> <span class="comment">// use my int</span>
<span class="keyword">else</span>
<span class="identifier">runWithNoMax</span><span class="special">();</span>
<span class="special">}</span>
</pre>
</div>
</div>
</div>
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<td align="left"><p><small>Last revised: May 23, 2014 at 14:35:08 GMT</small></p></td>
<td align="left"><p><small>Last revised: June 03, 2014 at 14:35:30 GMT</small></p></td>
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