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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"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></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"><</span><span class="identifier">T</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></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"><</span><span class="keyword">char</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></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"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></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"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 Andrzej Krzemieński<p>
+ Distributed under the Boost Software License, Version 1.0. (See accompanying
+ file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
+ </p>
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