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| <dt><span class="section"><a href="tutorial.html#boost_optional.tutorial.motivation">Motivation</a></span></dt> |
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| use Optional</a></span></dt> |
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| operators</a></span></dt> |
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| references</a></span></dt> |
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| Factories</a></span></dt> |
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| Safety Guarantees</a></span></dt> |
| <dt><span class="section"><a href="../boost_optional/tutorial/type_requirements.html">Type requirements</a></span></dt> |
| <dt><span class="section"><a href="../boost_optional/tutorial/performance_considerations.html">Performance |
| considerations</a></span></dt> |
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| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="boost_optional.tutorial.motivation"></a><a class="link" href="tutorial.html#boost_optional.tutorial.motivation" title="Motivation">Motivation</a> |
| </h3></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> |
| </div> |
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| <td align="left"></td> |
| <td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2018 Andrzej Krzemieński<p> |
| Distributed under the Boost Software License, Version 1.0. (See accompanying |
| file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) |
| </p> |
| </div></td> |
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