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26<div class="titlepage"><div><div><h3 class="title">
27<a name="boost_optional.tutorial.when_to_use_optional"></a><a class="link" href="when_to_use_optional.html" title="When to use Optional">When to
28      use Optional</a>
29</h3></div></div></div>
30<p>
31        It is recommended to use <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
32        in situations where there is exactly one, clear (to all parties) reason for
33        having no value of type <code class="computeroutput"><span class="identifier">T</span></code>,
34        and where the lack of value is as natural as having any regular value of
35        <code class="computeroutput"><span class="identifier">T</span></code>. One example of such situation
36        is asking the user in some GUI form to optionally specify some limit on an
37        <code class="computeroutput"><span class="keyword">int</span></code> value, but the user is allowed
38        to say 'I want the number not to be constrained by the maximum'. For another
39        example, consider a config parameter specifying how many threads the application
40        should launch. Leaving this parameter unspecified means that the application
41        should decide itself. For yet another example, consider a function returning
42        the index of the smallest element in a <code class="computeroutput"><span class="identifier">vector</span></code>.
43        We need to be prepared for the situation, where the <code class="computeroutput"><span class="identifier">vector</span></code>
44        is empty. Therefore a natural signature for such function would be:
45      </p>
46<pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span>
47<span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">size_t</span><span class="special">&gt;</span> <span class="identifier">find_smallest_elem</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;&amp;</span> <span class="identifier">vec</span><span class="special">);</span>
48</pre>
49<p>
50        Here, having received an empty <code class="computeroutput"><span class="identifier">vec</span></code>
51        and having no <code class="computeroutput"><span class="identifier">size_t</span></code> to return
52        is not a <span class="emphasis"><em>failure</em></span> but a <span class="emphasis"><em>normal</em></span>,
53        albeit irregular, situation.
54      </p>
55<p>
56        Another typical situation is to indicate that we do not have a value yet,
57        but we expect to have it later. This notion can be used in implementing solutions
58        like lazy initialization or a two-phase initialization.
59      </p>
60<p>
61        <code class="computeroutput"><span class="identifier">optional</span></code> can be used to take
62        a non-<a href="http://www.sgi.com/tech/stl/DefaultConstructible.html" target="_top"><code class="computeroutput"><span class="identifier">DefaultConstructible</span></code></a> type <code class="computeroutput"><span class="identifier">T</span></code> and create a sibling type with a default
63        constructor. This is a way to add a <span class="emphasis"><em>null-state</em></span> to any
64        type that doesn't have it already.
65      </p>
66<p>
67        Sometimes type <code class="computeroutput"><span class="identifier">T</span></code> already
68        provides a built-in null-state, but it may still be useful to wrap it into
69        <code class="computeroutput"><span class="identifier">optional</span></code>. Consider <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code>.
70        When you read a piece of text from a GUI form or a DB table, it is hardly
71        ever that the empty string indicates anything else but a missing text. And
72        some data bases do not even distinguish between a null string entry and a
73        non-null string of length 0. Still, it may be practical to use <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">string</span><span class="special">&gt;</span></code>
74        to indicate in the returned type that we want to treat the empty string in
75        a special dedicated program path:
76      </p>
77<pre class="programlisting"><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="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&gt;</span> <span class="identifier">name</span> <span class="special">=</span> <span class="identifier">ask_user_name</span><span class="special">())</span> <span class="special">{</span>
78    <span class="identifier">assert</span><span class="special">(*</span><span class="identifier">name</span> <span class="special">!=</span> <span class="string">""</span><span class="special">);</span>
79    <span class="identifier">logon_as</span><span class="special">(*</span><span class="identifier">name</span><span class="special">);</span>
80<span class="special">}</span>
81<span class="keyword">else</span> <span class="special">{</span>
82    <span class="identifier">skip_logon</span><span class="special">();</span>
83<span class="special">}</span>
84</pre>
85<p>
86        In the example above, the assertion indicates that if we choose to use this
87        technique, we must translate the empty string state to an optional object
88        with no contained value (inside function <code class="computeroutput"><span class="identifier">ask_user_name</span></code>).
89      </p>
90<h5>
91<a name="boost_optional.tutorial.when_to_use_optional.h0"></a>
92        <span class="phrase"><a name="boost_optional.tutorial.when_to_use_optional.not_recommended_usages"></a></span><a class="link" href="when_to_use_optional.html#boost_optional.tutorial.when_to_use_optional.not_recommended_usages">Not
93        recommended usages</a>
94      </h5>
95<p>
96        It is not recommended to use <code class="computeroutput"><span class="identifier">optional</span></code>
97        to indicate that we were not able to compute a value because of a <span class="emphasis"><em>failure</em></span>.
98        It is difficult to define what a failure is, but it usually has one common
99        characteristic: an associated information on the cause of the failure. This
100        can be the type and member data of an exception object, or an error code.
101        It is a bad design to signal a failure and not inform about the cause. If
102        you do not want to use exceptions, and do not like the fact that by returning
103        error codes you cannot return the computed value, you can use <a href="https://github.com/ptal/Boost.Expected" target="_top">Expected</a>
104        library. It is sort of <a href="../../../../../variant/index.html" target="_top">Boost.Variant</a>
105        that contains either a computed value or a reason why the computation failed.
106      </p>
107<p>
108        Sometimes the distinction into what is a failure and what is a valid but
109        irregular result is blurry and depends on a particular usage and personal
110        preference. Consider a function that converts a <code class="computeroutput"><span class="identifier">string</span></code>
111        to an <code class="computeroutput"><span class="keyword">int</span></code>. Is it a failure that
112        you cannot convert? It might in some cases, but in other you may call it
113        exactly for the purpose of figuring out if a given <code class="computeroutput"><span class="identifier">string</span></code>
114        is convertible, and you are not even interested in the resulting value. Sometimes
115        when a conversion fails you may not consider it a failure, but you need to
116        know why it cannot be converted; for instance at which character it is determined
117        that the conversion is impossible. In this case returning <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
118        will not suffice. Finally, there is a use case where an input string that
119        does not represent an <code class="computeroutput"><span class="keyword">int</span></code> is
120        not a failure condition, but during the conversion we use resources whose
121        acquisition may fail. In that case the natural representation is to both
122        return <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span></code> and
123        signal failure:
124      </p>
125<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">convert1</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">string</span><span class="special">&amp;</span> <span class="identifier">str</span><span class="special">);</span> <span class="comment">// throws</span>
126<span class="identifier">expected</span><span class="special">&lt;</span><span class="identifier">ErrorT</span><span class="special">,</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;&gt;</span> <span class="identifier">convert2</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">string</span><span class="special">&amp;</span> <span class="identifier">str</span><span class="special">);</span> <span class="comment">// return either optional or error</span>
127</pre>
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129<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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131<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2018 Andrzej Krzemieński<p>
132        Distributed under the Boost Software License, Version 1.0. (See accompanying
133        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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