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<div class="titlepage"><div><div><h2 class="title" style="clear: both">No-value policies</h2></div></div></div>
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<p>In the previous section we have seen that calling member function <code>.value()</code> on
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object of type <code>outcome<T></code> that did not contain a value caused an exception to
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be thrown. This is only part of the story. The full semantics of member function
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<code>.value()</code> are a bit more complex.</p>
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<p>Let’s consider <code>result<T></code> first. It is an alias on <code>basic_result<T, EC, NVP></code>,
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where <code>EC</code> is the type storing error information and defaulted to
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<code>std::error_code</code> and <code>NVP</code> is a <em>no-value policy</em> and defaulted to
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<code>policy::default_policy<T, EC, void></code>.</p>
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<p>The semantics of <code>basic_result::value()</code> are:</p>
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<ol>
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<li>Calls <code>NVP::wide_value_check(*this)</code>.</li>
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<li>If <code>this->has_value() == true</code> then returns a reference to the contained value,
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otherwise the behavior is undefined.</li>
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</ol>
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<p>Thus, the semantics of function <code>.value()</code> depend on the no-value policy. The
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default policy (<code>policy::default_policy<T, EC, void></code>) for <code>EC</code> of type
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<code>std::error_code</code> does the following:</p>
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<ul>
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<li>If <code>r.has_value() == false</code> throws exception <code>std::system_error{r.error()}</code>,</li>
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<li>otherwise no effect.</li>
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</ul>
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<div class="notices note" style="background: url('../images/note.png') top left no-repeat padding-box padding-box;">
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<div class="notices heading">note</div>
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<div class="notices message"><p>Class templates <code>basic_result<T, EC, NVP></code> and <code>basic_outcome<T, EC, EP, NVP></code>
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never use exceptions. Any exception-related logic is provided through no-value
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policies.</p>
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</div>
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</div>
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<p>When designing your own success-or-failure type using templates
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<code>basic_result<></code> or <code>basic_outcome<></code> you have to decide what no-value policy
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you want to use. Either create your own, or use one of the predefined policies.</p>
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<p>You can also use two predefined aliases for <code>basic_result<></code>:</p>
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<ul>
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<li><a href="reference/result/#standardese-outcome_v2_xxx__unchecked-R-S-" class="api-reference"><i class="fa fa-book" aria-hidden="true"></i> unchecked<T, EC></a>
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: it uses policy <a href="reference/policies/all_narrow" class="api-reference"><i class="fa fa-book" aria-hidden="true"></i> all_narrow</a>
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, where any observation of a missing value or error is undefined behavior;</li>
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<li><a href="reference/result/#standardese-outcome_v2_xxx__checked-R-S-" class="api-reference"><i class="fa fa-book" aria-hidden="true"></i> checked<T, EC></a>
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:
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it uses policy <a href="reference/policies/throw_bad_result_access" class="api-reference"><i class="fa fa-book" aria-hidden="true"></i> throw_bad_result_access<EC></a>
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, where any observation of a missing value or error throws <a href="reference/bad_access/#standardese-outcome_v2_xxx__bad_result_access_with-S-" class="api-reference"><i class="fa fa-book" aria-hidden="true"></i> bad_result_access_with<EC></a>
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or <a href="reference/bad_access/#standardese-outcome_v2_xxx__bad_result_access" class="api-reference"><i class="fa fa-book" aria-hidden="true"></i> bad_result_access</a>
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respectively.</li>
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</ul>
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</div><p><small>Last revised: December 05, 2018 at 14:18:52 UTC</small></p>
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