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<title>No-value policies - Boost.Outcome documentation</title>
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<div class="titlepage"><div><div><h1 style="clear: both">No-value policies</h1></div></div></div>
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<p>In the previous section we have seen that it would be useful if calling member
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function <code>.value()</code> on object of type <code>outcome<T></code> that did not contain a value,
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would cause an exception to be thrown according to some user-defined policy.</p>
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<p>Let us consider <code>result<T></code> first. It is an alias to <a href="../../reference/types/basic_result.html" class="api-reference"><code>basic_result<T, E, NoValuePolicy></code></a>
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,
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where <code>E</code> is the type storing error information and defaulted to
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<code>std::error_code</code>/<code>boost::system::error_code</code>, and <code>NoValuePolicy</code>
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is a <em>no-value policy</em> defaulted to <a href="../../reference/aliases/default_policy.html" class="api-reference"><code>default_policy<T, EC, EP></code></a>
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.</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>NoValuePolicy::wide_value_check(*this)</code>.</li>
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<li>Return a reference to the contained value. If no value is actually stored,
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your program has entered undefined behaviour.</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><sup class="footnote-ref" id="fnref:1"><a href="#fn:1">1</a></sup> 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 <a href="../../reference/types/basic_result.html" class="api-reference"><code>basic_result<T, E, NoValuePolicy></code></a>
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and <a href="../../reference/types/basic_outcome.html" class="api-reference"><code>basic_outcome<T, EC, EP, NoValuePolicy></code></a></p>
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<p>never use exceptions. Any exception-related logic is provided exclusively
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through no-value 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 <a href="../../tutorial/essential/no-value/builtin.html">use one of the predefined policies</a>.</p>
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<p>You can also use one of the two other predefined aliases for <code>basic_result<></code>:</p>
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<ul>
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<li><a href="../../reference/aliases/unchecked.html" class="api-reference"><code>unchecked<T, E = varies></code></a>
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: it uses policy <a href="../../reference/policies/all_narrow.html" class="api-reference"><code>all_narrow</code></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/aliases/checked.html" class="api-reference"><code>checked<T, E = varies></code></a>
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:
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it uses policy <a href="../../reference/policies/throw_bad_result_access.html" class="api-reference"><code>throw_bad_result_access<EC></code></a>
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, where any observation of a missing value or error throws <a href="../../reference/types/bad_result_access_with.html" class="api-reference"><code>bad_result_access_with<EC></code></a>
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or <a href="../../reference/types/bad_result_access.html" class="api-reference"><code>bad_result_access</code></a>
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respectively.</li>
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</ul>
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<div class="footnotes">
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<hr />
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<ol>
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<li id="fn:1">Similar overloads exist for throwing <code>boost::system::system_error</code> when <code>EC</code> is <code>boost::system::error_code</code>.
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<a class="footnote-return" href="#fnref:1"><sup>[return]</sup></a></li>
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</ol>
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</div>
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</div><p><small>Last revised: February 08, 2019 at 22:18:08 UTC</small></p>
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