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<title>value_or_error Concept - Boost.Outcome documentation</title>
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<div class="titlepage"><div><div><h1 style="clear: both">value_or_error Concept</h1></div></div></div>
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<p>Something not really mentioned until now is how Outcome interoperates with the proposed
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<a href="https://wg21.link/P0323" class="api-reference" target="_blank"><i class="fa fa-book" aria-hidden="true"></i> P0323 <code>std::expected<T, E></code></a>
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, whose design lands in between <a href="../../../reference/aliases/unchecked.html" class="api-reference"><code>unchecked<T, E = varies></code></a>
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and <a href="../../../reference/aliases/checked.html" class="api-reference"><code>checked<T, E = varies></code></a>
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(both of which are type aliases hard coding no-value
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policies <a href="../../../tutorial/essential/no-value/builtin.html">as previously covered in this tutorial</a>).</p>
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<p>Expected and Outcome are <a href="../../../faq.html#why-doesn-t-outcome-duplicate-std-expected-t-e-s-design">isomorphic to one another in design intent</a>, but interoperation
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for code using Expected and Outcome ought to be seamless thanks to the <a href="https://wg21.link/P0786">proposed <code>ValueOrError</code>
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concept framework</a>, a subset of which Outcome implements.</p>
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<p>The <a href="../../../reference/types/basic_result/explicit_valueorerror_converting_constructor.html" class="api-reference"><code>explicit basic_result(concepts::value_or_error<T, E> &&)</code></a>
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and <a href="../../../reference/types/basic_outcome/explicit_valueorerror_converting_constructor.html" class="api-reference"><code>explicit basic_outcome(concepts::value_or_error<T, E> &&)</code></a>
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constructors will explicitly construct from any type matching the <a href="../../../reference/types/basic_result/explicit_valueorerror_converting_constructor.html" class="api-reference"><code>concepts::value_or_error<T, E></code></a>
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concept, which includes <code>std::expected<A, B></code>, if <code>A</code> is constructible to <code>X</code>, and <code>B</code> is
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constructible to <code>Y</code>. The <code>value_or_error</code> concept in turn is true if and only if the input type has:</p>
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<ol>
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<li>A <code>.has_value()</code> observer returning a <code>bool</code>.</li>
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<li><code>.value()</code> and <code>.error()</code> observers.</li>
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</ol>
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<h2 id="implementation">Implementation</h2>
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<p>Outcome’s machinery for implementing <code>concepts::value_or_error</code> conversion is user extensible by injection
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of specialisations of the <a href="../../../reference/converters/value_or_error.html" class="api-reference"><code>value_or_error<T, U></code></a>
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type into the <code>BOOST_OUTCOME_V2_NAMESPACE::convert</code> namespace.</p>
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<p>Outcome’s default <code>convert::value_or_error<T, U></code> implementation explicitly
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excludes Outcome <code>result</code> and <code>outcome</code> types from the default mechanism as
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there is a major gotcha: the <code>value_or_error</code> matched type’s <code>.value()</code> is often
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not callable in constexpr as it can throw, which makes this conversion mechanism
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pretty much useless for constexpr code. Besides, <code>outcome</code> has a converting
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constructor overload for <code>result</code> inputs which is constexpr capable.</p>
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<p>Note that if you do enable <code>outcome</code> inputs, a <code>result</code> will match an input
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<code>outcome</code>, but silently dropping any exception state. This is probably undesirable.</p>
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<p>Examples of how to implement your own <code>convert::value_or_error<T, U></code> converter
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is demonstrated in the worked example, next.</p>
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</div><p><small>Last revised: December 15, 2020 at 12:22:39 UTC</small></p>
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