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<title>value_or_none<T> - Boost.Outcome documentation</title>
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<div class="titlepage"><div><div><h1 style="clear: both"><code>value_or_none<T></code></h1></div></div></div>
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<p>If on C++ 20 or the Concepts TS is enabled, a boolean concept matching types with a public <code>.has_value()</code> observer which returns <code>bool</code>, and a public <code>.value()</code> observer function.</p>
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<p>If without Concepts, a static constexpr bool which is true for types matching the same requirements, using a SFINAE based emulation.</p>
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<p>This concept matches optional-like types such as
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<a href="https://en.cppreference.com/w/cpp/utility/optional" class="api-reference" target="_blank"><i class="fa fa-book" aria-hidden="true"></i> <code>std::optional<T></code></a>
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. Note it also matches
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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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, which also has an optional-like interface. You may thus wish to preferentially match <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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for any given <code>T</code>.</p>
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<p><em>Namespace</em>: <code>BOOST_OUTCOME_V2_NAMESPACE::concepts</code></p>
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<p><em>Header</em>: <code><boost/outcome/convert.hpp></code></p>
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<p><em>Legacy</em>: This was named <code>convert::ValueOrNone<T></code> in Outcome v2.1 and earlier. Define <a href="../../reference/macros/enable_legacy_support_for.html" class="api-reference"><code>BOOST_OUTCOME_ENABLE_LEGACY_SUPPORT_FOR</code></a>
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to less than <code>220</code> to enable.</p>
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</div><p><small>Last revised: December 15, 2020 at 12:22:39 UTC</small></p>
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