1<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> 2<html><meta http-equiv="Content-Type" content="text/html; charset=utf-8"> 3<title>value_or_none<T> - Boost.Outcome documentation</title> 4<link rel="stylesheet" href="../../css/boost.css" type="text/css"> 5<meta name="generator" content="Hugo 0.52 with Boostdoc theme"> 6<meta name="viewport" content="width=device-width,initial-scale=1.0"/> 7 8<link rel="icon" href="../../images/favicon.ico" type="image/ico"/> 9<body><div class="spirit-nav"> 10<a accesskey="p" href="../../reference/concepts/value_or_error.html"><img src="../../images/prev.png" alt="Prev"></a> 11 <a accesskey="u" href="../../reference/concepts.html"><img src="../../images/up.png" alt="Up"></a> 12 <a accesskey="h" href="../../index.html"><img src="../../images/home.png" alt="Home"></a><a accesskey="n" href="../../reference/converters.html"><img src="../../images/next.png" alt="Next"></a></div><div id="content"> 13 <div class="titlepage"><div><div><h1 style="clear: both"><code>value_or_none<T></code></h1></div></div></div> 14 <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> 15 16<p>If without Concepts, a static constexpr bool which is true for types matching the same requirements, using a SFINAE based emulation.</p> 17 18<p>This concept matches optional-like types such as 19<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> 20. Note it also matches 21<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> 22, 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>ValueOrError<T, E></code></a> 23 for any given <code>T</code>.</p> 24 25<p><em>Namespace</em>: <code>BOOST_OUTCOME_V2_NAMESPACE::concepts</code></p> 26 27<p><em>Header</em>: <code><boost/outcome/convert.hpp></code></p> 28 29<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> 30 to <code>210</code> or lower to enable.</p> 31 32 33 </div><p><small>Last revised: May 24, 2020 at 12:48:39 +0100</small></p> 34<hr> 35<div class="spirit-nav"> 36<a accesskey="p" href="../../reference/concepts/value_or_error.html"><img src="../../images/prev.png" alt="Prev"></a> 37 <a accesskey="u" href="../../reference/concepts.html"><img src="../../images/up.png" alt="Up"></a> 38 <a accesskey="h" href="../../index.html"><img src="../../images/home.png" alt="Home"></a><a accesskey="n" href="../../reference/converters.html"><img src="../../images/next.png" alt="Next"></a></div></body> 39</html> 40
