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<meta name="author" content="Yaito Kakeyama, Nana Sakisaka">
<title>rvariant</title>
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<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/11.11.1/styles/default.min.css">
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<body class="book toc2 toc-left">
<div id="header">
<h1>rvariant<a href="https://github.com/iris-cpp/iris" target="_blank" rel="noopener"><span class="image"><img src="https://img.shields.io/badge/GitHub-iris-purple?logo=github" alt="GitHub"></span></a></h1>
<div class="details">
<span id="author" class="author">Yaito Kakeyama</span><br>
<span id="author2" class="author">Nana Sakisaka</span><br>
</div>
<div id="toc" class="toc2">
<div id="toctitle">rvariant</div>
<ul class="sectlevel1">
<li><a href="#introduction">Introduction</a>
<ul class="sectlevel2">
<li><a href="#motivation">Motivation</a></li>
<li><a href="#goals">Project Goals</a></li>
<li><a href="#comparison">Comparison of Variant Libraries</a></li>
</ul>
</li>
<li><a href="#rationale">Rationale</a>
<ul class="sectlevel2">
<li><a href="#why-is-recursive-wrapper-needed-1">Why is recursive_wrapper needed?</a></li>
<li><a href="#why-is-recursive-wrapper-needed-2">Why can’t I just use std::unique_ptr instead of recursive_wrapper?</a></li>
<li><a href="#why-is-rvariant-not-a-wrapper-of-std">Why can’t rvariant be a thin wrapper around std::variant?</a></li>
</ul>
</li>
<li><a href="#feature-cheat-sheet">Feature Cheat Sheet</a>
<ul class="sectlevel1">
<li><a href="#basic-features">Basic Features</a></li>
<li><a href="#advanced-features">Advanced Features</a></li>
</ul>
</li>
<li><a href="#rvariant">Reference <span class="slug">[rvariant]</span></a>
<ul class="sectlevel1">
<li><a href="#rvariant.general">General <span class="slug">[rvariant.general]</span></a></li>
<li><a href="#rvariant.syn">Header <iris/rvariant.hpp> synopsis <span class="slug">[rvariant.syn]</span></a></li>
<li><a href="#rvariant.rvariant">Class template <code>rvariant</code> <span class="slug">[rvariant.rvariant]</span></a>
<ul class="sectlevel2">
<li><a href="#rvariant.rvariant.general">General <span class="slug">[rvariant.rvariant.general]</span></a></li>
<li><a href="#rvariant.ctor">Constructors <span class="slug">[rvariant.ctor]</span></a></li>
<li><a href="#rvariant.dtor">Destructor <span class="slug">[rvariant.dtor]</span></a></li>
<li><a href="#rvariant.assign">Assignment <span class="slug">[rvariant.assign]</span></a></li>
<li><a href="#rvariant.mod">Modifiers <span class="slug">[rvariant.mod]</span></a></li>
<li><a href="#rvariant.status">Value status <span class="slug">[rvariant.status]</span></a></li>
<li><a href="#rvariant.subset">Subset <span class="slug">[rvariant.subset]</span></a></li>
<li><a href="#rvariant.swap">Swap <span class="slug">[rvariant.swap]</span></a></li>
</ul>
</li>
<li><a href="#rvariant.helper"><code>rvariant</code> helper classes <span class="slug">[rvariant.helper]</span></a></li>
<li><a href="#rvariant.flex">Flexibility traits <span class="slug">[rvariant.flex]</span></a></li>
<li><a href="#rvariant.get">Value access <span class="slug">[rvariant.get]</span></a></li>
<li><a href="#rvariant.visit">Visitation <span class="slug">[rvariant.visit]</span></a></li>
<li><a href="#rvariant.hash">Hash support <span class="slug">[rvariant.hash]</span></a></li>
<li><a href="#rvariant.io">I/O <span class="slug">[rvariant.io]</span></a>
<ul class="sectlevel2">
<li><a href="#rvariant.io.ostream"><code>operator<<</code> support</a></li>
<li><a href="#rvariant.io.format"><code>std::formatter</code> support</a></li>
</ul>
</li>
<li><a href="#rvariant.recursive">Class template <code>recursive_wrapper</code> <span class="slug">[rvariant.recursive]</span></a>
<ul class="sectlevel2">
<li><a href="#rvariant.recursive.general">General <span class="slug">[rvariant.recursive.general]</span></a></li>
<li><a href="#rvariant.recursive.ctor">Constructors</a></li>
</ul>
</li>
<li><a href="#rvariant.recursive.helper"><code>recursive_wrapper</code> helper utilities <span class="slug">[rvariant.recursive.helper]</span></a></li>
</ul>
</li>
<li><a href="#additional-information">Additional Information</a>
<ul class="sectlevel1">
<li><a href="#visitation-technique">Visitation Technique in Depth</a></li>
<li><a href="#license">License</a></li>
</ul>
</li>
</ul>
</div>
</div>
<div id="content">
<div id="preamble">
<div class="sectionbody">
<div class="paragraph">
<p><code>rvariant</code> is a standard-compatible variant library that supports recursive types.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code class="language-cpp" data-lang="cpp">// A common pattern for representing recursive ASTs using recursive variants.
struct BinaryExpr;
using Expr = iris::<a href="#rvariant.rvariant">rvariant</a><int, double, iris::<a href="#rvariant.recursive">recursive_wrapper</a><BinaryExpr>>;
enum class Op;
struct BinaryExpr { Expr lhs, rhs; Op op{}; };
Expr expr{BinaryExpr{Expr{42}, Expr{3.14}}};
expr.visit(iris::overloaded{
[](int const&) { /* ... */ },
[](double const&) { /* ... */ },
[](BinaryExpr const&) { /* ... */ },
});</code></pre>
</div>
</div>
<h3 id="supported-environments" class="discrete">Supported Environments</h3>
<div class="ulist">
<ul>
<li>
<p>GCC 14</p>
</li>
<li>
<p>Clang 21-22 (libc++)</p>
</li>
<li>
<p>MSVC 2022 and 2026</p>
</li>
<li>
<p>C++23 and C++26</p>
</li>
</ul>
</div>
</div>
</div>
<div class="sect1">
<h2 id="introduction"><a class="anchor" href="#introduction"></a>Introduction</h2>
<div class="sectionbody">
<div class="sect2">
<h3 id="motivation"><a class="anchor" href="#motivation"></a>Motivation</h3>
<div class="paragraph">
<p>Since its introduction in 2004, <code>boost::variant</code> has been used for a wide range of type-safe union use cases.</p>
</div>
<div class="paragraph">
<p>Starting with C++17, most of those can be replaced with <code>std::variant</code>, <em>except</em> for <strong>recursive types</strong>. As a result, many users of generic frameworks that require recursive variants—​most notably Boost.Spirit—​have continued using <code>boost::variant</code>, despite its significant impact on compile times.</p>
</div>
<div class="paragraph">
<p>The compile-time slowness of <code>boost::variant</code> stems from long-standing technical debt: it relies heavily on preprocessor magic in Boost.MPL. This wizardry is so tightly coupled with <code>boost::variant</code>'s internals that any attempt to modernize it would be unrealistic; it would effectively require a complete rewrite.</p>
</div>
<div class="paragraph">
<p>Until 2025, no one had managed to introduce a modern alternative to either Boost or the C++ standard. <code>rvariant</code> fills this gap with a new implementation that supports recursive types while remaining API-compatible with <code>std::variant</code>.</p>
</div>
</div>
<div class="sect2">
<h3 id="goals"><a class="anchor" href="#goals"></a>Project Goals</h3>
<div class="olist arabic">
<ol class="arabic">
<li>
<p><strong>Provide a modern, efficient, and battle-tested recursive variant library</strong> to enable developers to avoid using Boost.Variant in new applications.</p>
</li>
<li>
<p><strong>Replace existing usages of Boost.Variant</strong> in established frameworks.</p>
</li>
<li>
<p><strong>Explore potential directions for future standardization,</strong> while gaining implementation experience with the modernized design.</p>
</li>
</ol>
</div>
</div>
<div class="sect2">
<h3 id="comparison"><a class="anchor" href="#comparison"></a>Comparison of Variant Libraries</h3>
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<div class="content">
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