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<title>C++17 reflection - Better Enums</title>
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<meta name="description" content="Approximate implementation of N4428 enum reflection based on
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Better Enums." />
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<meta name="author" content="Anton Bachin" />
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<a class="first" href="https://raw.githubusercontent.com/aantron/better-enums/master/enum.h"
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download>Download</a>
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<a href="https://github.com/aantron/better-enums">GitHub</a>
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<a href="../index.html">Home</a>
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<section>
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<h1><a href="../index.html">Better Enums</a></h1>
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<h2>Reflective compile-time enums for <span class="cpp">C++</span></h2>
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<h3>Open-source under the BSD license</h3>
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</section>
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<section class="notes">
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<p>Version 0.10.1</p>
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<p>To install, just add <code>enum.h</code> to your project.</p>
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Visit the GitHub repo for issues, feedback, and the latest development.
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<a href="https://raw.githubusercontent.com/aantron/better-enums/master/enum.h"
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download>Download <code>enum.h</code></a>
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<a href="https://github.com/aantron/better-enums">GitHub</a>
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<p>
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This page is an advanced demo showing the kind of code you can write on top of
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Better Enums. It's a valid program — you can
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<a href="https://github.com/aantron/better-enums/blob/master/example/105-c++17-reflection.cc">download</a> it and try it out. The program runs as part of
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the automated test suite.
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</p>
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<h2>C++17 reflection</h2>
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<p>Better Enums can be used to approximately implement the enums portion of the
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<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4428.pdf"><span class="cpp">C++</span><span class="eleven">17</span> reflection proposal N4428</a> in <span class="cpp">C++</span><span class="eleven">11</span>. The implementation is
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<em>approximate</em> in the following senses:</p>
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<ul>
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<li>It only applies to Better Enums, not built-in enums.</li>
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<li><code>enum_traits<E>::enumerators::get<I>::identifier</code> is a non-<code>constexpr</code>
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function rather than a <code>constexpr</code> variable. I could make it a <code>constexpr</code>
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variable as in the proposal, but that requires
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<a href="../OptInFeatures.html#CompileTimeNameTrimming">compile-time name trimming</a> to be enabled for the Better Enum
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on which <code>get</code> is used. Since that's an opt-in feature, I can't guarantee it.
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I preferred not to write feature-detection code, in order to keep the
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implementation simple.</li>
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<li>The return type of <code>identifier</code> is <code>const char*</code> instead of the proposed
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<code>std::string_literal</code>, because I don't have an implementation of the latter
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available. I'm also ignoring the requirements on encoding, and just taking
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whatever the preprocessor provides.</li>
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</ul>
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<p>With that out of the way, we can look at a simple example.</p>
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<hr>
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<p>The implementation is defined in <a href="https://github.com/aantron/better-enums/blob/master/extra/better-enums/n4428.h"><code>extra/better-enums/n4428.h</code></a>. Let's
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assume that <code>extra/</code> has been added as a directory to search for include files.</p>
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<pre>#include <iostream>
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<em>#include</em> <<em>enum.h</em>>
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<em>#include</em> <<em>better-enums/n4428.h</em>></pre><hr>
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<p>Let's declare an enum:</p>
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<pre><em>ENUM</em>(<em>Channel</em>, <em>char</em>, <em>Red</em> = <em>1</em>, <em>Green</em>, <em>Blue</em>)</pre><p>N4428 proposes three <code>constexpr</code> traits, of which we have two implemented
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exactly — that is, as <code>constexpr</code>:</p>
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<pre><em>constexpr std::size_t size</em> =
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<em>std</em>::<em>enum_traits</em><<em>Channel</em>>::<em>enumerators</em>::<em>size</em>;
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<em>constexpr Channel value_0</em> =
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<em>std</em>::<em>enum_traits</em><<em>Channel</em>>::<em>enumerators</em>::<em>get</em><<em>0</em>>::<em>value</em>;
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<em>constexpr Channel value_1</em> =
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<em>std</em>::<em>enum_traits</em><<em>Channel</em>>::<em>enumerators</em>::<em>get</em><<em>1</em>>::<em>value</em>;</pre><p>Let's check the results:</p>
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<pre>static_assert(<em>size</em> == <em>3</em>, "");
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static_assert(<em>value_0</em> == +<em>Channel::Red</em>, "");
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static_assert(<em>value_1</em> == +<em>Channel::Green</em>, "");</pre><p>Finally, we can try using <code>identifier</code>:</p>
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<pre>int main()
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{
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std::cout
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<< <em>std</em>::<em>enum_traits</em><<em>Channel</em>>::<em>enumerators</em>::<em>get</em><<em>2</em>>::<em>identifier</em>()
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<< std::endl;
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return 0;
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}</pre><p>That prints <code>Blue</code>, as you would expect.</p>
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<section class="tutorial-footer">
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<p class="up">
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Return to the <a href="../index.html#CompileTimeDemos">demo index</a>.
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<footer>
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Copyright © 2015 Anton Bachin. Released under the BSD 2-clause license.
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See
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<a href="https://github.com/aantron/better-enums/blob/master/doc/LICENSE">
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LICENSE</a>.
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<br />
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This page is part of the documentation for Better Enums 0.10.1.
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