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<title>C++17 reflection proposal - Better Enums</title>
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<link rel="canonical" href="http://aantron.github.io/better-enums/demo/C++17ReflectionProposal.html" />
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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/0.11.3/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 0.11.3</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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</section>
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<section class="buttons">
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<a href="https://raw.githubusercontent.com/aantron/better-enums/0.11.3/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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<div class="main">
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<p>
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This is an example of code you can write on top of Better Enums. It's a valid
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program — you can <a href="https://github.com/aantron/better-enums/blob/0.11.3/example/105-c++17-reflection.cc">download</a> it and try it out. The
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program is also part of the test suite.
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</p>
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<h2>C++17 reflection proposal</h2>
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<p><em>You can try this demo <a href="http://melpon.org/wandbox/permlink/QelcwZNLi4gIx8Ux">live online</a>.</em></p>
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<p>Better Enums can be used to 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>. N4428 proposes the
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following traits interface:</p>
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<pre class="comment"><em>namespace std {
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template <typename E>
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struct enum_traits {
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struct enumerators {
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constexpr static size_t size;
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template <size_t I>
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struct get {
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constexpr string_literal identifier;
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constexpr static E value;
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};
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};
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};
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}</em></pre><p>So, the basic usage would be:</p>
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<pre class="comment"><em>enum class Foo {A, B, C};
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constexpr size_t size =
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std::enum_traits<Foo>::enumerators::size;
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constexpr Foo value_0 =
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std::enum_traits<Foo>::enumerators::get<0>::value;
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constexpr string_literal name_1 =
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std::enum_traits<Foo>::enumerators::get<1>::identifier;</em></pre><p>Resulting in the values <code>3</code>, <code>Foo::A</code>, and <code>"B"</code>, respectively.</p>
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<hr>
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<p>The interface is implemented in the optional header file
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<a href="https://github.com/aantron/better-enums/blob/0.11.3/extra/better-enums/n4428.h"><code>extra/better-enums/n4428.h</code></a>. There is a necessary difference: the
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interface is only available for enums declared through the <code>BETTER_ENUM</code> macro.
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This is because the macro is what generates the information necessary for
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reflection.</p>
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<a id="Demo"></a><h3>Demo</h3>
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<p>So, with that out of the way, we can do a little test. Let's assume that
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<code>extra/</code> has been added as a directory to search for include files.</p>
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<pre>#ifndef BETTER_ENUMS_CONSTEXPR_TO_STRING
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#define BETTER_ENUMS_CONSTEXPR_TO_STRING
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#endif
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#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>BETTER_ENUM</em>(<em>Channel</em>, <em>char</em>, <em>Red</em> = <em>1</em>, <em>Green</em>, <em>Blue</em>)</pre><p>...and try N4428:</p>
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<pre>constexpr std::size_t <em>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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constexpr Channel <em>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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constexpr Channel <em>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>;
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constexpr const char *<em>identifier_2</em> =
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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>;</pre><p>...and 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>, "");
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int main()
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{
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std::cout << <em>identifier_2</em> << 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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<a id="Quirk"></a><h3>Quirk</h3>
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<p>The reason for the <code>#define</code> in the code above is that there is one quirk:
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the interface above is available only for Better Enums for which
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<a href="../OptInFeatures.html#CompileTimeNameTrimming">compile-time name trimming</a> is enabled — those declared when
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<code>BETTER_ENUMS_CONSTEXPR_TO_STRING</code> was defined, or declared with the <code>SLOW_ENUM</code>
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variant of <code>BETTER_ENUM</code>. As mentioned on the linked page, the reason
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compile-time name trimming is not the default is that, while still pretty fast,
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it is four times slower than program-startup-time name trimming. The latter is
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the default.</p>
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<p>Despite the above, a variation on the interface is available for enums without
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compile-time name trimming:</p>
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<pre class="comment"><em>namespace std {
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template <typename E>
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struct enum_traits {
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struct enumerators {
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constexpr static size_t size;
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template <size_t I>
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struct get {
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constexpr const char *identifier;
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constexpr static E value;
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};
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</em>// For enums without compile-time name trimming.<em>
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template <size_t I>
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struct get_alt {
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static const char* identifier();
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constexpr static E value;
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};
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};
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};
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}</em></pre><p>As you can see, the difference is that <code>identifier</code> is a non-<code>constexpr</code>
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function, and you have to access it through <code>get_alt<I></code>.</p>
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<pre class="comment">// Without compile-time name trimming.
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<em>BETTER_ENUM(Depth, int, HighColor, TrueColor)
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int main()
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{
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std::cout
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<< std::enum_traits<Depth>::enumerators::get_alt<1>::identifier()
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<< std::endl;
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return 0;
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}</em></pre><a id="TheFuture"></a><h3>The future</h3>
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<p>N4428 is the fourth in a series of revisions: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3815.html">N3815</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4027.pdf">N4027</a>,
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<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4113.pdf">N4113</a>, N4428. If there are more revisions that change the proposal for
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enums, I will try to implement those as well.</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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</section>
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Copyright © 2015-2019 Anton Bachin. Released under the BSD 2-clause
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license. See
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<a href="https://github.com/aantron/better-enums/blob/0.11.3/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 0.11.3.
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