mirror of
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Made N4428 implementation more exact.
See http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4428.pdf
This commit is contained in:
Anton Bachin
parent
30efe7ccd7
commit
f404ea709b
@ -1,74 +1,188 @@
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## C++17 reflection
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## C++17 reflection proposal
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Better Enums can be used to approximately implement the enums portion of the
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[$cxx17 reflection proposal N4428][n4428] in $cxx11. The implementation is
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*approximate* in the following senses:
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*You can try this demo [live online][live].*
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- It only applies to Better Enums, not built-in enums.
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- `enum_traits<E>::enumerators::get<I>::identifier` is a non-`constexpr`
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function rather than a `constexpr` variable. I could make it a `constexpr`
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variable as in the proposal, but that requires
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[compile-time name trimming][slow-enum] to be enabled for the Better Enum
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on which `get` is used. Since that's an opt-in feature, I can't guarantee it.
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I preferred not to write feature-detection code, in order to keep the
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implementation simple.
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- The return type of `identifier` is `const char*` instead of the proposed
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`std::string_literal`, because I don't have an implementation of the latter
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available. I'm also ignoring the requirements on encoding, and just taking
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whatever the preprocessor provides.
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Better Enums can be used to implement the enums portion of the
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[$cxx17 reflection proposal N4428][n4428] in $cxx11. N4428 proposes the
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following traits interface:
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With that out of the way, we can look at a simple example.
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~~~comment
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<em>namespace std {
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[n4428]: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4428.pdf
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[slow-enum]: ${prefix}OptInFeatures.html#CompileTimeNameTrimming
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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>
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~~~
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So, the basic usage would be:
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~~~comment
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<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>
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~~~
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Resulting in the values `3`, `Foo::A`, and `"B"`, respectively.
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---
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The implementation is defined in [`extra/better-enums/n4428.h`][header]. Let's
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assume that `extra/` has been added as a directory to search for include files.
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The optional Better Enums header file [`extra/better-enums/n4428.h`][header]
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implements this interface, though with two necessary differences.
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1. The interface is only available for Better Enums, i.e. enums declared through
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the `ENUM` macro. This is because the macro is what generates the information
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necessary for reflection.
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2. The type of `identifier` is `const char*`, not the ${cxx17} proposed
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`string_literal`.
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### Demo
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So, with that out of the way, we can do a little test. Let's assume that
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`extra/` has been added as a directory to search for include files.
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#ifndef BETTER_ENUMS_CONSTEXPR_TO_STRING
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#define BETTER_ENUMS_CONSTEXPR_TO_STRING
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#endif
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#include <iostream>
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<em>#include</em> <<em>enum.h</em>>
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<em>#include</em> <<em>better-enums/n4428.h</em>>
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[header]: https://github.com/aantron/better-enums/blob/$ref/extra/better-enums/n4428.h
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---
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Let's declare an enum:
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<em>ENUM</em>(<em>Channel</em>, <em>char</em>, <em>Red</em> = <em>1</em>, <em>Green</em>, <em>Blue</em>)
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N4428 proposes three `constexpr` traits, of which we have two implemented
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exactly — that is, as `constexpr`:
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...and try N4428:
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<em>constexpr std::size_t size</em> =
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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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<em>constexpr Channel value_0</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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<em>constexpr Channel value_1</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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Let's check the results:
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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>;
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...and check the results:
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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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Finally, we can try using `identifier`:
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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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std::cout << <em>identifier_2</em> << std::endl;
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return 0;
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}
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That prints `Blue`, as you would expect.
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---
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So, Better Enums can be used in $cxx11 to test N4428, and maybe start developing
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libraries on top of it. N4428 is very low-level, so it needs additional code
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over it to be truly useful. Whether developing now is a good idea is debatable,
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since N4428 is still only a proposal. But, it's an interesting thing to
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consider.
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Also, Better Enums can be implemented almost completely *in terms of* N4428, so
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the two interfaces are in some vaguely mathematical sense "equivalent." If N4428
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is accepted, I may implement a variant of Better Enums on top of it, since it
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will remove many limitations.
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### Quirk
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The reason for the `#define` 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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[compile-time name trimming][slow-enum] is enabled, i.e. those declared when
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`BETTER_ENUMS_CONSTEXPR_TO_STRING` was defined, or declared with the `SLOW_ENUM`
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variant of `ENUM`. As mentioned on the linked page, the reason compile-time name
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trimming is not the default is that, while still pretty fast, it is four times
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slower than program-startup-time name trimming. The latter is the default.
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Despite the above, a variation on the interface is available for enums without
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compile-time name trimming:
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~~~comment
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<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>
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~~~
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As you can see, the difference is that `identifier` is a non-`constexpr`
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function, and you have to access it through `get_alt<I>`.
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~~~comment
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// Without compile-time name trimming.
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<em>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>
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~~~
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### The future
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N4428 is the fourth in a series of revisions: [N3815][n3815], [N4027][n4027],
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[N4113][n4113], N4428. If there are more revisions that change the proposal for
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enums, I will try to implement those as well.
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[n4428]: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4428.pdf
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[n4113]: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4113.pdf
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[n4027]: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4027.pdf
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[n3815]: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3815.html
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[slow-enum]: ${prefix}OptInFeatures.html#CompileTimeNameTrimming
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[header]: https://github.com/aantron/better-enums/blob/$ref/extra/better-enums/n4428.h
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[live]: http://melpon.org/wandbox/permlink/pNEx7UEWqDtqFAwm
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%% description = Approximate implementation of N4428 enum reflection based on
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Better Enums.
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@ -1,27 +1,60 @@
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// This file was generated automatically.
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// C++17 reflection
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// C++17 reflection proposal
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//
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// Better Enums can be used to approximately implement the enums portion of the
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// C++17 reflection proposal N4428 in C++11. The implementation is approximate
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// in the following senses:
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// Better Enums can be used to implement the enums portion of the C++17
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// reflection proposal N4428 in C++11. N4428 proposes the following traits
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// interface:
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//
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// 1. It only applies to Better Enums, not built-in enums.
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// 2. enum_traits<E>::enumerators::get<I>::identifier is a non-constexpr
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// function rather than a constexpr variable. I could make it a constexpr
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// variable as in the proposal, but that requires compile-time name
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// trimming to be enabled for the Better Enum on which get is used. Since
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// that's an opt-in feature, I can't guarantee it. I preferred not to write
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// feature-detection code, in order to keep the implementation simple.
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// 3. The return type of identifier is const char* instead of the proposed
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// std::string_literal, because I don't have an implementation of the
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// latter available. I'm also ignoring the requirements on encoding, and
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// just taking whatever the preprocessor provides.
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// namespace std {
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//
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// With that out of the way, we can look at an example.
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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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//
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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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//
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// }
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//
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// So, the basic usage would be:
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//
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// enum class Foo {A, B, C};
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//
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// constexpr size_t size =
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// std::enum_traits<Foo>::enumerators::size;
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//
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// constexpr Foo value_0 =
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// std::enum_traits<Foo>::enumerators::get<0>::value;
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//
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// constexpr string_literal name_1 =
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// std::enum_traits<Foo>::enumerators::get<1>::identifier;
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//
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// Resulting in the values 3, Foo::A, and "B", respectively.
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// The implementation is defined in extra/better-enums/n4428.h. Let's assume
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// that extra/ has been added as an include file path.
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// The optional Better Enums header file extra/better-enums/n4428.h implements
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// this interface, though with two necessary differences.
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//
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// 1. The interface is only available for Better Enums, i.e. enums declared
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// through the ENUM macro. This is because the macro is what generates the
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// information necessary for reflection.
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// 2. The type of identifier is const char*, not the ${cxx17}-proposed
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// string_literal.
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//
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// Demo
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//
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// So, with that out of the way, we can do a little test. Let's assume that
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// extra/ has been added as a directory to search for include files.
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#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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#include <enum.h>
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@ -32,31 +65,101 @@
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ENUM(Channel, char, Red = 1, Green, Blue)
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// N4428 requires three constexpr traits, of which we have two implemented
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// exactly, that is, as constexpr:
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// ...and try N4428:
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constexpr std::size_t size =
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std::enum_traits<Channel>::enumerators::size;
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constexpr Channel value_0 =
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std::enum_traits<Channel>::enumerators::get<0>::value;
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constexpr Channel value_1 =
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std::enum_traits<Channel>::enumerators::get<1>::value;
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// Let's check the results:
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constexpr const char *identifier_2 =
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std::enum_traits<Channel>::enumerators::get<2>::identifier;
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// ...and check the results:
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static_assert(size == 3, "");
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static_assert(value_0 == +Channel::Red, "");
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static_assert(value_1 == +Channel::Green, "");
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// Finally, we can try using identifier:
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int main()
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{
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std::cout
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<< std::enum_traits<Channel>::enumerators::get<2>::identifier()
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<< std::endl;
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std::cout << identifier_2 << std::endl;
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return 0;
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}
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// That prints Blue, as you would expect.
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// So, Better Enums can be used in C++11 to test N4428, and maybe start
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// developing libraries on top of it. N4428 is very low-level, so it needs
|
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// additional code over it to be truly useful. Whether developing now is a good
|
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// idea is debatable, since N4428 is still only a proposal. But, it's an
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// interesting thing to consider.
|
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//
|
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// Also, Better Enums can be implemented almost completely in terms of N4428, so
|
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// the two interfaces are in some vaguely mathematical sense "equivalent." If
|
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// N4428 is accepted, I may implement a variant of Better Enums on top of it,
|
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// since it will remove many limitations.
|
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//
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// Quirk
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//
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// The reason for the #define in the code above is that there is one quirk: the
|
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// interface above is available only for Better Enums for which compile-time
|
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// name trimming is enabled, i.e. those declared when
|
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// BETTER_ENUMS_CONSTEXPR_TO_STRING was defined, or declared with the SLOW_ENUM
|
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// variant of ENUM. As mentioned on the linked page, the reason compile-time
|
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// name trimming is not the default is that, while still pretty fast, it is four
|
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// times slower than program-startup-time name trimming. The latter is the
|
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// default.
|
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//
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// Despite the above, a variation on the interface is available for enums
|
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// without compile-time name trimming:
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//
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// namespace std {
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//
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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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//
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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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//
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// // For enums without compile-time name trimming.
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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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//
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// }
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//
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// As you can see, the difference is that identifier is a non-constexpr
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// function, and you have to access it through get_alt<I>.
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//
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// // Without compile-time name trimming.
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// ENUM(Depth, int, HighColor, TrueColor)
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//
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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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//
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// return 0;
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// }
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//
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// The future
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//
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// N4428 is the fourth in a series of revisions: N3815, N4027, N4113, N4428. If
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// there are more revisions that change the proposal for enums, I will try to
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// implement those as well.
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@ -4,11 +4,11 @@
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// This file provides an implementation of the enum reflection interface
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// proposed in
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// http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4428.pdf
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// over Better Enums.
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// on top of Better Enums.
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// See further discussion and a demonstration of usage at
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// example/107-c++17-reflection.cc, or visit:
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// http://aantron.github.io/better-enums/demo/C++17Reflection.html
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// http://aantron.github.io/better-enums/demo/C++17ReflectionProposal.html
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#pragma once
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@ -27,13 +27,19 @@ struct enum_traits {
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template <size_t Index>
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struct get {
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constexpr static Enum value = Enum::_values()[Index];
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constexpr static const char *identifier = Enum::_names()[Index];
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};
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// In the proposal, this is a constexpr variable of type
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// string_literal. It is possible to declare "identifier" that way
|
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// with Better Enums, but only for enums that have compile-time name
|
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// trimming enabled. Since compile-time name trimming currently has
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// a severe performance impact, and is disabled by default, I chose
|
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// to implement "identifier" as a function.
|
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// The above declarations implement N4428 (except for const char*
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// instead of string_literal). The alternative get below is needed for
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// Better Enums that weren't generated with compile-time name trimming,
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// i.e. without BETTER_ENUMS_CONSTEXPR_TO_STRING declared and not
|
||||
// through the SLOW_ENUM macro. That is currently the default for
|
||||
// Better Enums, since compile-time name trimming is pretty slow, so you
|
||||
// would want to use get_alt instead of get.
|
||||
template <size_t Index>
|
||||
struct get_alt {
|
||||
constexpr static Enum value = Enum::_values()[Index];
|
||||
static const char* identifier() { return Enum::_names()[Index]; };
|
||||
};
|
||||
};
|
||||
|
||||
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Reference in New Issue
Block a user