Simple implementation of N4428 enum reflection.

doc/demo/105-c++17-reflection.md

@@ -0,0 +1,74 @@
+## C++17 reflection
+
+Better Enums can be used to approximately implement the enums portion of the
+[$cxx17 reflection proposal N4428][n4428] in $cxx11. The implementation is
+*approximate* in the following senses:
+
+- It only applies to Better Enums, not built-in enums.
+- `enum_traits<E>::enumerators::get<I>::identifier` is a non-`constexpr`
+  function rather than a `constexpr` variable. I could make it a `constexpr`
+  variable as in the proposal, but that requires
+  [compile-time name trimming][slow-enum] to be enabled for the Better Enum
+  on which `get` is used. Since that's an opt-in feature, I can't guarantee it.
+  I preferred not to write feature-detection code, in order to keep the
+  implementation simple.
+- The return type of `identifier` is `const char*` instead of the proposed
+  `std::string_literal`, because I don't have an implementation of the latter
+  available. I'm also ignoring the requirements on encoding, and just taking
+  whatever the preprocessor provides.
+
+With that out of the way, we can look at a simple example.
+
+[n4428]: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4428.pdf
+[slow-enum]: ${prefix}OptInFeatures.html#CompileTimeNameTrimming
+
+---
+
+The implementation is defined in [`extra/better-enums/n4428.h`][header]. Let's
+assume that `extra/` has been added as a directory to search for include files.
+
+    #include <iostream>
+    <em>#include</em> <<em>enum.h</em>>
+    <em>#include</em> <<em>better-enums/n4428.h</em>>
+
+[header]: https://github.com/aantron/better-enums/blob/$ref/extra/better-enums/n4428.h
+
+---
+
+Let's declare an enum:
+
+    <em>ENUM</em>(<em>Channel</em>, <em>char</em>, <em>Red</em> = <em>1</em>, <em>Green</em>, <em>Blue</em>)
+
+N4428 proposes three `constexpr` traits, of which we have two implemented
+exactly &mdash; that is, as `constexpr`:
+
+    <em>constexpr std::size_t   size</em> =
+        <em>std</em>::<em>enum_traits</em><<em>Channel</em>>::<em>enumerators</em>::<em>size</em>;
+
+    <em>constexpr Channel       value_0</em> =
+        <em>std</em>::<em>enum_traits</em><<em>Channel</em>>::<em>enumerators</em>::<em>get</em><<em>0</em>>::<em>value</em>;
+    <em>constexpr Channel       value_1</em> =
+        <em>std</em>::<em>enum_traits</em><<em>Channel</em>>::<em>enumerators</em>::<em>get</em><<em>1</em>>::<em>value</em>;
+
+Let's check the results:
+
+    static_assert(<em>size</em> == <em>3</em>, "");
+
+    static_assert(<em>value_0</em> == +<em>Channel::Red</em>, "");
+    static_assert(<em>value_1</em> == +<em>Channel::Green</em>, "");
+
+Finally, we can try using `identifier`:
+
+    int main()
+    {
+        std::cout
+            << <em>std</em>::<em>enum_traits</em><<em>Channel</em>>::<em>enumerators</em>::<em>get</em><<em>2</em>>::<em>identifier</em>()
+            << std::endl;
+
+        return 0;
+    }
+
+That prints `Blue`, as you would expect.
+
+%% description = Approximate implementation of N4428 enum reflection based on
+Better Enums.

doc/template/cxx17.tmpl

@@ -0,0 +1 @@
+<span class="cpp">C++</span><span class="eleven">17</span>

doc/template/ref.tmpl

@@ -1 +1 @@
-0.10.1
+master

example/105-c++17-reflection.cc

@@ -0,0 +1,62 @@
+// This file was generated automatically.
+
+// C++17 reflection
+//
+// Better Enums can be used to approximately implement the enums portion of the
+// C++17 reflection proposal N4428 in C++11. The implementation is approximate
+// in the following senses:
+//
+//   1. It only applies to Better Enums, not built-in enums.
+//   2. enum_traits<E>::enumerators::get<I>::identifier is a non-constexpr
+//      function rather than a constexpr variable. I could make it a constexpr
+//      variable as in the proposal, but that requires compile-time name
+//      trimming to be enabled for the Better Enum on which get is used. Since
+//      that's an opt-in feature, I can't guarantee it. I preferred not to write
+//      feature-detection code, in order to keep the implementation simple.
+//   3. The return type of identifier is const char* instead of the proposed
+//      std::string_literal, because I don't have an implementation of the
+//      latter available. I'm also ignoring the requirements on encoding, and
+//      just taking whatever the preprocessor provides.
+//
+// With that out of the way, we can look at an example.
+
+// The implementation is defined in extra/better-enums/n4428.h. Let's assume
+// that extra/ has been added as an include file path.
+
+#include <iostream>
+#include <enum.h>
+#include <better-enums/n4428.h>
+
+
+// Let's declare an enum:
+
+ENUM(Channel, char, Red = 1, Green, Blue)
+
+// N4428 requires three constexpr traits, of which we have two implemented
+// exactly, that is, as constexpr:
+
+constexpr std::size_t   size =
+    std::enum_traits<Channel>::enumerators::size;
+
+constexpr Channel       value_0 =
+    std::enum_traits<Channel>::enumerators::get<0>::value;
+constexpr Channel       value_1 =
+    std::enum_traits<Channel>::enumerators::get<1>::value;
+
+// Let's check the results:
+
+static_assert(size == 3, "");
+
+static_assert(value_0 == +Channel::Red, "");
+static_assert(value_1 == +Channel::Green, "");
+
+// Finally, we can try using identifier:
+
+int main()
+{
+    std::cout
+        << std::enum_traits<Channel>::enumerators::get<2>::identifier()
+        << std::endl;
+
+    return 0;
+}

extra/better-enums/n4428.h

@@ -0,0 +1,46 @@
+// This file is part of Better Enums, released under the BSD 2-clause license.
+// See doc/LICENSE for details, or visit http://github.com/aantron/better-enums.
+
+// This file provides an implementation of the enum reflection interface
+// proposed in
+//     http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4428.pdf
+// over Better Enums.
+
+// See further discussion and a demonstration of usage at
+// example/107-c++17-reflection.cc, or visit:
+//     http://aantron.github.io/better-enums/demo/C++17Reflection.html
+
+#pragma once
+
+#ifndef BETTER_ENUMS__N4428_H
+#define BETTER_ENUMS__N4428_H
+
+
+
+namespace std {
+
+template <typename Enum>
+struct enum_traits {
+    struct enumerators {
+        constexpr static const size_t   size = Enum::_size();
+
+        template <size_t Index>
+        struct get {
+            constexpr static Enum       value = Enum::_values()[Index];
+
+            // In the proposal, this is a constexpr variable of type
+            // string_literal. It is possible to declare "identifier" that way
+            // with Better Enums, but only for enums that have compile-time name
+            // trimming enabled. Since compile-time name trimming currently has
+            // a severe performance impact, and is disabled by default, I chose
+            // to implement "identifier" as a function.
+            static const char* identifier() { return Enum::_names()[Index]; };
+        };
+    };
+};
+
+}
+
+
+
+#endif // #ifndef BETTER_ENUMS__N4428_H

test/CMakeLists.txt

@@ -89,10 +89,11 @@ endforeach(TEST)
 set(EXAMPLES
     1-hello-world 2-conversions 3-iterate 4-switch 5-iostreams 6-safety
     7-representation 8-constexpr 101-special-values 102-any-underlying
-    103-bitset 104-quine)
+    103-bitset 104-quine 105-c++17-reflection)
 
 set(SKIPPED_FOR_CXX98
-    8-constexpr 101-special-values 102-any-underlying 103-bitset 104-quine)
+    8-constexpr 101-special-values 102-any-underlying 103-bitset 104-quine
+    105-c++17-reflection)
 
 set(SKIPPED_FOR_STRICT_CONVERSION 4-switch 102-any-underlying)
 
@@ -112,6 +113,7 @@ endforeach(EXAMPLE)
 # Add compiler flags.
 
 include_directories(..)
+include_directories(../extra)
 
 if(CMAKE_COMPILER_IS_GNUCXX OR CMAKE_CXX_COMPILER_ID MATCHES Clang)
     include(CheckCXXCompilerFlag)

test/expect/105-c++17-reflection

@@ -0,0 +1 @@
+Blue