Added support for flag enums; handle unidentified enums

Author: liuzicheng1987

docs/enums.md

@@ -1,6 +1,8 @@
 # Enums
 
-reflect-cpp supports scoped enumerations.
+reflect-cpp supports scoped enumerations. They can either come in the form of normal enumerations or flag enums.
+
+## Normal enumerations
 
 Example:
 
@@ -38,7 +40,7 @@ enum struct Color2 { red, green, blue, yellow };
 enum Color3 { red, green, blue, yellow };
 ```
 
-2. You cannot have more than 100 values and if you explicitly assign values, they must be between 0 and 99.
+2. You cannot have more than 128 values and if you explicitly assign values, they must be between 0 and 127.
 
 ```cpp
 /// OK
@@ -50,6 +52,97 @@ enum struct Color2 { red, green, blue = 50, yellow };
 /// unsupported - red is a negative value
 enum Color3 { red = -10, green, blue, yellow };
 
-/// unsupported - red is greater than 99
+/// unsupported - red is greater than 127
 enum Color3 { red = 200, green, blue, yellow };
-```
+```
+
+## Flag enums
+
+Sometimes the enumerations are not mutually exclusive - sticking with the metaphor of colors, it is perfectly
+possible for things to have more than one color. C++ programmers sometimes like to model this using a flag enum.
+
+Flag enums work as follows:
+
+1. The bitwise OR operator must be defined on them. If an enum has the bitwise OR operator defined on it, it will be treated as a flag enum.
+2. The most important values of the enum must be 1 or 2^N, N being a positive integer (in other words 1,2,4,8,16,32,...).
+
+Example:
+
+```cpp
+// The important colors must be 1 or 2^N
+enum class Color {
+  red = 256,
+  green = 512,
+  blue = 1024,
+  yellow = 2048,
+  orange = (256 | 2048)  // red + yellow = orange
+};
+
+// The bitwise OR operator must be defined - this is how reflect-cpp knows that
+// this is a flag enum.
+inline Color operator|(Color c1, Color c2) {
+  return static_cast<Color>(static_cast<int>(c1) | static_cast<int>(c2));
+}
+```
+
+In this particular example, the important colors red, green, blue and yellow are all in the form of 2^N. 
+Other colors are ok as well, if they are expressed as combinations of the 2^N-colors. 
+
+When something is a flag enum, then you can also do this:
+
+```cpp
+const auto circle =
+    Circle{.radius = 2.0, .color = Color::blue | Color::green};
+```
+
+Which will then be represented as follows:
+
+```json
+{"radius":2.0,"color":"blue|green"}
+```
+
+Using orange is fine as well:
+
+```cpp
+const auto circle =
+    Circle{.radius = 2.0, .color = Color::orange};
+```
+
+But it will be represented in terms of the base colors (which are in the form 2^N):
+
+```json
+{"radius":2.0,"color":"red|yellow"}
+```
+
+You can also combine orange with another color:
+
+```cpp
+const auto circle =
+    Circle{.radius = 2.0, .color = Color::blue | Color::orange};
+```
+
+Which will result in this:
+
+```json
+{"radius":2.0,"color":"red|blue|yellow"}
+```
+
+## What happens if an enum cannot be matched?
+
+If an enum cannot be matched, it will be written as an integer. This works for both normal enums as well as flag enums.
+
+For instance, if you use the `Color` flag enum as described in the previous section, you can do something like this:
+
+```cpp
+// Enums are not supposed to be used like this, but it is defined behavior for scoped enumerations.
+const auto circle = Circle{.radius = 2.0, .color = static_cast<Color>(10000)};
+```
+
+This will be represented as follows:
+
+```json
+{"radius":2.0,"color":"16|red|green|blue|8192"}
+```
+
+This works, because 16 + 256 + 512 + 1024 + 8192 = 10000. Flag enums are *always* represented in terms of 2^N-numbers.
+

include/rfl/internal/enums/LiteralConverter.hpp

@@ -8,12 +8,13 @@
 #include <type_traits>
 
 #include "rfl/Literal.hpp"
+#include "rfl/define_literal.hpp"
 
 namespace rfl {
 namespace internal {
 namespace enums {
 
-template <class EnumType, class LiteralType, size_t N>
+template <class EnumType, class LiteralType, size_t N, auto... _enums>
 struct Names {
   /// Contains a collection of enums as compile-time strings.
   using Literal = LiteralType;
@@ -22,10 +23,17 @@ struct Names {
   constexpr static size_t size = N;
 
   /// A list of all the possible enums
-  std::array<EnumType, N> enums_;
+  constexpr static std::array<EnumType, N> enums_ =
+      std::array<EnumType, N>{_enums...};
 
   static_assert(N == 0 || LiteralType::size() == N,
                 "Size of literal and enum do not match.");
+
+  template <class NewLiteral, auto _new_enum>
+  using AddOneType = std::conditional_t<
+      N == 0, Names<EnumType, NewLiteral, 1, _new_enum>,
+      Names<EnumType, define_literal_t<LiteralType, NewLiteral>, N + 1,
+            _enums..., _new_enum>>;
 };
 
 }  // namespace enums

include/rfl/internal/enums/Names.hpp

@@ -0,0 +1,132 @@
+#ifndef RFL_INTERNAL_ENUMS_STRINGCONVERTER_HPP_
+#define RFL_INTERNAL_ENUMS_STRINGCONVERTER_HPP_
+
+#include <algorithm>
+#include <array>
+#include <string>
+#include <string_view>
+#include <type_traits>
+
+#include "rfl/Result.hpp"
+#include "rfl/internal/enums/get_enum_names.hpp"
+#include "rfl/internal/enums/is_flag_enum.hpp"
+#include "rfl/internal/strings/join.hpp"
+#include "rfl/internal/strings/split.hpp"
+#include "rfl/type_name_t.hpp"
+
+namespace rfl {
+namespace internal {
+namespace enums {
+
+template <class EnumType>
+class StringConverter {
+ private:
+  static constexpr bool is_flag_enum_ = is_flag_enum<EnumType>;
+
+  static constexpr auto names_ = get_enum_names<EnumType, is_flag_enum_>();
+
+  using NamesLiteral = typename decltype(names_)::Literal;
+
+ public:
+  /// Transform an enum to a matching string.
+  static std::string enum_to_string(const EnumType _enum) {
+    if constexpr (is_flag_enum_) {
+      return flag_enum_to_string(_enum);
+    } else {
+      return enum_to_single_string(_enum);
+    }
+  }
+
+  /// Transforms a string to the matching enum.
+  static Result<EnumType> string_to_enum(const std::string& _str) {
+    static_assert(names_.size != 0,
+                  "No enum could be identified. Please choose enum values "
+                  "between 0 to 127 or for flag enums choose 1,2,4,8,16,...");
+    if constexpr (is_flag_enum_) {
+      return string_to_flag_enum(_str);
+    } else {
+      return single_string_to_enum(_str);
+    }
+  }
+
+ private:
+  /// Iterates through the enum bit by bit and matches it against the flags.
+  static std::string flag_enum_to_string(const EnumType _e) {
+    using T = std::underlying_type_t<EnumType>;
+    auto val = static_cast<T>(_e);
+    int i = 0;
+    std::vector<std::string> flags;
+    while (val != 0) {
+      const auto bit = val & static_cast<T>(1);
+      if (bit == 1) {
+        auto str = enum_to_single_string(
+            static_cast<EnumType>(static_cast<T>(1) << i));
+        flags.emplace_back(std::move(str));
+      }
+      ++i;
+      val >>= 1;
+    }
+    return strings::join("|", flags);
+  }
+
+  /// This assumes that _enum can be exactly matched to one of the names and
+  /// does not have to be combined using |.
+  static std::string enum_to_single_string(const EnumType _enum) {
+    const auto to_str = [](const auto _l) { return _l.str(); };
+
+    for (size_t i = 0; i < names_.size; ++i) {
+      if (names_.enums_[i] == _enum) {
+        return NamesLiteral::from_value(
+                   static_cast<typename NamesLiteral::ValueType>(i))
+            .transform(to_str)
+            .value();
+      }
+    }
+
+    return std::to_string(static_cast<std::underlying_type_t<EnumType>>(_enum));
+  }
+
+  /// Finds the enum matching the literal.
+  static EnumType literal_to_enum(const NamesLiteral _lit) noexcept {
+    return names_.enums_[_lit.value()];
+  }
+
+  /// This assumes that _enum can be exactly matched to one of the names and
+  /// does not have to be combined using |.
+  static Result<EnumType> single_string_to_enum(const std::string& _str) {
+    const auto r = NamesLiteral::from_string(_str).transform(literal_to_enum);
+    if (r) {
+      return r;
+    } else {
+      try {
+        return static_cast<EnumType>(std::stoi(_str));
+      } catch (std::exception& exp) {
+        return Error(exp.what());
+      }
+    }
+  }
+
+  /// Only relevant if this is a flag enum - combines the different matches
+  /// using |.
+  static Result<EnumType> string_to_flag_enum(
+      const std::string& _str) noexcept {
+    using T = std::underlying_type_t<EnumType>;
+    const auto split = strings::split(_str, "|");
+    auto res = static_cast<T>(0);
+    for (const auto& s : split) {
+      const auto r = single_string_to_enum(s);
+      if (r) {
+        res |= static_cast<T>(*r);
+      } else {
+        return r;
+      }
+    }
+    return static_cast<EnumType>(res);
+  }
+};
+
+}  // namespace enums
+}  // namespace internal
+}  // namespace rfl
+
+#endif