add compile time enum_index (#159)

doc/reference.md

@@ -108,11 +108,14 @@ constexpr optional<E> enum_cast(string_view value, BinaryPredicate p) noexcept(i
 ```cpp
 template <typename E>
 constexpr E enum_value(size_t index) noexcept;
+
+template <typename E, size_t I>
+constexpr E enum_value() noexcept;
 ```
 
 * Returns enum value at specified index.
-
-* No bounds checking is performed: the behavior is undefined if `index >= number of enum values`.
+  * `enum_value(value)` no bounds checking is performed: the behavior is undefined if `index >= number of enum values`.
+  * `enum_value<value>()` check if `I >= number of enum values`, occurs the compilation error `magic_enum::enum_value out of range`.
 
 * Examples
 
@@ -120,7 +123,12 @@ constexpr E enum_value(size_t index) noexcept;
   int i = 1;
   Color color = magic_enum::enum_value<Color>(i);
   // color -> Color::BLUE
-  ````
+  ```
+
+  ```cpp
+  Color color = magic_enum::enum_value<Color, 1>();
+  // color -> Color::BLUE
+  ```
 
 ## `enum_values`
 
@@ -184,27 +192,23 @@ constexpr string_view enum_name() noexcept;
 
 * Returns name from enum value as `string_view` with null-terminated string.
   * If enum value does not have name or [out of range](limitations.md), `enum_name(value)` returns empty string.
-  * If enum value does not have name, `enum_name<value>()` occurs the compilation error `"Enum value does not have a name."`.
+  * If enum value does not have name, `enum_name<value>()` occurs the compilation error `magic_enum::enum_name enum value does not have a name`.
 
 * `enum_name<value>()` is much lighter on the compile times and is not restricted to the enum_range [limitation](limitations.md).
 
 * Examples
 
-  * Enum value to string.
+  ```cpp
+  Color color = Color::RED;
+  auto color_name = magic_enum::enum_name(color);
+  // color_name -> "RED"
+  ```
 
-    ```cpp
-    Color color = Color::RED;
-    auto color_name = magic_enum::enum_name(color);
-    // color_name -> "RED"
-    ```
-
-  * Static storage enum variable to string.
-
-    ```cpp
-    constexpr Color color = Color::BLUE;
-    constexpr auto color_name = magic_enum::enum_name<color>();
-    // color_name -> "BLUE"
-    ```
+  ```cpp
+  constexpr Color color = Color::BLUE;
+  constexpr auto color_name = magic_enum::enum_name<color>();
+  // color_name -> "BLUE"
+  ```
 
 ## `enum_flags_name`
 
@@ -264,19 +268,27 @@ constexpr array<pair<E, string_view>, N> enum_entries() noexcept;
 
 ```cpp
 template <typename E>
-constexpr optional<size_t> enum_index() noexcept;
+constexpr optional<size_t> enum_index(E value) noexcept;
+
+template <auto V>
+constexpr size_t enum_index() noexcept;
 ```
 
 * Obtains index in enum values from enum value.
-
-* Returns `optional<size_t>` with index.
+  * `enum_index(value)` returns `optional<size_t>` with index.
+  * `enum_index<value>()` returns index. If enum value does not have a index, occurs the compilation error `magic_enum::enum_index enum value does not have a index`.
 
 * Examples
 
   ```cpp
   constexpr auto color_index = magic_enum::enum_index(Color::BLUE);
-  // color_index -> color_index.value() -> 1
-  // color_index -> color_index.has_value() -> true
+  // color_index.value() -> 1
+  // color_index.has_value() -> true
+  ```
+
+  ```cpp
+  constexpr auto color_index = magic_enum::enum_index<Color::BLUE>();
+  // color_index -> 1
   ```
 
 ## `enum_contains`
@@ -330,7 +342,7 @@ constexpr string_view enum_type_name() noexcept;
 
 ```cpp
 template <typename... Es>
-[[nodiscard]] constexpr optional<enum_fuse_t> enum_fuse(Es... values);
+[[nodiscard]] constexpr optional<enum_fuse_t> enum_fuse(Es... values) noexcept;
 ```
 
 * Returns a typesafe bijective mix of several enum values. This can be used to emulate 2D switch/case statements.
@@ -339,7 +351,7 @@ template <typename... Es>
 
 * Switch/case statement over an incomplete enum is a Visual Studio warning C4064
   * You have to silent (/wd4064) or ignore it.
-  * Alternatively, define `MAGIC_ENUM_NO_TYPESAFE_ENUM_FUSE` to disable type-safety (`enum_fuse_t` equals `std::uintmax_t`).
+  * Alternatively, define `MAGIC_ENUM_NO_TYPESAFE_ENUM_FUSE` to disable type-safety (`enum_fuse_t` equals `uintmax_t`).
 
 * Examples
 

include/magic_enum.hpp

@@ -661,7 +661,7 @@ using underlying_type_t = typename underlying_type<T>::type;
 template <typename E>
 [[nodiscard]] constexpr auto enum_type_name() noexcept -> detail::enable_if_enum_t<E, string_view> {
   constexpr string_view name = detail::type_name_v<std::decay_t<E>>;
-  static_assert(name.size() > 0, "Enum type does not have a name.");
+  static_assert(!name.empty(), "magic_enum::enum_type_name enum type does not have a name.");
 
   return name;
 }
@@ -691,7 +691,10 @@ template <typename E>
 // Returns enum value at specified index.
 template <typename E, std::size_t I>
 [[nodiscard]] constexpr auto enum_value() noexcept -> detail::enable_if_enum_t<E, std::decay_t<E>> {
-  return enum_value<std::decay_t<E>>(I);
+  using D = std::decay_t<E>;
+  static_assert(I < detail::count_v<D>, "magic_enum::enum_value out of range.");
+
+  return enum_value<D>(I);
 }
 
 // Returns std::array with enum values, sorted by enum value.
@@ -705,7 +708,7 @@ template <typename E>
 template <auto V>
 [[nodiscard]] constexpr auto enum_name() noexcept -> detail::enable_if_enum_t<decltype(V), string_view> {
   constexpr string_view name = detail::enum_name_v<std::decay_t<decltype(V)>, V>;
-  static_assert(name.size() > 0, "Enum value does not have a name.");
+  static_assert(!name.empty(), "magic_enum::enum_name enum value does not have a name.");
 
   return name;
 }
@@ -888,6 +891,15 @@ template <typename E>
   return {}; // Invalid value or out of range.
 }
 
+// Obtains index in enum values from static storage enum variable.
+template <auto V>
+[[nodiscard]] constexpr auto enum_index() noexcept -> detail::enable_if_enum_t<decltype(V), std::size_t> {
+  constexpr auto index = enum_index<std::decay_t<decltype(V)>>(V);
+  static_assert(index.has_value(), "magic_enum::enum_index enum value does not have a index.");
+
+  return index.value();
+}
+
 // Checks whether enum contains enumerator with such enum value.
 template <typename E>
 [[nodiscard]] constexpr auto enum_contains(E value) noexcept -> detail::enable_if_enum_t<E, bool> {

test/test.cpp

@@ -230,12 +230,14 @@ TEST_CASE("enum_index") {
   constexpr auto cr = enum_index(Color::RED);
   Color cg = Color::GREEN;
   REQUIRE(cr.value() == 0);
+  REQUIRE(enum_index<Color::RED>() == 0);
   REQUIRE(enum_index<Color&>(cg).value() == 1);
   REQUIRE(enum_index(cm[2]).value() == 2);
   REQUIRE_FALSE(enum_index(static_cast<Color>(0)).has_value());
 
   constexpr auto no = enum_index(Numbers::one);
   REQUIRE(no.value() == 0);
+  REQUIRE(enum_index<Numbers::one>() == 0);
   REQUIRE(enum_index(Numbers::two).value() == 1);
   REQUIRE(enum_index(Numbers::three).value() == 2);
   REQUIRE_FALSE(enum_index(Numbers::many).has_value());
@@ -243,6 +245,7 @@ TEST_CASE("enum_index") {
 
   constexpr auto dr = enum_index(Directions::Right);
   Directions dl = Directions::Left;
+  REQUIRE(enum_index<Directions::Left>() == 0);
   REQUIRE(enum_index<Directions&>(dl).value() == 0);
   REQUIRE(enum_index<const Directions>(Directions::Down).value() == 1);
   REQUIRE(enum_index(Directions::Up).value() == 2);
@@ -260,6 +263,7 @@ TEST_CASE("enum_index") {
 #endif
 
   constexpr auto nt = enum_index(number::three);
+  REQUIRE(enum_index<number::one>() == 0);
   REQUIRE(enum_index(number::one).value() == 0);
   REQUIRE(enum_index(number::two).value() == 1);
   REQUIRE(nt.value() == 2);