type_traits: type_list_index[_v]

docs/md/core.md

@@ -762,6 +762,7 @@ Here is a (possibly incomplete) list of the functionalities that come with a
 type list:
 
 * `type_list_element[_t]` to get the N-th element of a type list.
+* `type_list_index[_v]` to get the index of a given element of a type list.
 * `type_list_cat[_t]` and a handy `operator+` to concatenate type lists.
 * `type_list_unique[_t]` to remove duplicate types from a type list.
 * `type_list_contains[_v]` to know if a type list contains a given type.

src/entt/core/type_traits.hpp

@@ -121,22 +121,22 @@ struct type_list_element;
 /**
  * @brief Provides compile-time indexed access to the types of a type list.
  * @tparam Index Index of the type to return.
- * @tparam Type First type provided by the type list.
+ * @tparam First First type provided by the type list.
  * @tparam Other Other types provided by the type list.
  */
-template<std::size_t Index, typename Type, typename... Other>
-struct type_list_element<Index, type_list<Type, Other...>>
+template<std::size_t Index, typename First, typename... Other>
+struct type_list_element<Index, type_list<First, Other...>>
     : type_list_element<Index - 1u, type_list<Other...>> {};
 
 /**
  * @brief Provides compile-time indexed access to the types of a type list.
- * @tparam Type First type provided by the type list.
+ * @tparam First First type provided by the type list.
  * @tparam Other Other types provided by the type list.
  */
-template<typename Type, typename... Other>
-struct type_list_element<0u, type_list<Type, Other...>> {
+template<typename First, typename... Other>
+struct type_list_element<0u, type_list<First, Other...>> {
     /*! @brief Searched type. */
-    using type = Type;
+    using type = First;
 };
 
 /**
@@ -147,6 +147,58 @@ struct type_list_element<0u, type_list<Type, Other...>> {
 template<std::size_t Index, typename List>
 using type_list_element_t = typename type_list_element<Index, List>::type;
 
+/*! @brief Primary template isn't defined on purpose. */
+template<typename, typename>
+struct type_list_index;
+
+/**
+ * @brief Provides compile-time type access to the types of a type list.
+ * @tparam Type Type to look for and for which to return the index.
+ * @tparam First First type provided by the type list.
+ * @tparam Other Other types provided by the type list.
+ */
+template<typename Type, typename First, typename... Other>
+struct type_list_index<Type, type_list<First, Other...>> {
+    /*! @brief Unsigned integer type. */
+    using value_type = std::size_t;
+    /*! @brief Compile-time position of the given type in the sublist. */
+    static constexpr value_type value = 1u + type_list_index<Type, type_list<Other...>>::value;
+};
+
+/**
+ * @brief Provides compile-time type access to the types of a type list.
+ * @tparam Type Type to look for and for which to return the index.
+ * @tparam Other Other types provided by the type list.
+ */
+template<typename Type, typename... Other>
+struct type_list_index<Type, type_list<Type, Other...>> {
+    static_assert(type_list_index<Type, type_list<Other...>>::value == sizeof...(Other), "Non-unique type");
+    /*! @brief Unsigned integer type. */
+    using value_type = std::size_t;
+    /*! @brief Compile-time position of the given type in the sublist. */
+    static constexpr value_type value = 0u;
+};
+
+/**
+ * @brief Provides compile-time type access to the types of a type list.
+ * @tparam Type Type to look for and for which to return the index.
+ */
+template<typename Type>
+struct type_list_index<Type, type_list<>> {
+    /*! @brief Unsigned integer type. */
+    using value_type = std::size_t;
+    /*! @brief Compile-time position of the given type in the sublist. */
+    static constexpr value_type value = 0u;
+};
+
+/**
+ * @brief Helper variable template.
+ * @tparam List Type list.
+ * @tparam Type Type to look for and for which to return the index.
+ */
+template<typename Type, typename List>
+inline constexpr std::size_t type_list_index_v = type_list_index<Type, List>::value;
+
 /**
  * @brief Concatenates multiple type lists.
  * @tparam Type Types provided by the first type list.

test/entt/core/type_traits.cpp

@@ -78,6 +78,10 @@ TEST(TypeList, Functionalities) {
     static_assert(std::is_same_v<entt::type_list_element_t<1u, type>, char>);
     static_assert(std::is_same_v<entt::type_list_element_t<0u, other>, double>);
 
+    static_assert(entt::type_list_index_v<int, type> == 0u);
+    static_assert(entt::type_list_index_v<char, type> == 1u);
+    static_assert(entt::type_list_index_v<double, other> == 0u);
+
     static_assert(std::is_same_v<entt::type_list_diff_t<entt::type_list<int, char, double>, entt::type_list<float, bool>>, entt::type_list<int, char, double>>);
     static_assert(std::is_same_v<entt::type_list_diff_t<entt::type_list<int, char, double>, entt::type_list<int, char, double>>, entt::type_list<>>);
     static_assert(std::is_same_v<entt::type_list_diff_t<entt::type_list<int, char, double>, entt::type_list<int, char>>, entt::type_list<double>>);