jwt_object design

include/jwt/detail/meta.hpp

@@ -2,11 +2,15 @@
 #define CPP_JWT_META_HPP
 
 #include <type_traits>
+#include "jwt/string_view.hpp"
 
 namespace jwt {
 namespace detail {
 namespace meta {
 
+/**
+ * The famous void_t trick.
+ */
 template <typename... T>
 struct make_void
 {
@@ -16,7 +20,16 @@ struct make_void
 template <typename... T>
 using void_t = typename make_void<T...>::type;
 
+/**
+ * A type tag representing an empty tag.
+ * To be used to represent a `result-not-found`
+ * situation.
+ */
+struct empty_type {};
 
+
+/**
+ */
 template <typename T, typename=void>
 struct has_create_json_obj_member: std::false_type
 {
@@ -34,6 +47,91 @@ struct has_create_json_obj_member<T,
 {
 };
 
+/**
+ * Checks if the type `T` models MappingConcept.
+ *
+ * Requirements on type `T` for matching the requirements:
+ *  a. Must be able to construct jwt::string_view from the
+ *     `key_type` of the map.
+ *  b. Must be able to construct jwt::string_view from the
+ *    `mapped_type` of the map.
+ *  c. The type `T` must have an access operator i.e. operator[].
+ *  d. The type `T` must have `begin` and `end` member functions
+ *     for iteration.
+ *
+ *  NOTE: Requirements `a` and `b` means that the concept
+ *  type can only hold values that are string or constructible
+ *  to form a string_view (basically C strings and std::string)
+ */
+template <typename T, typename=void>
+struct is_mapping_concept: std::false_type
+{
+};
+
+template <typename T>
+struct is_mapping_concept<T,
+  void_t<
+    typename std::enable_if<
+      std::is_constructible<jwt::string_view, typename T::key_type>::value,
+      void
+    >::type,
+
+    typename std::enable_if<
+      std::is_constructible<jwt::string_view, typename T::mapped_type>::value,
+      void
+    >::type,
+
+    decltype(
+      std::declval<T&>().operator[](std::declval<const typename T::key_type&>()),
+      std::declval<T&>().begin(),
+      std::declval<T&>().end(),
+      (void)0
+    )
+
+  >
+  >: std::true_type
+{
+};
+
+
+/**
+ * Checks if the type `T` models the ParameterConcept.
+ *
+ * Requirements on type `T` for matching the requirements:
+ *  a. The type must have a `get` method.
+ */
+template <typename T, typename=void>
+struct is_parameter_concept: std::false_type
+{
+};
+
+template <typename T>
+struct is_parameter_concept<T,
+  void_t<
+    decltype(
+      std::declval<T&>().get(),
+      (void)0
+    )
+  >
+  >: std::true_type
+{
+};
+
+/**
+ */
+template <bool... V>
+struct bool_pack {};
+
+/**
+ */
+template <bool... B>
+using all_true = std::is_same<bool_pack<true, B...>, bool_pack<B..., true>>;
+
+/**
+ */
+template <typename... T>
+using are_all_params = all_true<is_parameter_concept<T>{}...>;
+
 
 } // END namespace meta
 } // END namespace detail