added object_cache and test

include/matador/utils/identifier.hpp

@@ -23,7 +23,6 @@ public:
   virtual void serialize(uint16_t &, const field_attributes &) = 0;
   virtual void serialize(uint32_t &, const field_attributes &) = 0;
   virtual void serialize(uint64_t &, const field_attributes &) = 0;
-  virtual void serialize(const char *, const field_attributes &) = 0;
   virtual void serialize(std::string &, const field_attributes &) = 0;
   virtual void serialize(null_type_t &, const field_attributes &) = 0;
 };
@@ -149,7 +148,7 @@ public:
   }
 
   explicit identifier(const char *id)
-  : id_(std::make_shared<pk<const char *> >(id)) {
+  : id_(std::make_shared<pk<std::string> >(id)) {
   }
 
   identifier(const identifier &x);
@@ -164,7 +163,7 @@ public:
   }
 
   identifier &operator=(const char *value) {
-    id_ = std::make_shared<pk<const char *> >(value);
+    id_ = std::make_shared<pk<std::string> >(value);
     return *this;
   }
 
@@ -220,4 +219,13 @@ struct id_pk_hash {
 /// @endcond
 }
 
+namespace std {
+template<>
+struct hash<matador::utils::identifier> {
+  size_t operator()(const matador::utils::identifier &id) const noexcept {
+    return id.hash();
+  }
+};
+} // namespace std
+
 #endif //MATADOR_IDENTIFIER_HPP