fable: Provide fable/fable_fwd.hpp header for forward declarations

This follows the pattern set by other libraries, such as Boost
and nlohmann/json.

fable/include/fable/fable.hpp

@@ -25,6 +25,7 @@
 
 #pragma once
 
+#include <fable/fable_fwd.hpp>
 #include <fable/conf.hpp>
 #include <fable/confable.hpp>
 #include <fable/enum.hpp>

fable/include/fable/fable_fwd.hpp

@@ -0,0 +1,51 @@
+/*
+ * Copyright 2023 Robert Bosch GmbH
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+/**
+ * \file fable/fable_fwd.hpp
+ */
+
+#include <string>
+
+#include <nlohmann/detail/value_t.hpp>
+#include <nlohmann/json_fwd.hpp>
+
+namespace fable {
+
+// from json.hpp
+using Json = nlohmann::json;
+using JsonPointer = nlohmann::json_pointer<std::string>;
+using JsonType = nlohmann::detail::value_t;
+
+// from conf.hpp
+class Conf;
+
+// from error.hpp
+class Error;
+class ConfError;
+class SchemaError;
+
+// from schema.hpp
+class Schema;
+
+// from confable.hpp
+class Confable;
+
+// from environment.hpp
+class Environment;
+
+}  // namespace fable

fable/include/fable/json.hpp

@@ -65,15 +65,15 @@ using Json = nlohmann::json;
  * This makes it easier to talk/write about operations that deal with
  * JSON pointers.
  */
-using JsonPointer = Json::json_pointer;
+using JsonPointer = nlohmann::json_pointer<std::string>;
 
 /**
  * The JsonType type maps to nlohmann::json::value_t.
  *
  * This makes it easier to talk/write about operations that deal on the
  * underlying type that is stored in a Json value.
  */
-using JsonType = Json::value_t;
+using JsonType = nlohmann::detail::value_t;
 
 /**
  * Return a string representation of a JSON type.
@@ -100,7 +100,7 @@ std::string to_string(JsonType);
  * See the documentation on each of these global variables for more details.
  */
 template <typename InputType>
-inline Json parse_json(InputType&& input) {
+Json parse_json(InputType&& input) {
   return Json::parse(std::forward<InputType>(input), nullptr, NLOHMANN_JSON_USE_EXCEPTIONS,
                      NLOHMANN_JSON_ALLOW_COMMENTS);
 }