Add a FileWriter interface (#7821)

* Add a FileWriter interface

* Change interface

* Provide 2 impl for File interface: FileManager & FileNameManager

* Update

* update

* Update

* Add file_writer file

* Update

* Format files

* Update based on review

* Update

* Format bzl file

* Add LoadFile function

* Format

---------

Co-authored-by: Derek Bailey <derekbailey@google.com>

src/file_writer.cpp

@@ -0,0 +1,47 @@
+/*
+ * Copyright 2023 Google Inc. All rights reserved.
+ *
+ * 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.
+ */
+
+#include <fstream>
+#include <set>
+#include <string>
+
+#include "flatbuffers/file_manager.h"
+
+namespace flatbuffers {
+
+class FileWriter : public FileManager {
+ public:
+  bool SaveFile(const std::string &absolute_file_name,
+                const std::string &content) override {
+    std::ofstream ofs(absolute_file_name, std::ofstream::out);
+    if (!ofs.is_open()) return false;
+    ofs.write(content.c_str(), content.size());
+    return !ofs.bad();
+  }
+
+  bool Loadfile(const std::string &absolute_file_name, std::string *output) {
+    if (DirExists(absolute_file_name.c_str())) return false;
+    std::ifstream ifs(absolute_file_name, std::ifstream::in);
+    if (!ifs.is_open()) return false;
+    // This is slower, but works correctly on all platforms for text files.
+    std::ostringstream oss;
+    oss << ifs.rdbuf();
+    *output = oss.str();
+    return !ifs.bad();
+  }
+};
+
+}  // namespace flatbuffers