Implement --file-names-only (#8788)

* flatc builds and seems to work, some of the extra targets are having linker errors

* fix build system

* pipeline failures

* un-rename files

* refactor to use unique_ptr

* typo

* rm make_unique, add comments

* fix cmake

---------

Co-authored-by: Wouter van Oortmerssen <aardappel@gmail.com>

include/flatbuffers/code_generator.h

@@ -19,12 +19,14 @@
 
 #include <string>
 
+#include "flatbuffers/file_manager.h"
 #include "flatbuffers/idl.h"
 
 namespace flatbuffers {
 
 struct CodeGenOptions {
   std::string output_path;
+  FileSaver* file_saver{nullptr};
 };
 
 // A code generator interface for producing converting flatbuffer schema into

include/flatbuffers/file_manager.h

@@ -17,30 +17,52 @@
 #ifndef FLATBUFFERS_FILE_MANAGER_H_
 #define FLATBUFFERS_FILE_MANAGER_H_
 
+#include <cstddef>
 #include <set>
 #include <string>
 
-#include "flatbuffers/util.h"
-
 namespace flatbuffers {
 
 // A File interface to write data to file by default or
 // save only file names
-class FileManager {
+class FileSaver {
  public:
-  FileManager() = default;
-  virtual ~FileManager() = default;
+  FileSaver() = default;
+  virtual ~FileSaver() = default;
 
-  virtual bool SaveFile(const std::string& absolute_file_name,
-                        const std::string& content) = 0;
+  virtual bool SaveFile(const char* name, const char* buf, size_t len,
+                        bool binary) = 0;
 
-  virtual bool LoadFile(const std::string& absolute_file_name,
-                        std::string* buf) = 0;
+  bool SaveFile(const char* name, const std::string& buf, bool binary) {
+    return SaveFile(name, buf.c_str(), buf.size(), binary);
+  }
+
+  virtual void Finish() {}
 
  private:
   // Copying is not supported.
-  FileManager(const FileManager&) = delete;
-  FileManager& operator=(const FileManager&) = delete;
+  FileSaver(const FileSaver&) = delete;
+  FileSaver& operator=(const FileSaver&) = delete;
+  // Rule of 5
+  FileSaver(FileSaver&&) = default;
+  FileSaver& operator=(FileSaver&&) = default;
+};
+
+class RealFileSaver final : public FileSaver {
+ public:
+  bool SaveFile(const char* name, const char* buf, size_t len,
+                bool binary) final;
+};
+
+class FileNameSaver final : public FileSaver {
+ public:
+  bool SaveFile(const char* name, const char* buf, size_t len,
+                bool binary) final;
+
+  void Finish() final;
+
+ private:
+  std::set<std::string> file_names_{};
 };
 
 }  // namespace flatbuffers

include/flatbuffers/idl.h

@@ -25,6 +25,7 @@
 #include <vector>
 
 #include "flatbuffers/base.h"
+#include "flatbuffers/file_manager.h"
 #include "flatbuffers/flatbuffers.h"
 #include "flatbuffers/flexbuffers.h"
 #include "flatbuffers/hash.h"
@@ -634,6 +635,10 @@ inline bool operator<(const IncludedFile& a, const IncludedFile& b) {
 
 // Container of options that may apply to any of the source/text generators.
 struct IDLOptions {
+  // file saver
+  // shared pointer since this object gets copied and modified.
+  FileSaver* file_saver = nullptr;
+
   // field case style options for C++
   enum CaseStyle { CaseStyle_Unchanged = 0, CaseStyle_Upper, CaseStyle_Lower };
   enum class ProtoIdGapAction { NO_OP, WARNING, ERROR };

include/flatbuffers/util.h

@@ -448,12 +448,16 @@ bool LoadFile(const char* name, bool binary, std::string* buf);
 // If "binary" is false data is written using ifstream's
 // text mode, otherwise data is written with no
 // transcoding.
+// NOTE: this function is deprecated in favor of FileSaver class,
+// but left here for compatibility.
 bool SaveFile(const char* name, const char* buf, size_t len, bool binary);
 
 // Save data "buf" into file "name" returning true if
 // successful, false otherwise.  If "binary" is false
 // data is written using ifstream's text mode, otherwise
 // data is written with no transcoding.
+// NOTE: this function is deprecated in favor of FileSaver class,
+// but left here for compatibility.
 inline bool SaveFile(const char* name, const std::string& buf, bool binary) {
   return SaveFile(name, buf.c_str(), buf.size(), binary);
 }