Add virtual converter output interface

typefactory/converter.h

@@ -2,21 +2,32 @@
 #define CONVERTER_H
 
 #include <memory>
+#include <cstddef>
 #include <string>
+#include <vector>
 
 class Converter {
 public:
   explicit Converter (std::string input_file_name);
-  ~Converter();
+  virtual ~Converter();
 
   Converter (const Converter &) = delete;
   Converter &operator= (const Converter &) = delete;
 
-  bool save (const std::string &output_file_name);
+  virtual bool save (const std::string &output_file_name) = 0;
   [[nodiscard]] int error_code() const noexcept;
 
 protected:
   bool parse();
+  void set_error_code (int error_code) noexcept;
+
+  [[nodiscard]] const std::vector<std::string> &titles() const noexcept;
+  [[nodiscard]] std::size_t quote_count (const std::string &title) const;
+  [[nodiscard]] const std::string &quote_meta (const std::string &title,
+                                               std::size_t index) const;
+  [[nodiscard]] const std::vector<std::string> &quote_text_lines (
+    const std::string &title,
+    std::size_t index) const;
 
 private:
   struct Impl;