Rework book server and client.

examples/book_server/book.h

@@ -0,0 +1,24 @@
+#ifndef BOOK_H_
+#define BOOK_H_
+
+#include <iosfwd>
+#include <string>
+
+#include "boost/filesystem/path.hpp"
+
+struct Book {
+  std::string id;
+  std::string title;
+  double price;
+  std::string photo;  // Name only
+
+  bool IsNull() const {
+    return id.empty();
+  }
+};
+
+std::ostream& operator<<(std::ostream& os, const Book& book);
+
+extern const Book kNullBook;
+
+#endif  // BOOK_H_