Demonstrate how to save and read a protobuf message AddressBook to/from pb or pbtxt files.

pb_utils.cc

#include <iostream>
#include <fstream>
#include <string>
#include <sstream>


#include <google/protobuf/text_format.h>

// for read_from_pbtxt_nocopy
#include <fcntl.h>
#include <google/protobuf/io/zero_copy_stream_impl.h>


#include "addressbook.pb.h"

using namespace std;
using namespace tutorial;



bool save_to_pb(const AddressBook& address_book, string path) {
  fstream fout(path, ios::out | ios::trunc | ios::binary);
  if (!fout.is_open()) return false;
  return address_book.SerializeToOstream(&fout);
}

bool read_from_pb(AddressBook& address_book, string path) {
  fstream fin(path, ios::in | ios::trunc | ios::binary);
  if (!fin.is_open()) return false;
  return address_book.ParseFromIstream(&fin);
}

bool save_to_pbtxt(AddressBook& address_book, string path) {
  string content;
  if (google::protobuf::TextFormat::PrintToString(address_book, &content)) {
    fstream out(path, ios::out | ios::trunc);
    if (!out.is_open()) return false;
    out << content;
    out.close();
    return true;
  } else {
    return false;
  }
}

bool read_from_pbtxt(AddressBook& address_book, string path) {
  ifstream fin(path);
  if (!fin.is_open()) return false;
  stringstream ss;
  ss << fin.rdbuf();
  return google::protobuf::TextFormat::ParseFromString(ss.str(), &address_book);    
}


bool read_from_pbtxt_nocopy(AddressBook& address_book, string path) {
  auto fd = open(path.c_str(), O_RDONLY);
  if (fd < 0) {
    return false;
  }
  google::protobuf::io::FileInputStream fin(fd);
  fin.SetCloseOnDelete(true);
  return google::protobuf::TextFormat::Parse(&fin, &address_book);
}

When opening a file for reading, ios::trunc does not make sense.

Thanks @SandSnip3r . You are right. It's an error.