I have an application where number of messages is a priory unknown. I can
open output file and write 0 in the beginning:
Writer::Writer(const string &filename):
_output(filename.c_str(), ios::out | ios::trunc | ios::binary),
_events_written(0)
{
_raw_out.reset(new
::google::protobuf::io::OstreamOutputStream(&_output));
_coded_out.reset(new
::google::protobuf::io::CodedOutputStream(_raw_out.get()));
_coded_out->WriteVarint32(_events_written);
}
and then in the destructor reset the position in the ofstream to the
beginning to write the number of the events:
Writer::~Writer()
{
_coded_out.reset();
_raw_out.reset();
_output.seekp(0);
// Small trick to save number of the events
//
_raw_out.reset(new
::google::protobuf::io::OstreamOutputStream(&_output));
_coded_out.reset(new
::google::protobuf::io::CodedOutputStream(_raw_out.get()));
_coded_out->WriteVarint32(_events_written);
_coded_out.reset();
_raw_out.reset();
_output.close();
}
Then read the beginning of the file with number of the events:
Reader::Reader(const string &filename):
_input(filename.c_str(), ios::in | ios::binary),
_is_good(true),
_events_written(0)
{
_raw_in.reset(new ::google::protobuf::io::IstreamInputStream(&_input));
_coded_in.reset(new
::google::protobuf::io::CodedInputStream(_raw_in.get()));
_coded_in->ReadVarint32(&_events_written);
}
Everything seems fine. Then events can be read one by one like:
bool Reader::read(Event &event)
{
event.Clear();
uint32_t message_size;
if (!_coded_in->ReadVarint32(&message_size))
{
_is_good = false;
return false;
}
if (0 < message_size)
{
string message;
if (!_coded_in->ReadString(&message, message_size) ||
!event.ParseFromString(message))
return false;
}
return true;
}
Unfortunately, ReadVarint32 fails for some reason in the Reader::read(...)
method.
The code works fine in case I do not use seekp in the Writer::~Writer().
What is the proper way to seek to the beginning of the file and store number
of entries?
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