Why protobuf only read the last message as input result?

问题

Usually, we use protobuf to communicate a message, for multiple times, each with different message content. But I found seems the reader side process the whole message, and only the last one is used, like below:

$cat 30.proto
message hello
{
    required int32 f1=1;
    required int32 f2=2;
    optional int32 f3=3;
}

$cat 30.cpp
#include "30.pb.h"
#include<fstream>
#include<iostream>
using namespace std;
int main()
{
    fstream fo("./log30.data",ios::binary|ios::out);
    hello p1,p2,p3;
    p1.set_f1(1);
    p1.set_f2(2);
    p2.set_f1(3);
    p2.set_f2(4);
    p3.set_f1(5);
    p3.set_f2(6);
    p1.SerializeToOstream(&fo);
    p2.SerializeToOstream(&fo);
    p3.SerializeToOstream(&fo);
    fo.close();

    fstream fi("./log30.data",ios::binary|ios::in);
    hello pi;
    pi.ParseFromIstream(&fi);
    cout<<pi.f1()<<pi.f2()<<endl;
    return 0;
}

Compile and run, the program outputs: 56

Well, I expected that when I first parse from log30.data, the "pi" should read the first object and thus print "12". To my supprise, seems the Parse will go to the end of the message and give the final one.

My question is, we use pb as a rpc encoding/decoding channel to convey many messages between peers, if only one message is being parsed, what's its really usage in production level? Maybe my interpretation is incorrect, please kindly correct and explain.

Thanks a lot

回答1:

You must only write one message to the output stream. The API does not provide a record-separating container format!

Thus when you read the data everything is assumed to be part of the same message, and the usual protocol of retaining only the last value for single-item fields is followed.

来源：https://stackoverflow.com/questions/41626128/why-protobuf-only-read-the-last-message-as-input-result