protobuf: default value of optional enum

Summary

The default value is helpful when using enum in protobuf; but there're some tips I'd like to highlight here:

1. If the modifier is required, set_xx() is still necessary before serializing
2. xx() will return the default value
3. has_xx() still return false if set_xx() is not executed

Example of protobuf

message Msg {
    enum Type {
        A = 1;
        B = 2;
    }

    optional Type type = 1 [ default = B ];
}

The following code slice shows that the default value of type will be B, and has_type() returns false if set_type() is not executed.

#include <iostream>
#include "opt.pb.h"

using namespace std;

int main(int argc, char** argv)
{
    Msg msg;
    if (msg.has_type()) {
        cout << "default type is Enabled!" << endl;
    } else {
        cout << "default type is Optional" << endl;
    }

    string buf;

    msg.SerializeToString(&buf);

    msg.ParseFromString(buf);

    if (msg.has_type()) {
        cout << "default type is Enabled!" << endl;
    } else {
        cout << "default type is Optional" << endl;
    }

    Msg::Type t = msg.type();

    cout << t << endl;

    msg.set_type(Msg::A);
    if (msg.has_type()) {
        cout << "default type is Enabled!" << endl;
    } else {
        cout << "default type is Optional" << endl;
    }

    return 0;
}