ProtocolBuffer object ByteSize is wrong

Question

I have the following method:

void ConnectionManager::SendAll()
{
for (int t = 0; t < (int)m_Connections.size(); ++t)
{
    if (m_Connections[t].socket != INVALID_SOCKET)
    {
        // Create the object
        MessageID message;
        message.set_type(MessageID::Type::MessageID_Type_PLAYERDATA);

        for (int i = 0; i < (int)m_Connections.size(); ++i)
        {
            if (m_Connections[i].playerData.username() != google::protobuf::internal::kEmptyString)
            {
                auto temp = message.add_playerdata();   
                temp = &m_Connections[i].playerData;
            }
        }

        if (message.playerdata_size() > 0)
        {
            // Serialize to byte array
            int size = message.ByteSize();
            void* buffer = malloc(size);
            message.SerializeToArray(buffer, size);

            Send(m_Connections[t].socket, buffer, size);
        }
    }
}
}

Now the problem lies at the end of this method, at the line:

int size = message.ByteSize();

I know that the data is loaded correctly (or it should be at least) but the size isn't right. It should be 30 and it returns 2. I have no idea what I'm doing wrong.

The data in m_Connections is available and should be reached by pointer temp. I think that, for some reason, the data is lost from the "message" object but I don't know how to solve it.

Kenton Varda · Accepted Answer · 2014-08-28 18:27:32Z

1

auto temp = message.add_playerdata();   
temp = &m_Connections[i].playerData;

These lines look wrong. add_playerdata() returns a pointer. That means that the second line is just setting temp to some other pointer, not doing anything to the message which temp points at. It's more obvious if you write out the type rather than use auto:

MessageID::PlayerData* temp = message.add_playerdata();   
temp = &m_Connections[i].playerData;

Maybe you wanted to do this instead:

*temp = m_Connections[i].playerData;

However, I do not see how this bug would lead to ByteSize() being 2. It looks like ByteSize() should only be 2 if you haven't added any players to the message, but then playerdata_size() would be zero, so you wouldn't get to the serialization step at all.

answered Aug 28, 2014 at 18:27

Kenton Varda

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3 Comments

Dries Over a year ago

Well the thing is that temp is a new item in a vector of type PlayerData* in the MessageID object "message". What I wanted to do in the line you copied was to assign the data from a vector I have created to the vector that is used in the MessageID object. I'll try your ideas.

Dries Over a year ago

Well, I haven't checked the data yet but apparently the line: *temp = m_Connections[i].playerData; solved the crash. Could you perhaps explain why?

Kenton Varda Over a year ago

Well, this is basic C... temp is a pointer, *temp is the thing the pointer points at. If you write temp = &foo;, you're only changing the pointer, to make it point at foo rather than at whatever it was pointing at before. Since you don't even use temp again after that, this has no effect. On the other hand, when you write *temp = foo;, you are copying the value of foo into the thing that temp points at.

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