Question

I can’t seem to find a clear distinction between the MergeFrom* and the ParseFrom* methods of the MessageLite class in protobuf.

I am trying to minimize the amount of data copying I have to do, so I have written the following code below to decode a length-prefixed-message:

bool StreamMessageDelimiter::receiveWithLengthPrefix(Message& message)
{
    google::protobuf::uint32 messageSize;
    auto_ptr<google::protobuf::uint8> prefixBuf(new google::protobuf::uint8[sizeof(messageSize)]);
    int receivedBytes = receiveNBytes(prefixBuf.get(), sizeof(messageSize));
    if(receivedBytes != sizeof(messageSize))
    {
        return false;
    }

    CodedInputStream prefixInput(prefixBuf.get(), sizeof(messageSize));
    prefixInput.ReadLittleEndian32(&messageSize);

    google::protobuf::uint8* payloadBuf = new google::protobuf::uint8[messageSize];
    receivedBytes = receiveNBytes(payloadBuf, messageSize);
    if(receivedBytes != messageSize)
    {
        return false;
    }

    ArrayInputStream rawInput(payloadBuf, messageSize);
    CodedInputStream codedInput(&rawInput);

    if(!message.MergeFromCodedStream(&codedInput))
    {
        return false;
    }

    return true;
}

My question is does using the MergeFromCodedStream cause message to take ownership of the payloadBuf, or does message make a copy of the underlying data? If message does indeed make a copy, then I obviously should use an auto_ptr for payloadBuf like I did for the prefixBuf.

Thanks for the input!

Answer 1

Check the documentation for ArrayInputStream:

“data” remains the property of the caller but must remain valid until
the stream is destroyed.

So no, it doesn’t take ownership, and you should make sure to release the memory at the right time.

I think you might be confused by the use of the word Merge in the function names. It does not come to say “data is merged from the passed buffer” (which I think caused you to consider ownership), but rather “data is merged into the message”. So, Parse calls Clear() before filling the message, whereas Merge uses the message you passed directly as-is.