Understanding protocols - protocols

Question

guys need some insight here.
I know the definition of a protocol, being new to this c++ programming is quite a challenging
task.I am creating a Multi-threaded chat using SDL/C++, this is a learning experience for me
and now i have encounter a hump in which I need to overcome but understanding it is a little more difficult than I had thought.I need to make a chat protocol of some sort, I think...but am stump. Up until this point i have been sending messages in strings of characters.Now that am improving the application to the point where clients can register and login, I need a better way to communicating with my clients and server.
thank you.

Answer 1

Create objects that represent a message, then serialize the object, send it over the network, then deserialize at the other end.
For example, you could create a class called LoginMessage that contains two fields. One for a user name, and one for a password. To login, you would do something like:
LoginMessage *msg = new LoginMessage();
msg->username = "Fred";
msg->password = "you'll never guess";
char *serialized_msg = serialize(msg);
// send the bytes over the network
You would do something similar at the other end to convert the byte stream back into an object.
There are APIs for creating message objects and serializing them for you. Here are two popular ones. Both should suit your needs.
Protocol Buffers by Google
Thrift By Facebook
If you want the serialized messages to be readable, you can use YAML. Google has an API called yaml-cpp for serializing data to YAML format.
UPDATE:
Those APIs are for making your own protocol. They just handle the conversion of messages from object form to byte stream form. They do have feature for the actual transport of the messages over the network, but you don't need to use those features. How you design your protocol it up to you. But if you want to create messages by hand, you can do that too.
I'll give you some ideas for creating your own message format.
This is one way to do it.
Have the first 4 bytes of the message represent the length of the message as an unsigned integer. This is necessary to figure out where one message ends and where the next one starts. You will need to convert between host and network byte order when reading and writing to/from these four bytes.
Have the 5th byte represent the message type. For example, you could use a 1 to indicate a login request, a 2 to indicate a login response, and 3 to indicate a chat message. This byte is necessary for interpreting the meaning of the remaining bytes.
The remaining bytes would contain the message contents. For example, if it was a login message, you would encode the username and password into these bytes somehow. If it is a chat message, these bytes would contain the chat text.