Parse an id and instanciate an associated class - c++

Question

Im building a protocol in c++ for a TCP socket communication.
When I catched the id of an incoming packet, I would like to make a generic code that find which known packet (in my protocol) that I received.
class NetworkMessage {
public:
virtual ~NetworkMessage();
virtual void serialize(NetworkBuffer &buffer) = 0;
virtual void deserialize(NetworkBuffer &buffer) = 0;
virtual int32_t getProtocolId() const = 0;
};
Example of message
class HelloConnectMessage: public NetworkMessage {
public:
static constexpr int32_t PROTOCOL_ID = 1;
void serialize(NetworkBuffer &buffer) override;
void deserialize(NetworkBuffer &buffer) override;
int32_t getProtocolId() const override;
int32_t getHelloVar() const;
private:
int32_t _hellovar;
};
I wanted to map the protocolId to the associated child class of NetworkMessage, but we can't hold class types like in Java.
I was about to make something like the following code, that can't compile since it's just an idea of what I really want.
std::unordered_map<int, class<? extends NetworkMessage>> messages;
My alternative for now, below:
static NetworkMessage build_packet(NetworkBuffer &buffer) {
int32_t id = buffer.readInt();
switch(id) {
case HelloConnectMessage::PROTOCOL_ID:
HelloConnectMessage msg;
msg.deserialize(buffer);
return msg;
default:
throw std::runtime_error("invalid packet");
}
}
But this is not enough generic.. I will waste time rewriting always the same code for each message.. Would appreciate any help, im not familiar to c++

Answer 1

Populate your map with factory functions. Something like this:
template <typename Msg>
std::unique_ptr<NetworkMessage> MakeMessage() { return std::make_unique<Msg>(); }
using Factory_t = std::unique_ptr<NetworkMessage>(*)();
std::unordered_map<int, Factory_t> messages = {
{HelloConnectMessage::PROTOCOL_ID, MakeMessage<HelloConnectMessage>},
...
};
std::unique_ptr<NetworkMessage> build_packet(NetworkBuffer &buffer) {
...
auto msg = messages[id]();
msg->deserialize(buffer);
return msg;
}