Circular references between two classes

Question

I know this must be a n00b question, but I have to implement a mockup client-server sequential interaction application, and because the number of client-server calls varies, I cannot just iterate the steps in an external function, always fetching data from the client and then forwarding it to the server and vice-versa, so I need to make my Server and Client classes be aware of each other so that they can call their public methods between themselves. One approach would be to design both as Singletons, but I was hoping to do it in a simpler way, more precisely using a circular reference: the client stores a reference to the server and the server stores a reference to the client. I am aware that this might not be a good approach, and it could cause the call stack to explode when it becomes too deep, so any improvements to my design are welcomed.

In order to achieve the described implementation, I thought that I could use std::shared_ptr, because std::unique_ptr won't work if I also want to prevent the two variables from main from getting clobbered when I call the two setters (right?). So, this is what I have (simplified code):

#include <iostream>
#include <memory>

class Server;

class Client
{
public:
    void SetServer (const Server &server);
private:
    std::shared_ptr<const Server> server;
};

void Client::SetServer (const Server &server)
{
    this->server = std::shared_ptr<const Server>(&server);
}

class Server
{
public:
    void SetClient (const Client &client);
private:
    std::shared_ptr<const Client> client;
};

void Server::SetClient (const Client &client)
{
    this->client = std::shared_ptr<const Client>(&client);
}

int main ()
{
    Server server;
    Client client;

    server.SetClient(client);
    client.SetServer(server);

    //Here I ask the client to start interacting with the server.
    //The process will terminate once the client
    //exhausts all the data it needs to send to the server for processing

    return 0;
}

Unfortunately, my code seems to try to call the Client and Server (implicit) destructors multiple times, or some similar nasty thing, and I'm certain that this is caused by my poor understanding of how std::shared_ptr is supposed to work. Please advise.

Answer 1

You allocate your Server and Client instances on the stack, and they will be deleted when main() exits. You don't want std::shared_ptr deleting them as well. Thus, there are two solutions:

1. Use unmanaged pointers within Client and Server, and manage their lifetimes externally.

2. Use managed pointers everywhere, including main(). Do note that shared_ptr implies ownership. In the current design, a Server owns the client, but also a Client owns the server. That's a circular reference: by default, they'll never be freed, unless you reset one of those pointers while you still can.

   As an example, you can decide that the clients keep the server alive, thus last vanishing client will bring down the server if there are no other shared_ptrs pointing at it. The server would have a weak_ptr to the client(s), but the clients would have a shared_ptr to the server.

class Client;
class Server;

class Client
{
public:
    void SetServer (const std::shared_ptr<const Server> &server);
private:
    std::shared_ptr<const Server> server;
};

void Client::SetServer (const std::shared_ptr<const Server> &server)
{
    this->server = server;
}

class Server
{
public:
    void SetClient (const std::weak_ptr<const Client> &client);
private:
    std::weak_ptr<const Client> client;
};

void Server::SetClient (const std::weak_ptr<const Client> &client)
{
    this->client = client;
}


int main()
{
  std::shared_ptr<Server> server(new Server);
  std::shared_ptr<Client> client(new Client);

  server->SetClient(client);
  client->SetServer(server);

  // do stuff

  return 0;
}

Answer 2

Your variables have automatic lifetime, and will be destroyed when they go out of scope.

Therefore, using any of the lifetime-managing smart pointers is wrong, because it will call delete a second time.