Mediator Pattern

聊天服务器与用户之间的关系，一个用户向另一个用户发送的消息由聊天服务器执行，用户不关注发送细节。

// Mediator
class User;

class Mediator
{
public:
    Mediator() = default;
    virtual ~Mediator() = default;

    virtual void registerUser(User *user) = 0;
    virtual void unregisterUser(User *user) = 0;
    virtual bool relayMsg(const std::string &from, const std::string &to, const std::string &msg) = 0;
};

class ChatService : public Mediator
{
public:
    ChatService();
    ~ChatService() override;

    void registerUser(User *user) override { m_users.emplace_back(user); }
    void unregisterUser(User *user) override { m_users.remove(user); }
    bool relayMsg(const std::string &from, const std::string &to, const std::string &msg) override
    {
        for (User *user : m_users) {
            if (user->id() != to) {
                continue;
            }
            user->recv(from, msg);
        }
        return true;
    }

protected:
    std::list<User *> m_users;
};

// User
class Mediator;

class User
{
public:
    User() = default;
    virtual ~User() = default;

    virtual const std::string &id() const = 0;
    virtual bool send(const std::string &to, const std::string &msg) = 0;
    virtual void recv(const std::string &from, const std::string &msg) = 0;
};

class GeneralUser : public User
{
public:
    explicit GeneralUser(const std::string &id, Mediator *mediator = nullptr);
    ~GeneralUser() override;

    const std::string &id() const override { return m_id; }
    bool send(const std::string &to, const std::string &msg) override
    {
        if (m_mediator == nullptr) {
            return false;
        }
        return m_mediator->relayMsg(m_id, to, msg);
    }
    void recv(const std::string &from, const std::string &msg) override
    {
        // recived a message
    }

protected:
    Mediator *m_mediator;
    std::string m_id;
};

// main
#include "mediator.h"
#include "user.h"

int main(int argc, char *argv[])
{
    ChatService chatService;
    GeneralUser userA("13579", &chatService);
    GeneralUser userB("02468", &chatService);
    GeneralUser userC("12345", &chatService);

    chatService.registerUser(&userA);
    chatService.registerUser(&userB);
    chatService.registerUser(&userC);

    userA.send("02468", "Hello, 02468!");
    userA.send("12345", "Hello, 12345!");

    userB.send("13579", "Hello, 13579!");
    userB.send("12345", "Hello, 12345!");

    userC.send("13579", "Hello, 13579!");
    userC.send("02468", "Hello, 02468!");

    chatService.unregisterUser(&userC);

    userA.send("02468", "Hello, 02468!");
    userA.send("12345", "Hello, 12345!");

    userB.send("13579", "Hello, 13579!");
    userB.send("12345", "Hello, 12345!");

    return 0;
}

• 中介者模式