Generic messaging - c++

Question

I am working on a messaging system on C++. I have;
class MessageData
{
public:
typedef std::vector<std::shared_ptr<MessageData>> MessageList;
virtual int getValue(std::shared_ptr<int>) { throw "Not implemented!"; };
virtual float getValue(std::shared_ptr<float>) { throw "Not implemented!"; };
virtual std::string getValue(std::shared_ptr<std::string>) { throw "Not implemented!"; };
...
...
virtual ~MessageData() {};
};
template <typename T>
class Message : public MessageData
{
T val;
public:
static std::shared_ptr<Message<T>> Make(T val) { return std::make_shared<Message<T>>(val); };
static T Get(std::shared_ptr<MessageData> in) { return in->getValue(std::make_shared<T>()); };
Message(T i) { val = i; };
T getValue(std::shared_ptr<T> out) override { return *out = val; }
~Message() {};
};
Using these, I can send/receive generic messages of different length conveniently using e.g;
sendMessage(MessageData::MessageList{
Message<std::string>::Make("paint"),
Message<int>::Make(14),
Message<float>::Make(129.3f),
...
});
Then I get the values;
sendMessage(MessageData::MessageList data) {
auto a = Message<std::string>::Get(data[0]);
auto b = Message<int>::Get(data[1]);
auto c = Message<float>::Get(data[2]);
...
}
The downside is that I have to list all the types I need to use in MessageData class. This isn't a big deal as I can limit the types I want to support but I'm really curious about how to templatize the type list without using a 3rd party library. Or is there a completely different and better method that I can use with similar clean syntax and type safety to pass messages around?

Answer 1

One way to make your code more generic is:
template <typename ... Ts>
class MessageDataImp;
template <typename T>
class MessageDataImp<T>
{
public:
virtual ~MessageDataImp() = default;
virtual T getValue(std::shared_ptr<T>) { throw "Not implemented!"; };
};
template <typename T, typename ... Ts>
class MessageDataImp<T, Ts...> : public MessageDataImp<T>, public MessageDataImp<Ts...>
{
public:
using MessageDataImp<T>::getValue;
using MessageDataImp<Ts...>::getValue;
};
template <typename ... Ts>
class MessageDataTs : public MessageDataImp<Ts...>
{
public:
typedef std::vector<std::shared_ptr<MessageDataTs<Ts...>>> MessageList;
};
using MessageData = MessageDataTs<int, float, std::string>;

Answer 2

I think I've developed a decent solution to my problem.
class MessageData {
public:
typedef std::vector<std::shared_ptr<MessageData>> MessageList;
virtual ~MessageData() {};
};
template<typename T>
class Message : public MessageData {
T val;
public:
template<typename U>
friend U GetMessage(std::shared_ptr<MessageData> in);
Message(T i) { val = i; };
};
template<typename T>
T GetMessage(std::shared_ptr<MessageData> in) {
std::shared_ptr<Message<T>> tmp = std::dynamic_pointer_cast<Message<T>>(in);
if (tmp) {
return tmp->val;
}
throw "Incorrect type!";
};
template<typename T>
std::shared_ptr<Message<T>> MakeMessage(T val)
{
return std::make_shared<Message<T>>(val);
};
Then send & receive values using;
sendMessage(MessageData::MessageList{
MakeMessage(std::string("paint")),
MakeMessage(14),
MakeMessage(129.3f),
...
});
sendMessage(MessageData::MessageList data) {
auto a = GetMessage<std::string>(data[0]);
auto b = GetMessage<int>(data[1]);
auto c = GetMessage<float>(data[2]);
...
}

Answer 3

Assuming that it's a simple multiple-reader, multiple-writer message bus based on a non-prioritised queue, I think I'd start with something like this:-
Note that I have used boost::variant/optional. These could easily be replaced with std:: versions if you have those available.
I have used variant because it efficiently caters for most use cases with compile-time safety.
The std/boost::any version would require significant (and possibly unwelcome) care for users of your bus.
#include <iostream>
#include <string>
#include <queue>
#include <thread>
#include <condition_variable>
#include <boost/variant.hpp>
#include <boost/optional.hpp>
template<class Mutex> auto get_lock(Mutex& m) { return std::unique_lock<Mutex>(m); }
template<class...Types>
struct message_bus
{
using message_type = boost::variant<Types...>;
void push(message_type msg) {
auto lock = get_lock(mutex_);
messages_.push(std::move(msg));
lock.unlock();
activity_.notify_one();
}
boost::optional<message_type> wait_pop()
{
boost::optional<message_type> result;
auto lock = get_lock(mutex_);
activity_.wait(lock, [this] { return this->stopped_ or not this->messages_.empty(); });
if (not messages_.empty())
{
result = std::move(messages_.front());
messages_.pop();
}
return result;
}
void signal_stop()
{
auto lock = get_lock(mutex_);
stopped_ = true;
lock.unlock();
activity_.notify_all();
}
std::queue<message_type> messages_;
std::mutex mutex_;
std::condition_variable activity_;
bool stopped_ = false;
};
static std::mutex emit_mutex;
template<class T>
void emit(const T& t)
{
auto lock = get_lock(emit_mutex);
std::cout << std::this_thread::get_id() << ": " << t << std::endl;;
}
int main()
{
using bus_type = message_bus<std::string, int>;
bus_type mb;
std::vector<std::thread> threads;
for (int i = 0 ; i < 10 ; ++i)
{
threads.emplace_back([&]
{
for(;;)
{
auto message = mb.wait_pop();
if (not message)
break;
boost::apply_visitor([](auto&& data) { emit(data); }, message.value());
}
});
}
for (int i = 0 ; i < 1000 ; ++i)
{
mb.push("string: " + std::to_string(i));
mb.push(i);
}
mb.signal_stop();
for (auto& t : threads) if (t.joinable()) t.join();
}