concurrent.hpp

#include <exception>
#include <future>
#include <thread>
#include <tbb/concurrent_queue.h>

namespace {
    template<typename Future, typename F, typename T>
    void set_value(std::promise<Future>& p, F& f, T& t) {
        p.set_value(f(t));
    }

    template<typename F, typename T>
    void set_value(std::promise<void>& p, F& f, T& t) {
        f(t);
        p.set_value();
    }
}

template<typename T>
class concurrent {
public:
    concurrent(T t = T{})
        : t{t}
        , thrd{[=]{
            while (!done) {
                std::function<void()> f;
                q.pop(f);
                f();
            }
        }}
    { /* empty */ }

    ~concurrent() {
        q.push([=]{ done = true; });
        thrd.join();
    }

    template<typename F>
    auto operator()(F f) const -> std::future<decltype(f(t))> {
        auto p = std::make_shared<std::promise<decltype(f(t))>>();
        auto r = p->get_future();

        q.push([=]{
            try { set_value(*p, f, t); }
            catch(...) { p->set_exception(std::current_exception()); }
        });

        return r;
    }

private:
    /* mutable */ T t;
    mutable tbb::concurrent_bounded_queue<std::function<void()>> q;
    bool done = false;
    std::thread thrd;
};

main.cpp

#include <iostream>
#include <vector>
#include "concurrent.hpp"

int main() {
    using namespace std;

    concurrent<ostream&> async_cout{cout};
    vector<future<void>> v;

    for (int i = 0; i < 5; i++) {
        v.push_back(async([&, i]{

            async_cout([=](ostream& cout){
                cout << to_string(i) << " " << to_string(i);
                cout << "\n";
            });

            async_cout([=](ostream& cout){
                cout << "Hi from " << i << endl;
            });
        }));
    }

    for (auto& f : v)
        f.wait();
    cout << "Done\n";
    async_cout([](ostream& cout){ cout << "Done\n"; });

    /*
    concurrent<string> s{"start\n"};
    vector<future<void>> v;

    for (int i = 0; i < 5; i++) {
        v.push_back(async([&, i]{
            s([=](string& s){
                s += to_string(i) + " " + to_string(i);
                s += "\n";
            });

            s([](string& s){
                cout << s;
            });
        }));
    }

    for (auto& f : v)
        f.wait();
    cout << "Done\n";
    */
}