C++ Callbacks

callback.cpp

#include <functional>
#include <vector>
#include <memory>

template<typename... X> struct Callback;
template <typename R, typename ...ARGS> struct Callback<R(ARGS...)>
{
    struct BaseFx
    {
        virtual R call(ARGS...) = 0;
    };

    template <typename FX> struct Fx : public BaseFx 
    {
        Fx(FX f) : f(f) {}
        virtual R call(ARGS... args)
        {
            return f(args...);
        };
        FX f;
    };

    using Holder = std::shared_ptr<std::function<void(std::nullptr_t)>>;

    template <typename FX> Holder add(FX f)
    {
        callbacks.push_back(std::make_shared<Fx<FX>>(f));
        auto cb = callbacks.back();
        return Holder(nullptr, [=](std::nullptr_t) {
            callbacks.erase(remove(callbacks.begin(), callbacks.end(), cb));
        });
    };

    void call(ARGS... args)
    {
        for(auto &cb : callbacks)
            cb->call(args...);
    }

    std::vector<std::shared_ptr<BaseFx>> callbacks;    
};

int main()
{
    // Outputs 5.0
    Callback<void(float, float)> test;
    {
        auto holder = test.add([](float x, float y) {
            printf("%.1f\n", x+y);
        });
        test.call(3.0f, 2.0f);
    }
    test.call(1.0f, 5.0f);
    return 0;
}