Manipulator base class

gistfile1.cpp

#include <iostream>
#include <functional>

class manipulator
{
public:
    template<typename Stream, typename F, typename... Args>
    manipulator(Stream& os, F&& f, Args&&... args)
        : callback(std::bind(std::forward<F>(f), std::ref(os), std::forward<Args>(args)...))
    { callback();}
private:
    std::function<void ()> callback;
};

class manipulator_wrapper
{
    class inner;
public:
    manipulator_wrapper(std::string str)
        : value(str)
    {
    }
    
    template<class Stream>
    friend manipulator_wrapper::inner operator<<(Stream& os, const manipulator_wrapper& mw);
private:
    class inner : public manipulator
    {
    public:
        inner(std::ostream& os, std::string str)
            : manipulator(os, &inner::go, str)
        { }
    private:
        static void go(std::ostream& os, std::string str)
        {
            os << "Your message is: " << str;
        }
    };
    std::string value;
};

template<class Stream>
manipulator_wrapper::inner operator<<(Stream& os, const manipulator_wrapper& mw)
{
    return manipulator_wrapper::inner(os, mw.value);
}

manipulator_wrapper display(std::string str)
{
    return manipulator_wrapper(str);
}

int main()
{
    std::cout << display("hello");
}