/*****************/
/* Wrapper class */
/*****************/

#include <boost/optional.hpp>
#include <boost/utility/declval.hpp>
#include <boost/utility/result_of.hpp>

template <typename Func>
class tr1_functor
{
public:
    // tr1 result_of support
    template <typename Sig>
    struct result;

    template <typename This, typename... Args>
    struct result<This(Args...)>
    {
        typedef decltype(boost::declval<This>()(boost::declval<Args>()...)) type;
    };

    // ctors
    tr1_functor() {}
    tr1_functor(Func const& f) : f_(f) {}

    // assignment
    tr1_functor& operator=(tr1_functor const& other)
    {
        if (f_) {
            f_ = boost::none;
        }
        if (other.f_) {
            f_ = boost::in_place(other.f_.get());
        }
        return *this;
    }

    // invoke
    // precondition: f_ is initialized
    template <typename... Args>
    typename result<Func(Args&&...)>::type operator()(Args&&... args)
    {
        return (*f_)(static_cast<Args&&>(args)...);
    }
    
    // precondition: f_ is initialized
    template <typename... Args>
    typename result<Func const(Args&&...)>::type operator()(Args&&... args) const
    {
        return (*f_)(static_cast<Args&&>(args)...);
    }

private:
    // data
    boost::optional<Func> f_;
};

template <typename Func>
tr1_functor<Func> make_tr1_functor(Func const& f)
{
    return tr1_functor<Func>(f);
}


/***************/
/* Sample code */
/***************/
#include <iostream>
#include <boost/foreach.hpp>
#include <boost/range/irange.hpp>
#include <boost/range/adaptor/transformed.hpp>

int main (int argc, char* argv[])
{    
    using boost::adaptors::transformed;
    
    BOOST_FOREACH(
        int i
      , boost::irange(0,5)
      | transformed( make_tr1_functor([](int i){ return i * 100; }) )
    )
    {
        std::cout << i << " ";
    }
    // Output: "0 100 200 300 400"
    
    return 0;
}