Constructor arguments from tuple

You need some template meta-programming machinery to achieve that.

The easiest way to realize the argument dispatch is to exploit pack expansion on expressions which contain a packed compile-time sequence of integers. The template machinery is needed to build such a sequence (also see the remark at the end of this answer for more information on a proposal to standardize such a sequence).

Supposing to have a class (template) index_range that encapsulates a compile-time range of integers [M, N) and a class (template) index_list that encapsulates a compile-time list of integers, this is how you would use them:

template<typename T, typename... Args>
struct foo
{
    tuple<Args...> args;

    // Allows deducing an index list argument pack
    template<size_t... Is>
    T gen(index_list<Is...> const&)
    {
        return T(get<Is>(args)...); // This is the core of the mechanism
    }

    T gen()
    {
        return gen(
            index_range<0, sizeof...(Args)>() // Builds an index list
            );
    }
};

And here is a possible implementation of index_range and index_list:

//===============================================================================
// META-FUNCTIONS FOR CREATING INDEX LISTS

// The structure that encapsulates index lists
template <size_t... Is>
struct index_list
{
};

// Collects internal details for generating index ranges [MIN, MAX)
namespace detail
{
    // Declare primary template for index range builder
    template <size_t MIN, size_t N, size_t... Is>
    struct range_builder;

    // Base step
    template <size_t MIN, size_t... Is>
    struct range_builder<MIN, MIN, Is...>
    {
        typedef index_list<Is...> type;
    };

    // Induction step
    template <size_t MIN, size_t N, size_t... Is>
    struct range_builder : public range_builder<MIN, N - 1, N - 1, Is...>
    {
    };
}

// Meta-function that returns a [MIN, MAX) index range
template<unsigned MIN, unsigned MAX>
using index_range = typename detail::range_builder<MIN, MAX>::type;

Also notice, that an interesting proposal by Jonathan Wakely exists to standardize an int_seq class template, which is something very similar to what I called index_list here.

C++17 has std::make_from_tuple for this:

template <typename T, typename... Args>
struct foo
{
  std::tuple<Args...> args;
  T gen() { return std::make_from_tuple<T>(args); }
};

C++14 will add standard support for index_sequence:

template<typename T, typename... Args>
struct foo {
  tuple<Args...> args;
  T gen() { return gen_impl(std::index_sequence_for<Args...>()); }
private:
  template <size_t... Indices>
  T gen_impl(std::index_sequence<Indices...>) { return T(std::get<Indices>(args)...); }
};

Use index_sequence to unpack a std::tuple (or std::pair, std::array, or anything else supporting the tuple interface):

#include <utility>
#include <tuple>

template <typename Tuple, std::size_t... Inds>
SomeClass help_make_SomeClass(Tuple&& tuple, std::index_sequence<Inds...>)
{
    return SomeClass(std::get<Inds>(std::forward<Tuple>(tuple))...);
}

template <typename Tuple>
SomeClass make_SomeClass(Tuple&& tuple)
{
    return help_make_SomeClass(std::forward<Tuple>(tuple),
        std::make_index_sequence<std::tuple_size<Tuple>::value>());
}

std::index_sequence and std::make_index_sequence will be in C++1y. If you can't find a header that defines them, you could use these:

template <std::size_t... Inds>
struct index_sequence {
    static constexpr std::size_t size()
    { return sizeof...(Inds); }
};

template <std::size_t N, std::size_t... Inds>
struct help_index_seq {
    typedef typename help_index_seq<N-1, N-1, Inds...>::type type;
};

template <std::size_t... Inds>
struct help_index_seq<0, Inds...> {
    typedef index_sequence<Inds...> type;
};

template <std::size_t N>
using make_index_sequence = typename help_index_seq<N>::type;

Live example, in C++11 mode: http://coliru.stacked-crooked.com/a/ed91a67c8363061b