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How to unzip a parameter of a variational pattern with a numerical sequence?

How (or is it possible) to unpack a packet of parameters with a numerical sequence? For instance,

template <typename C, typename... T>
C* init_from_tuple(bp::tuple tpl)
{
   return new C{bp::extract<T>("magic"(tpl))...}; // <--
}

which string <--should expand to

   return new C{bp::extract<T0>(tpl[0]),
                bp::extract<T1>(tpl[1]),
                .....
                bp::extract<Tn>(tpl[n])};

where n == sizeof...(T) - 1.

The goal is to create an __init__ function for Boost.Python that accepts a tuple with predefined types.

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2 answers

In fact, unpacking operations can immediately target two different packages of parameters (I think they should be the same length). Here we need a bunch of types and a packet of numbers.

Something similar to:

template <typename C, typename... T, size_t... N>
C* init_from_tuple_impl(bp::tuple tpl) {
  return new C{ bp::extract<T>(tpl[N])... };
}

We just need to create an index package:

template <size_t... N> struct Collection {};

template <typename C> struct ExtendCollection;

template <size_t... N>
struct ExtendCollection< Collection<N...> > {
  typedef Collection<N..., sizeof...(N)> type;
};

template <typename... T>
struct GenerateCollection;

template <>
struct GenerateCollection<> { typedef Collection<> type; };

template <typename T0, typename... T>
struct GenerateCollection<T0, T...> {
  typedef typename ExtendCollection<
    typename GenerateCollection<T...>::type
  >::type type;
};

And then use it:

template <typename C, typename... T, size_t... N>
C* init_from_tuple_impl(bp::tuple tpl, Collection<N...>) {
  return new C { bp::extract<T>(tpl[N])... };
}

template <typename C, typename... T>
C* init_from_tuple(bp::tuple tpl) {
  typename GenerateCollection<T...>::type collection;
  return init_from_tuple_impl<C, T...>(tpl, collection);
}

In action on Ideone .

, "" init_from_tuple_impl (, new):

template <typename C, typename... T, size_t... N>
C* init_from_tuple_impl(bp::tuple tpl, Collection<N...>) {
  return C { bp::extract<T>(tpl[N])... };
}

Ideone:

prog.cpp: In function 'C* init_from_tuple_impl(bp::tuple, Collection<N ...>)
[with
    C = bp::Object,
    T = {int, float, char},
    unsigned int ...N = {0u, 1u, 2u},
    bp::tuple = std::basic_string<char>
]':

, :)

+3

, , . , :

template <typename C, int N, typename... T>
C* init_from_tuple(bp::tuple tpl, T... args) // enable_if N == sizeof...(T)
{
    return new C{args...};
}

template <typename C, int N, typename T0, typename... T>
C* init_from_tuple(bp::tuple tpl, T... args) // enable_if N != sizeof...(T)
{
    return init_from_tuple<C, N + 1>(tpl, args, bp::extract<T0>(tpl[N]));
}

template <typename C, typename... T>
C* init_from_tuple(bp::tuple tpl, T... args)
{
    return init_from_tuple<C, 0, T...>(tpl, args);
}

boost enable_if, , , , , , .

+1

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