[boost] [container] container_detail::pair piecewise constructor

Subject: [boost] [container] container_detail::pair piecewise constructor
From: László Szécsi (szecsi_at_[hidden])
Date: 2012-11-14 12:53:04

Hi everyone,

boost::container::container_detail::pair's piecewise constructor is
missing, as acknowledged in a comment. This makes
container::flat_map::emplace less useful (perhaps useless) as the
following will not work:

boost::container::flat_multimap<TestKey, TestValue> testMap;

testMap.emplace(boost::container::container_detail::piecewise_construct,
 boost::make_tuple(30),
 boost::make_tuple(50));

where both TestKey and TestValue are non-copyable, moveable classes
with a constructor taking an int.

The compilation error message will correctly say that
boost::container::container_detail::pair does not have a constructor
with three arguments.

I tried to implement the missing piecewise constructor based on
std::pair's in the g++ standard libraries. I failed at it, so I would
like to request some assistance and education. This is where I got:

// from the standard lib
template<std::size_t _Num>
struct _Build_index_tuple
{ typedef typename _Build_index_tuple<_Num-1>::__type::__next __type; };

// from the standard lib
template<>
struct _Build_index_tuple<0>
{ typedef _Index_tuple<> __type; };

template <class... Args1, class... Args2>
pair(piecewise_construct_t, ::boost::tuple<Args1...> first_args,
::boost::tuple<Args2...> second_args)
  : pair(::boost::move(first_args), ::boost::move(second_args),
  typename _Build_index_tuple<sizeof...(Args1)>::__type(),
  typename _Build_index_tuple<sizeof...(Args2)>::__type())
  {}

template<typename... Args1, std::size_t... Indices1, typename...
Args2, std::size_t... Indices2>
explicit pair(tuple<Args1...> tuple1, tuple<Args2...> tuple2,
  _Index_tuple<Indices1...>, _Index_tuple<Indices2...>)
  : first(::boost::forward<Args1>(std::get<Indices1>(tuple1))...),
  second(::boost::forward<Args2>(std::get<Indices2>(tuple2))...)
  { }

This gets me a recursive error message far too long to quote, starting with:
error C2664: 'boost::container::container_detail::pair<Key,T>::pair(std::pair<Key,T>
&&)' : cannot convert parameter 3 from
'boost::tuples::tuple<int,boost::tuples::null_type // lot of
null_types omitted
 to 'boost::tuples::tuple<boost::tuples::null_type, // lot of null_types omitted

I used the VS2012 Nov CTP compiler.

I understand that variadic templates have not been very portable and
boost::unordered, which has a piecewise constructor implementation,
does it completely differently and much more painfully. But is there a
reason for the approach I tried not to work with boost at all?

Could you please help me fix this code?

Thank you very much,
Laszlo Szecsi