Empty nested tuples error

Question

#include <iostream>
#include <tuple>
int main(){

auto bt=std::make_tuple(std::tuple<>(),std::tuple<std::tuple<>>()); //Line 1
auto bt2=std::make_tuple(std::tuple<>(),std::tuple<>());             //Line 2
}

Why does Line 1 gives a compile error while Line 2 compiles fine? (tested in both Gcc&Clang)

Is there a possible workaround?

error message for clang

/usr/include/c++/4.6/tuple:150:50: error: ambiguous conversion from derived class 'std::_Tuple_impl<0, std::tuple<>,
      std::tuple<std::tuple<> > >' to base class 'std::_Head_base<0, std::tuple<>, true>':
    struct std::_Tuple_impl<0, class std::tuple<>, class std::tuple<class std::tuple<> > > -> _Tuple_impl<0UL + 1, class std::tuple<class std::tuple<> > > -> _Head_base<1UL, class std::tuple<class std::tuple<> >, std::is_empty<class tuple<class tuple<> > >::value> -> class std::tuple<class std::tuple<> > -> _Tuple_impl<0, class std::tuple<> > -> _Head_base<0UL, class std::tuple<>, std::is_empty<class tuple<> >::value>
    struct std::_Tuple_impl<0, class std::tuple<>, class std::tuple<class std::tuple<> > > -> _Head_base<0UL, class std::tuple<>, std::is_empty<class tuple<> >::value>
      _Head&            _M_head()       { return _Base::_M_head(); }
                                                 ^~~~~
/usr/include/c++/4.6/tuple:173:33: note: in instantiation of member function 'std::_Tuple_impl<0, std::tuple<>,
      std::tuple<std::tuple<> > >::_M_head' requested here
        _Base(std::forward<_Head>(__in._M_head())) { }
                                       ^
/usr/include/c++/4.6/tuple:334:9: note: in instantiation of member function 'std::_Tuple_impl<0, std::tuple<>,
      std::tuple<std::tuple<> > >::_Tuple_impl' requested here
      : _Inherited(static_cast<_Inherited&&>(__in)) { }
        ^
gcc_bug.cpp:5:10: note: in instantiation of member function
      'std::tuple<std::tuple<>, std::tuple<std::tuple<> > >::tuple' requested here
        auto bt=std::make_tuple(std::tuple<>(),std::tuple<std::tuple<>>());
                ^
1 error generated.

Answer 1

Looks like you found a bug in libstdc++! (This code works in clang with libc++). A reduced test case:

#include <tuple>

int main(){
    auto b = std::tuple<std::tuple<std::tuple<>>>{};
}

---

The problem is due to how std::tuple is implemented in libstdc++. The tuple implementation uses "recursion" with multiple-inheritance. You can think of tuple<X, Y, Z> as inheriting from both X and tuple<Y, Z>. This means tuple<tuple<>> will inherit from both tuple<> and tuple<> and that will cause an ambiguous base error. Of course the real problem isn't like this, because tuple<tuple<>> doesn't produce any error.

The real implementation that caused the error is like this:

template<size_t _Idx, typename _Head>
struct _Head_base : public _Head
{};

template<size_t _Idx, typename... _Elements>
struct _Tuple_impl;

template<size_t _Idx>
struct _Tuple_impl<_Idx> {};

template<size_t _Idx, typename _Head, typename... _Tail>
struct _Tuple_impl<_Idx, _Head, _Tail...>
    : public _Tuple_impl<_Idx + 1, _Tail...>,
      private _Head_base<_Idx, _Head>
{
    typedef _Tuple_impl<_Idx + 1, _Tail...> _Inherited;
    constexpr _Tuple_impl() = default;
    constexpr _Tuple_impl(_Tuple_impl&& __in) : _Inherited(std::move(__in)) {}
};

template<typename... _Elements>
struct tuple : public _Tuple_impl<0, _Elements...> {};

When we instantiate tuple<tuple<tuple<>>>, we get this inheritance hierarchy:

We see that _Tuple_impl<1> is reachable in two different paths. This is not yet the problem, the problem is in the move constructor, who invokes the move-conversion constructor of _Tuple_impl<1>. Which _Tuple_impl<1> do you want? The compiler doesn't know, so it chooses the give up.

(In your case it's because of _Head_base<0, tuple<>> as you are instantiating tuple<tuple<>, tuple<tuple<>>> instead, but the principle is the same.)

---

Why libc++ does not have the same problem? There are two main reasons:

1. tuple<T...> in libc++ use composition instead of inheritance to refer to __tuple_impl<...>.
2. As a result, the empty base class optimization in __tuple_leaf<tuple<tuple<>>> does not kick in, i.e. __tuple_leaf<tuple<tuple<>>> won't inherit from tuple<tuple<>>
3. Therefore, the ambiguous base class problem won't happen.
4. (and each base is unique as mentioned by @mitchnull, but that is not a main difference here.)

As we can see above, if tuple<...> uses inheritance instead of composition, OP's tuple<tuple<>, tuple<tuple<>>> will still inherit from __tuple_leaf<0, tuple<>> twice, which might be a problem.