I'm trying to nest boost's "map_list_of" in C++03, but apparently construction is ambiguous?

Question

Consider this:

#include <iostream>
#include <map>
#include <string>
#include <boost/assign/list_of.hpp>
using boost::assign::map_list_of;

const std::map<int, std::map<int, char> > test = map_list_of
    (100, map_list_of
        (1, 'a')
        (2, 'b')
    )
    (101, map_list_of
        (1, 'c')
        (2, 'd')
    )
;

int main()
{
    std::cout << test.find(101)->second.find(2)->second << "\n";
}

I wanted the result to be a program that, when executed, outputs d.

Instead, I get this:

$ clang++ -std=c++03 -O2 -Wall -pedantic -pthread main.cpp

In file included from main.cpp:1:
In file included from /usr/local/bin/../lib/gcc/x86_64-unknown-linux-gnu/4.9.2/../../../../include/c++/4.9.2/iostream:39:
In file included from /usr/local/bin/../lib/gcc/x86_64-unknown-linux-gnu/4.9.2/../../../../include/c++/4.9.2/ostream:38:
In file included from /usr/local/bin/../lib/gcc/x86_64-unknown-linux-gnu/4.9.2/../../../../include/c++/4.9.2/ios:40:
In file included from /usr/local/bin/../lib/gcc/x86_64-unknown-linux-gnu/4.9.2/../../../../include/c++/4.9.2/bits/char_traits.h:39:
In file included from /usr/local/bin/../lib/gcc/x86_64-unknown-linux-gnu/4.9.2/../../../../include/c++/4.9.2/bits/stl_algobase.h:64:
/usr/local/bin/../lib/gcc/x86_64-unknown-linux-gnu/4.9.2/../../../../include/c++/4.9.2/bits/stl_pair.h:119:22: error: call to constructor of 'std::map<int, char, std::less<int>, std::allocator<std::pair<const int, char> > >' is ambiguous
        : first(__p.first), second(__p.second) { }
                            ^      ~~~~~~~~~~
/usr/local/bin/../lib/gcc/x86_64-unknown-linux-gnu/4.9.2/../../../../include/c++/4.9.2/bits/stl_tree.h:1843:29: note: in instantiation of function template specialization 'std::pair<const int, std::map<int, char, std::less<int>, std::allocator<std::pair<const int, char> > > >::pair<int, boost::assign_detail::generic_list<std::pair<int, char> > >' requested here
          _M_insert_unique_(end(), *__first);
                                   ^
/usr/local/bin/../lib/gcc/x86_64-unknown-linux-gnu/4.9.2/../../../../include/c++/4.9.2/bits/stl_map.h:255:16: note: in instantiation of function template specialization 'std::_Rb_tree<int, std::pair<const int, std::map<int, char, std::less<int>, std::allocator<std::pair<const int, char> > > >, std::_Select1st<std::pair<const int, std::map<int, char, std::less<int>, std::allocator<std::pair<const int, char> > > > >, std::less<int>, std::allocator<std::pair<const int, std::map<int, char, std::less<int>, std::allocator<std::pair<const int, char> > > > > >::_M_insert_unique<std::_Deque_iterator<std::pair<int, boost::assign_detail::generic_list<std::pair<int, char> > >, std::pair<int, boost::assign_detail::generic_list<std::pair<int, char> > > &, std::pair<int, boost::assign_detail::generic_list<std::pair<int, char> > > *> >' requested here
        { _M_t._M_insert_unique(__first, __last); }
               ^
/usr/local/include/boost/assign/list_of.hpp:163:20: note: in instantiation of function template specialization 'std::map<int, std::map<int, char, std::less<int>, std::allocator<std::pair<const int, char> > >, std::less<int>, std::allocator<std::pair<const int, std::map<int, char, std::less<int>, std::allocator<std::pair<const int, char> > > > > >::map<std::_Deque_iterator<std::pair<int, boost::assign_detail::generic_list<std::pair<int, char> > >, std::pair<int, boost::assign_detail::generic_list<std::pair<int, char> > > &, std::pair<int, boost::assign_detail::generic_list<std::pair<int, char> > > *> >' requested here
            return Container( begin(), end() );
                   ^
/usr/local/include/boost/assign/list_of.hpp:142:20: note: in instantiation of function template specialization 'boost::assign_detail::converter<boost::assign_detail::generic_list<std::pair<int, boost::assign_detail::generic_list<std::pair<int, char> > > >, std::_Deque_iterator<std::pair<int, boost::assign_detail::generic_list<std::pair<int, char> > >, std::pair<int, boost::assign_detail::generic_list<std::pair<int, char> > > &, std::pair<int, boost::assign_detail::generic_list<std::pair<int, char> > > *> >::convert<std::map<int, std::map<int, char, std::less<int>, std::allocator<std::pair<const int, char> > >, std::less<int>, std::allocator<std::pair<const int, std::map<int, char, std::less<int>, std::allocator<std::pair<const int, char> > > > > > >' requested here
            return convert<Container>( c, tag_type() );
                   ^
/usr/local/include/boost/assign/list_of.hpp:436:49: note: in instantiation of function template specialization 'boost::assign_detail::converter<boost::assign_detail::generic_list<std::pair<int, boost::assign_detail::generic_list<std::pair<int, char> > > >, std::_Deque_iterator<std::pair<int, boost::assign_detail::generic_list<std::pair<int, char> > >, std::pair<int, boost::assign_detail::generic_list<std::pair<int, char> > > &, std::pair<int, boost::assign_detail::generic_list<std::pair<int, char> > > *> >::convert_to_container<std::map<int, std::map<int, char, std::less<int>, std::allocator<std::pair<const int, char> > >, std::less<int>, std::allocator<std::pair<const int, std::map<int, char, std::less<int>, std::allocator<std::pair<const int, char> > > > > > >' requested here
            return this-> BOOST_NESTED_TEMPLATE convert_to_container<Container>();
                                                ^
main.cpp:7:50: note: in instantiation of function template specialization 'boost::assign_detail::generic_list<std::pair<int, boost::assign_detail::generic_list<std::pair<int, char> > > >::operator map<std::map<int, std::map<int, char, std::less<int>, std::allocator<std::pair<const int, char> > >, std::less<int>, std::allocator<std::pair<const int, std::map<int, char, std::less<int>, std::allocator<std::pair<const int, char> > > > > > >' requested here
const std::map<int, std::map<int, char> > test = map_list_of
                                                 ^
/usr/local/bin/../lib/gcc/x86_64-unknown-linux-gnu/4.9.2/../../../../include/c++/4.9.2/bits/stl_map.h:171:7: note: candidate constructor
      map(const _Compare& __comp,
      ^
/usr/local/bin/../lib/gcc/x86_64-unknown-linux-gnu/4.9.2/../../../../include/c++/4.9.2/bits/stl_map.h:182:7: note: candidate constructor
      map(const map& __x)
      ^
1 error generated.

(similar results under GCC)

How can I resolve this?

I get a similar error even if I use std::map<int, char>(map_list_of(...)) instead of map_list_of(...) for those inner maps.

Answer 1

C++03 defines two constructors for map that can be called with one argument [lib.map]p2:

explicit map(const Compare& comp = Compare(),
             const Allocator& = Allocator());
// [...]
map(const map<Key,T,Compare,Allocator>& x);

boost's map_list_of creates an object of a generic_list class template instantiation, from the recent SVN:

template< class Key, class T >
inline assign_detail::generic_list< std::pair
    < 
        BOOST_DEDUCED_TYPENAME assign_detail::assign_decay<Key>::type, 
        BOOST_DEDUCED_TYPENAME assign_detail::assign_decay<T>::type
    > >
map_list_of( const Key& k, const T& t )

Where the primary generic_list template contains the following conversion operator:

template< class Container >
operator Container() const
{
    return this-> BOOST_NESTED_TEMPLATE convert_to_container<Container>();
}

Both map constructors are viable, as this operator allows conversion to both map and Compare. As far as I know, you cannot SFINAE-constrain a conversion operator in C++03.

---

The map is constructed explicitly when inserting a new node in the outer map. A pair of iterators is used to iterate over the inner generic_list to construct the outer map. Dereferencing this iterator yields a std::pair<int, boost::assign_detail::generic_list<std::pair<int, char> >. The node (value) type of the outer map is std::pair<int const, std::map<int, char> >.

Therefore, the compiler tries to construct the latter type from the former. In C++03, this pair constructor is not SFINAE-constrained, since that's not possible in C++03. [lib.pairs]p1

template<class U, class V> pair(const pair<U, V> &p);

libstdc++ implements this as follows:

template<class _U1, class _U2>
  pair(const pair<_U1, _U2>& __p)
  : first(__p.first), second(__p.second) { }

I'm not entirely sure if that's compliant, since [lib.pairs]p4

Effects: Initializes members from the corresponding members of the argument, performing implicit conversions as needed.

(But, as I said, SFINAE on ctors cannot be implemented in C++03.)

In C++11 and 14, this also fails, but for a different reason. Here, the pair constructors are SFINAE-constrained. But the constrain requires implicit convertibility (is_convertible), while the program has UB if the target pair of types cannot be constructed from the sources (is_constructible). I've written a bit more about this issue in another SO answer. Interestingly, a proposed solution N4387 to the issue mentioned in that other question says:

It should be noted here, that for the general case the std::is_constructible<T, U>::value requirement for the non-explicit constructor which is constrained on std::is_convertible<U, T>::value is not redundant, because it is possible to create types that can be copy-initialized but not direct-initialized

This is exactly the case we run into here: A map can be copy-initialized from a generic_list, since this makes the explicit constructor non-viable. But a map cannot be direct-initialized from generic_list, since this makes the conversion ambiguous.

As far as I can see, N4387 does not solve the problem in the OP. On the other hand, with uniform initialization, we have an alternative to map_list_of. And we can SFINAE-constrain conversion operators since C++11.

---

One solution is to eliminate the explicit constructor by only allowing implicit conversions:

template<typename T> T implicit_cast(T t) { return t; }

implicit_cast<InnerMap>( map_list_of(1, 'a')(2, 'b') )

But there's a more direct way: simply use the convert_to_container member function of generic_list's base class converter (also a class template):

map_list_of(1, 'a')(2, 'b').convert_to_container<InnerMap>()