Compilation error on ambiguous type in code using boost::spirit

Question

If I write this code it still compiles:

namespace MyNamespace
{

struct STreeConstructionRuleQuery : std::string {};
struct STreeConstructionRuleOperation : std::string {};
struct STreeConstructionRuleOperand : std::string {};
struct STreeConstructionRuleCondition : std::string {};

struct STreeConstructionRuleOperationWithOperands : boost::tuple<STreeConstructionRuleOperation, std::vector<STreeConstructionRuleOperand> > {};

struct STreeConstructionRule
{
    boost::optional<std::vector<std::vector<STreeConstructionRuleOperationWithOperands> > > m_sAssertion;
    STreeConstructionRuleQuery m_sQuery;
    STreeConstructionRuleOperationWithOperands m_sAction;
    boost::optional<STreeConstructionRuleCondition> m_sCondition;
};

}


BOOST_FUSION_ADAPT_STRUCT(
    MyNamespace::STreeConstructionRule,
    (boost::optional<std::vector<std::vector<MyNamespace::STreeConstructionRuleOperationWithOperands> > >, m_sAssertion)
    (MyNamespace::STreeConstructionRuleQuery, m_sQuery)
    (MyNamespace::STreeConstructionRuleOperationWithOperands, m_sAction)
    (boost::optional<MyNamespace::STreeConstructionRuleCondition>, m_sCondition)
)


namespace MyNamespace
{

namespace qi = boost::spirit::qi;
namespace ascii = boost::spirit::ascii;
namespace phoenix = boost::phoenix;

template <typename Iterator>
struct STreeContructionRulesGrammar : qi::grammar<Iterator, std::vector<STreeConstructionRule>(), ascii::space_type>
{
    qi::rule<Iterator, std::vector<STreeConstructionRule>(), ascii::space_type> m_oStart;
    qi::rule<Iterator, qi::unused_type(), ascii::space_type> m_oComment;
    qi::rule<Iterator, STreeConstructionRule(), ascii::space_type> m_oRule;

    STreeContructionRulesGrammar() : STreeContructionRulesGrammar::base_type(m_oStart)
    {
        m_oStart = *(m_oComment | m_oRule [phoenix::push_back(qi::_val, qi::_1)]);
        m_oComment = (("-->" >> *(qi::char_) >> "<--") | ('#' >> *(qi::char_ - qi::char_("\n\r")))); 
    }

};
}

But when I change the definition of STreeConstructionRuleOperand to

struct STreeConstructionRuleOperand : boost::variant<int> {};

I get the following compilation error (STLPort is used instead of STL):

1>d:\commonlib\include\boost\variant\variant.hpp(1373) : error C2666: 'boost::variant<T0_>::convert_construct' : 2 overloads have similar conversions
1>        with
1>        [
1>            T0_=int
1>        ]
1>        d:\commonlib\include\boost\variant\variant.hpp(1358): could be 'void boost::variant<T0_>::convert_construct<int,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_,boost::detail::variant::void_>(const boost::variant<T0_> &,long)'
1>        with
1>        [
1>            T0_=int
1>        ]
1>        d:\commonlib\include\boost\variant\variant.hpp(1289): or       'void boost::variant<T0_>::convert_construct<const T>(T &,int,boost::mpl::false_)'
1>        with
1>        [
1>            T0_=int,
1>            T=MyNamespace::STreeConstructionRuleOperand
1>        ]
1>        while trying to match the argument list '(const MyNamespace::STreeConstructionRuleOperand, long)'
1>        d:\commonlib\include\boost\fusion\container\vector\detail\vector_n.hpp(45) : see reference to function template instantiation 'boost::variant<T0_>::variant<MyNamespace::STreeConstructionRuleOperand>(const T &)' being compiled
1>        with
1>        [
1>            T0_=int,
1>            T=MyNamespace::STreeConstructionRuleOperand
1>        ]
1>        d:\commonlib\include\stlport\stl\_uninitialized.h(93) : see reference to function template instantiation 'void stlpd_std::_Destroy_Range<_OutputIter>(_ForwardIterator,_ForwardIterator)' being compiled
1>        with
1>        [
1>            _OutputIter=stlpd_std::vector<MyNamespace::STreeConstructionRuleOperationWithOperands> *,
1>            _ForwardIterator=stlpd_std::vector<MyNamespace::STreeConstructionRuleOperationWithOperands> *
1>        ]
1>        d:\commonlib\include\stlport\stl\_uninitialized.h(113) : see reference to function template instantiation '_OutputIter stlpd_std::priv::__ucopy<_InputIter,_OutputIter,ptrdiff_t>(_RandomAccessIter,_RandomAccessIter,_OutputIter,const stlpd_std::random_access_iterator_tag &,_Distance *)' being compiled
1>        with
1>        [
1>            _OutputIter=stlpd_std::vector<MyNamespace::STreeConstructionRuleOperationWithOperands> *,
1>            _InputIter=const stlpd_std::vector<MyNamespace::STreeConstructionRuleOperationWithOperands> *,
1>            _RandomAccessIter=const stlpd_std::vector<MyNamespace::STreeConstructionRuleOperationWithOperands> *,
1>            _Distance=ptrdiff_t
1>        ]
1>        d:\commonlib\include\stlport\stl\_vector.h(249) : see reference to function template instantiation '_OutputIter stlpd_std::priv::__ucopy_ptrs<const stlpd_std::vector<_Tp>*,stlpd_std::vector<_Tp>*>(_InputIter,_InputIter,_OutputIter,const stlpd_std::__false_type &)' being compiled
1>        with
1>        [
1>            _OutputIter=stlpd_std::vector<MyNamespace::STreeConstructionRuleOperationWithOperands> *,
1>            _Tp=MyNamespace::STreeConstructionRuleOperationWithOperands,
1>            _InputIter=const stlpd_std::vector<MyNamespace::STreeConstructionRuleOperationWithOperands> *
1>        ]
1>        d:\commonlib\include\stlport\stl\_vector.h(246) : while compiling class template member function 'stlpd_std::priv::_NonDbg_vector<_Tp,_Alloc>::_NonDbg_vector(const stlpd_std::priv::_NonDbg_vector<_Tp,_Alloc> &)'
1>        with
1>        [
1>            _Tp=stlpd_std::vector<MyNamespace::STreeConstructionRuleOperationWithOperands>,
1>            _Alloc=stlpd_std::allocator<stlpd_std::vector<MyNamespace::STreeConstructionRuleOperationWithOperands>>
1>        ]
1>        d:\commonlib\include\stlport\stl\debug\_iterator.h(378) : see reference to class template instantiation 'stlpd_std::priv::_NonDbg_vector<_Tp,_Alloc>' being compiled
1>        with
1>        [
1>            _Tp=stlpd_std::vector<MyNamespace::STreeConstructionRuleOperationWithOperands>,
1>            _Alloc=stlpd_std::allocator<stlpd_std::vector<MyNamespace::STreeConstructionRuleOperationWithOperands>>
1>        ]
1>        d:\commonlib\include\stlport\stl\debug\_vector.h(106) : see reference to class template instantiation 'stlpd_std::priv::__construct_checker<_Container>' being compiled
1>        with
1>        [
1>            _Container=stlpd_std::priv::_NonDbg_vector<stlpd_std::vector<MyNamespace::STreeConstructionRuleOperationWithOperands>,stlpd_std::allocator<stlpd_std::vector<MyNamespace::STreeConstructionRuleOperationWithOperands>>>
1>        ]
1>        d:\commonlib\include\boost\optional\optional.hpp(110) : see reference to class template instantiation 'stlpd_std::vector<_Tp>' being compiled
1>        with
1>        [
1>            _Tp=stlpd_std::vector<MyNamespace::STreeConstructionRuleOperationWithOperands>
1>        ]
1>        d:\commonlib\include\boost\optional\optional.hpp(113) : see reference to class template instantiation 'boost::optional_detail::aligned_storage<T>::dummy_u' being compiled
1>        with
1>        [
1>            T=stlpd_std::vector<stlpd_std::vector<MyNamespace::STreeConstructionRuleOperationWithOperands>>
1>        ]
1>        d:\commonlib\include\boost\optional\optional.hpp(450) : see reference to class template instantiation 'boost::optional_detail::aligned_storage<T>' being compiled
1>        with
1>        [
1>            T=stlpd_std::vector<stlpd_std::vector<MyNamespace::STreeConstructionRuleOperationWithOperands>>
1>        ]
1>        d:\commonlib\include\boost\optional\optional.hpp(457) : see reference to class template instantiation 'boost::optional_detail::optional_base<T>' being compiled
1>        with
1>        [
1>            T=stlpd_std::vector<stlpd_std::vector<MyNamespace::STreeConstructionRuleOperationWithOperands>>
1>        ]
1>        d:\preformator\sources\prefcore\preftreeconstructor.cpp(52) : see reference to class template instantiation 'boost::optional<T>' being compiled
1>        with
1>        [
1>            T=stlpd_std::vector<stlpd_std::vector<MyNamespace::STreeConstructionRuleOperationWithOperands>>
1>        ]

This is the only error the compiler reports. I don't see any ambiguity but the compiler does. What is the reason for this error and how it should be fixed?

Answer 1

Two things strike me here:

1. how would something that is used to be std::string, be assigned to something that is a now a variant<int>?

   The code shown doesn't include the code that would explain this (as it is probably in m_oRule or a subrule of that). But perhaps a variant<std::string> poses fewer problems, seeing how you had a std::string value to assign before.

2. You appear to abuse struct inheritance quite a lot to achieve typedef behaviour. Note that public inheritance is not the same as a typedef, especially with regards to the constructors (which are not inherited).

   Try replacing each use of

   struct A : baseType {};
   

   by the more usual idiom of

   typedef baseType A;
   

I actually expect the latter to solve the issue, since you are running into constructor overload trouble.

Regardless of that, I have only tested with gcc, boost_1_51_0 and GNU libstdc++. It compiles everything you posted without a hitch (but again, not you left out crucial elements, such as the rule m_oRule that does the actual assigning to RuleOpWithOperands).

Demonstration FauxTypedef:

Demonstrating the relevant problem (at least one) with inheritance instead of typedef:

struct Type {
    Type(int, double) { }
};

typedef Type          ProperTypedef;
struct  FauxTypedef : Type { };

int main()
{
    Type          a(1, 4.2); // okay
    ProperTypedef b(1, 4.2); // okay as well, obviously
    FauxTypedef   c(1, 4.2); // oops ouch!!! This won't compile
}

If you insist on having a _unique typeid for the typedef-ed name, you can resort to BOOST_STRONG_TYPEDEF