Re: [Boost-users] boost:any/boost::variant for integral/floating type

Subject: Re: [Boost-users] boost:any/boost::variant for integral/floating type
From: Larry Evans (cppljevans_at_[hidden])
Date: 2010-06-03 13:00:05

On 06/03/10 04:22, Mathieu Malaterre wrote:
> On Tue, Jun 1, 2010 at 4:42 PM, Larry Evans <cppljevans_at_[hidden]> wrote:
[snip]
>> Look out for the boost:;variant trap revealed by the post:
>>
>>  http://article.gmane.org/gmane.comp.lib.boost.user/58791
>>
>> The problem occurs when one component can be constructed from another
>> component. as suggested by this quote from the above post:
>>
>>> When bool is added to variant, the statement
>>> variant = "hello world" ;
>>> calls bool constructor.
>>>
>> I think something similar may happen here, but maybe the problem
>> can be solved by some order of the bound components.  Maybe putting
>> more "stringent" components either first or last.  That way, maybe
>> the correct one would be selected when the assignment is made.
> 
> Thanks for the warning. My base type are fundamental type, so I should
> not suffer from this implicit cast.
The 1st attachment, when compiled with gcc4.4 I,
gives errors in 2nd attachment.

When the '#if 1' is replaced with '#if 0' it compiles OK, indicating
that you must explicitly cast the argument type to avoid the
ambiguity error in this particular case.

HTH.

-Larry


In file included from /home/evansl/prog_dev/boost-svn/ro/trunk/boost/variant.hpp:17,
                 from variant_implicit_conversion.cpp:5:
/home/evansl/prog_dev/boost-svn/ro/trunk/boost/variant/variant.hpp: In member function 'void boost::variant<T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19>::convert_construct(T&, int, mpl_::false_) [with T = const double, T0_ = int, T1 = float, T2 = boost::detail::variant::void_, T3 = boost::detail::variant::void_, T4 = boost::detail::variant::void_, T5 = boost::detail::variant::void_, T6 = boost::detail::variant::void_, T7 = boost::detail::variant::void_, T8 = boost::detail::variant::void_, T9 = boost::detail::variant::void_, T10 = boost::detail::variant::void_, T11 = boost::detail::variant::void_, T12 = boost::detail::variant::void_, T13 = boost::detail::variant::void_, T14 = boost::detail::variant::void_, T15 = boost::detail::variant::void_, T16 = boost::detail::variant::void_, T17 = boost::detail::variant::void_, T18 = boost::detail::variant::void_, T19 = boost::detail::variant::void_]':
/home/evansl/prog_dev/boost-svn/ro/trunk/boost/variant/variant.hpp:1372:   instantiated from 'boost::variant<T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19>::variant(const T&) [with T = double, T0_ = int, T1 = float, T2 = boost::detail::variant::void_, T3 = boost::detail::variant::void_, T4 = boost::detail::variant::void_, T5 = boost::detail::variant::void_, T6 = boost::detail::variant::void_, T7 = boost::detail::variant::void_, T8 = boost::detail::variant::void_, T9 = boost::detail::variant::void_, T10 = boost::detail::variant::void_, T11 = boost::detail::variant::void_, T12 = boost::detail::variant::void_, T13 = boost::detail::variant::void_, T14 = boost::detail::variant::void_, T15 = boost::detail::variant::void_, T16 = boost::detail::variant::void_, T17 = boost::detail::variant::void_, T18 = boost::detail::variant::void_, T19 = boost::detail::variant::void_]'
variant_implicit_conversion.cpp:20:   instantiated from here
/home/evansl/prog_dev/boost-svn/ro/trunk/boost/variant/variant.hpp:1299: error: call of overloaded 'initialize(void*, const double&)' is ambiguous
/home/evansl/prog_dev/boost-svn/ro/trunk/boost/variant/detail/initializer.hpp:89: note: candidates are: static int boost::detail::variant::make_initializer_node::apply<BaseIndexPair, Iterator>::initializer_node::initialize(void*, typename boost::call_traits<typename boost::unwrap_recursive<typename boost::mpl::deref<Iterator>::type>::type>::param_type) [with BaseIndexPair = boost::mpl::pair<boost::detail::variant::make_initializer_node::apply<boost::mpl::pair<boost::detail::variant::initializer_root, mpl_::int_<0> >, boost::mpl::l_iter<boost::mpl::list2<int, float> > >::initializer_node, mpl_::int_<1> >, Iterator = boost::mpl::l_iter<boost::mpl::list1<float> >]
/home/evansl/prog_dev/boost-svn/ro/trunk/boost/variant/detail/initializer.hpp:89: note:                 static int boost::detail::variant::make_initializer_node::apply<BaseIndexPair, Iterator>::initializer_node::initialize(void*, typename boost::call_traits<typename boost::unwrap_recursive<typename boost::mpl::deref<Iterator>::type>::type>::param_type) [with BaseIndexPair = boost::mpl::pair<boost::detail::variant::initializer_root, mpl_::int_<0> >, Iterator = boost::mpl::l_iter<boost::mpl::list2<int, float> >]
make: *** [/home/evansl/prog_dev/boost-svn/ro/trunk/sandbox-local/build/gcc4_4n/boost-svn/ro/sandbox/variadic_templates/libs/composite_storage/sandbox/pack/variant_implicit_conversion.o] Error 1