Re: [Boost-users] Strange compiler behaviour with mpl::vector

From: Aleksey Gurtovoy (agurtovoy_at_[hidden])
Date: 2006-06-26 04:36:12

Ovanes Markarian writes:
> I posted the following message
> http://lists.boost.org/boost-users/2006/06/20271.php a few days ago,
> but did not get any replies.
>  
> I know there is an error in the const_ref class. But nevertheless the
> problem which occures is that my class accepts a vector of supported types
> as template parameter.
>  
> It has two constructors (no default ctor):
>  
> template<class Seq_>
> class my_class
> {
>     public:
>         typedef my_class<Seq_>        this_type;
>  
>         explicit inline my_class(const this_type& copy_from) :... {...}
>     
>         template<class TypeFromVector>
>         explicit inline my_class(typename const_ref<TypeFromVector>::type
> inst) :... {...}
> };
>  
>  
> Now if I try to initialize my_class with type declared in vector I get an
> error:
>  
> error C2664: 'discriminated_union<Seq_>::discriminated_union(const
> discriminated_union<Seq_> &)' : cannot convert parameter 1 from 't1' to
> 'const discriminated_union<Seq_> &'
>         with
>         [
>             Seq_=types_vector
>         ]
>         Reason: cannot convert from 't1' to 'const
> discriminated_union<Seq_>'
>         with
>         [
>             Seq_=types_vector
>         ]
>         No user-defined-conversion operator available that can perform this
> conversion, or the operator cannot be called

In the context of overload resolution, any dependent type expression
in a form of

    typename something<T, ...>::type

is "non-deducible", meaning that 'T' etc. has to be explicitly
specified or deduced from another argument(s), or the function
template won't even be considered for forming the list of candidate
functions. See 14.8.2 [temp.over] and 14.8.3 [temp.over] for more
details.

HTH,
-- 
Aleksey Gurtovoy
MetaCommunications Engineering