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

25 Jun 2006

      Sorry, in the error message you will read the type name discriminated_union.
This is the real type name, think of it as my_class.

Thanks and sorry again,
Ovanes

  _____  

From: Ovanes Markarian [mailto:om_boost@keywallet.com] 
Sent: Sunday, June 25, 2006 23:18
To: 'boost-users@lists.boost.org'
Subject: Strange compiler behaviour with mpl::vector

Hello all!

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        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::discriminated_union(const
discriminated_union &)' : cannot convert parameter 1 from 't1' to
'const discriminated_union &'
        with
        [
            Seq_=types_vector
        ]
        Reason: cannot convert from 't1' to 'const
discriminated_union'
        with
        [
            Seq_=types_vector
        ]
        No user-defined-conversion operator available that can perform this
conversion, or the operator cannot be called

No user-defined-conversion operator available that can perform this
conversion, or the operator cannot be called

Ok, if I define the ctor param without typename const_ref<...>::type
everything goes fine, if I use const_ref it fails. const_ref is defined as
follows:

template<class Ty>
struct const_ref 
 : boost::add_reference
  <
   typename boost::add_const
   <
    typename boost::remove_reference<Ty>::type
...
::type
{};

I can understand that the compiler is unable to make a decision which ctor
to use, but I do not get an ambiguaty error. It seems like the compiler
simply quits expanding templates and uses the last most successful ctor
matched before the error. This happens only when I use mpl::vector templated
type in conjunction with templated ctor. Other types like below work fine:

template
struct my_pair : 
 mpl::pair, Ty_>
{
 typedef my_pair     my_type;

 inline my_pair()
 {}

 explicit inline my_pair( typename const_ref::type ty)
  : second_(ty)
 {}

 explicit inline my_pair(const my_type& other)
  : second_(other.second_)
 {}

 inline my_type& operator =(const my_type& other)
 {
  second_=other.second_;
  return *this;
 }

 second  second_;
};

May be some of you have any suggestions.

Thanks a lot,

Ovanes Markarian