Re: [Boost-users] boost::is_const does not recognize constant type

From: Becher, Jochen (jochen.becher_at_[hidden])
Date: 2008-05-28 11:23:10

Hi John,

thank you, you are right, remove_reference fixes the problem. I do not understand why MSVC print the reference type without the reference symbol (I checked this twice) but internally it seems to be a reference type (and that is what you expect for typedef ...::reference).

My final code is this:

template<class ITERATOR>
class SecondIterator_TC : public boost::iterator_adaptor<
  SecondIterator_TC<ITERATOR>,
  ITERATOR, 
  typename ITERATOR::value_type::second_type, 
  boost::use_default, 
  typename boost::mpl::eval_if<
    typename boost::is_const<typename boost::remove_reference<
      typename ITERATOR::reference>::type>::type,
    boost::mpl::identity<typename ITERATOR::value_type::second_type const&>,
    boost::mpl::identity<typename ITERATOR::value_type::second_type &>
  >::type,
  boost::use_default>
{
...
};

It is my first attempt to use iterator_adaptor. It allows iterators over maps (and other iterators that dereference to a pair) to use like iterators on most other containers. The Reference template parameter now looks really complex, is there any simpler solution?

Regards, Jochen

-----Urspruengliche Nachricht-----
Von: boost-users-bounces_at_[hidden] [mailto:boost-users-bounces_at_[hidden]] Im Auftrag von John Maddock
Gesendet: Mittwoch, 28. Mai 2008 16:39
An: boost-users_at_[hidden]
Betreff: Re: [Boost-users] boost::is_const does not recognize constant type

Becher, Jochen wrote:
>
> The following type expression evaluates to the false_ type:
>
> boost::is_const<std::map<std::string,std::string>::const_iterator::refer
> ence>::type
>
>
>
> It should evaluate to the true type because ...::reference is
>
>
>
> const std::pair<const std::string,std::string>

Nope, wouldn't that be:

const std::pair<const std::string,std::string>&

which is a reference type and can be neither const nor volatile qualified 
(the const qualifier in the above refers to the thing being referenced not 
the reference itself).  If you feed it through remove_reference before 
passing to is_const then you should get true as the result.

HTH, John. 

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