Re: [Boost-bugs] [Boost C++ Libraries] #11032: boost::offset_ptr needs explicit ctor

Subject: Re: [Boost-bugs] [Boost C++ Libraries] #11032: boost::offset_ptr needs explicit ctor
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2015-03-11 20:35:07

#11032: boost::offset_ptr needs explicit ctor
-------------------------------+--------------------------
  Reporter:  stheophil@…       |      Owner:  igaztanaga
      Type:  Bugs              |     Status:  new
 Milestone:  To Be Determined  |  Component:  interprocess
   Version:  Boost 1.57.0      |   Severity:  Problem
Resolution:                    |   Keywords:
-------------------------------+--------------------------

Comment (by igaztanaga):

 I can't see the problem. offset_ptr has a constructor that allows
 conversions:
 {{{
 //!Constructor from other offset_ptr. If pointers of pointee types are
 //!convertible, offset_ptrs will be convertibles. Never throws.
 template<class T2>
 offset_ptr( const offset_ptr<T2, DifferenceType, OffsetType,
 OffsetAlignment> &ptr
           #ifndef BOOST_INTERPROCESS_DOXYGEN_INVOKED
           , typename ipcdetail::enable_if_c<
 ipcdetail::is_convertible<T2*, PointedType*>::value
             && ipcdetail::offset_ptr_maintains_address<T2,
 PointedType>::value
           >::type * = 0
           #endif
           )
    : internal(static_cast<OffsetType>
          (ipcdetail::offset_ptr_to_offset_from_other<0>(this, &ptr,
 ptr.get_offset())))
 {}

 //!Constructor from other offset_ptr. If pointers of pointee types are
 //!convertible, offset_ptrs will be convertibles. Never throws.
 template<class T2>
 offset_ptr( const offset_ptr<T2, DifferenceType, OffsetType,
 OffsetAlignment> &ptr
             , typename ipcdetail::enable_if_c<
 ipcdetail::is_convertible<T2*, PointedType*>::value
             && !ipcdetail::offset_ptr_maintains_address<T2,
 PointedType>::value
             >::type * = 0)
    : internal(static_cast<OffsetType>
 (ipcdetail::offset_ptr_to_offset<0>(static_cast<PointedType*>(ptr.get()),
 this)))
 {}

 }}}

 this allows upcasting (from derived to base). Did you mean that offset_ptr
 does not support downcasting (from base to derived)? It does that for good
 safety reasons as raw pointers don't allow those conversions. Do you want
 this to compile?

 offset_ptr<Base> pb = ...;
 offset_ptr<Derived>pd(p);

 whereas this is a compilation error

 Base *pb = ...
 Derived *pd = pb;

 ?

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/11032#comment:1>
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