Re: [boost] [iterator] reverse_iterator<counting_iterator<T>>

Subject: Re: [boost] [iterator] reverse_iterator<counting_iterator<T>>
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2010-04-13 11:09:24

Jeffrey Hellrung wrote:
> Stewart, Robert wrote:
> > Arno Schödl wrote:
> >> "Stewart, Robert" <Robert.Stewart_at_[hidden]
> >
> >> boost::prior(...) internally makes a copy of its iterator
> >> argument, decrements it and returns it by value, so far so
> >> good. counting_iterator::operator* then returns a reference
> >> into this (temporary) iterator. Returning from
> >> reverse_iterator::dereference() destroys the temporary
> >> iterator, but still returns the reference pointing into it.
> >
> > boost::prior's T is deduced as a reference type.  The copy
> is of a reference.  There's no temporary.
>
> You can think of the code as doing the equivalent of the following:
>
> reference dereference() const
> {
>      counting_iterator<T> temp_it = this->base(); // holds a T value
>      --temp_it;
>      return *temp_it; // return a reference to a member object of
> temp_it --> BAD
> }

Taking another look:

   reference
   dereference() const
   {
      return *boost::prior(this->base());
   }

this->base() is:

   Base const &
   base() const
   {
      return m_iterator;
   }

That returns Iterator const &, which is passed to boost::prior(), which is:

   template <class T>
   T
   prior(T x)
   {
      return --x;
   }

If T is deduced as Iterator const &, x is a reference, which was my earlier inference, but operator --(), which is non-const, would be applied to that reference to const and wouldn't compile.  That's the detail I missed earlier.

Clearly, the compiler deduces T as Iterator, copy constructs a temporary Iterator from the reference returned by this->base(), and then decrements that temporary.  That leads to the OP's concern and you see that I'm finally on board.

_____
Rob Stewart                           robert.stewart_at_[hidden]
Software Engineer, Core Software      using std::disclaimer;
Susquehanna International Group, LLP  http://www.sig.com

IMPORTANT: The information contained in this email and/or its attachments is confidential. If you are not the intended recipient, please notify the sender immediately by reply and immediately delete this message and all its attachments. Any review, use, reproduction, disclosure or dissemination of this message or any attachment by an unintended recipient is strictly prohibited. Neither this message nor any attachment is intended as or should be construed as an offer, solicitation or recommendation to buy or sell any security or other financial instrument. Neither the sender, his or her employer nor any of their respective affiliates makes any warranties as to the completeness or accuracy of any of the information contained herein or that this message or any of its attachments is free of viruses.