Re: [boost] [functional] [shared_ptr] boost::hash<shared_ptr<T>> Returns Only Two Values

Subject: Re: [boost] [functional] [shared_ptr] boost::hash<shared_ptr<T>> Returns Only Two Values
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2011-02-18 16:13:38

Daniel James wrote:
> On 18 February 2011 19:40, Stewart, Robert
> <Robert.Stewart_at_[hidden]> wrote:
>
> > The following is a program that demonstrates that a null
> > shared_ptr<int> produces a hash value of zero and that
> > (ostensibly) all non-null instances produce a hash value
> > of one.
>
> This is known, it happens because shared_ptr can be implicitly cast to
> bool, so it uses that. If you set BOOST_HASH_NO_IMPLICIT_CASTS it
> won't happen. This macro isn't set by default because I'm not sure if
> there will be any other consequences from preventing implicit casts,
> but it might be in a future version.

Thanks for the information.

We knew it was due to the Safe Bool Idiom, but it was surprising that there was no documentation on the issue or a suitable solution available.

Defining BOOST_HASH_NO_IMPLICIT_CASTS produces a static assertion failure which is certainly better than silent misbehavior.  It would be nice to ascertain whether allowing implicit casts is really worthwhile.

> > I expected that there'd be a shared_ptr specialization of
> > hash_value to make it work right, but I don't find one.
> > Shouldn't there be one?
>
> That's an issue for the shared_ptr implementation.

The following worked even without defining BOOST_HASH_NO_IMPLICIT_CASTS:

namespace boost
{
   template <class T>
   std::size_t
   hash_value(boost::shared_ptr<T> const & _ptr)
   {
      return boost::hash_value(_ptr.get());
   }
}

Peter, should this be added to boost/smart_ptr/shared_ptr_hash_value.hpp or some such?

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

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