Re: [Boost-users] scoped_ptr::release (again)

Subject: Re: [Boost-users] scoped_ptr::release (again)
From: Howard Hinnant (hinnant_at_[hidden])
Date: 2009-02-20 23:13:48

On Feb 20, 2009, at 10:53 PM, Yang Zhang wrote:

> Howard Hinnant wrote:
>> On Feb 20, 2009, at 1:27 PM, Yang Zhang wrote:
>>> From http://www.boost.org/doc/libs/1_36_0/libs/smart_ptr/scoped_ptr.htm 
>>> :
>>>
>>> "If scoped_ptr had a release() member, it would become possible to  
>>> transfer ownership of the held pointer, weakening its role as a  
>>> way of limiting resource lifetime to a given context. Use  
>>> std::auto_ptr where transfer of ownership is required. (supplied  
>>> by Dave Abrahams)"
>>>
>>> But there already is a way to escape the scope, via swap().
>>>
>>> Would it make more sense to just provide release() and have users  
>>> specify const if they want?  (const scoped_ptr/scoped_array would  
>>> effectively disallow swapping/releasing.)
>> <opinion>
>> http://home.roadrunner.com/~hinnant/unique_ptr03.html
>> (search for "scoped_ptr")
>> </opinion>
>
> Nice.  Thanks for the pointer (pun intended).  I take it boost does  
> not have unique_ptr, correct?  I checked in Boost.TR1 and found  
> nothing.  I did find boost::interprocess::unique_pointer, but that  
> appears to be different.

You might keep a close eye on boost::interprocess::unique_pointer.   
Ion knows what he's doing and is in sync with me.  His version is  
probably changing to take advantage of a newly developed generic move  
emulation library that he's been working on, but I am not sure.  I'm  
sure Ion will chime in. Whatever he says goes. I have 100% confidence  
in him in this matter.  :-)

> I did some digging, and I did find mention of this file on the gcc  
> mailing lists - I take it this implementation is being included in  
> GNU libstdc++?

I don't know.

> Lastly, I didn't see any rvalue references in this file.  I was  
> under the impression that the implementation of unique_ptr leverages  
> rvalue references.  Is this why there are const_casts present in the  
> code?

The referenced file is a pure C++03 emulation and doesn't contain C+ 
+0X code (e.g. rvalue references).  The emulation is not perfect and  
so yes, const_cast is used to in some places to emulate (as much as  
possible) C++0X language features.   A combined C++03/C++0X  
implementation is more likely to be found in a boost implementation  
such as boost::interprocess::unique_pointer (but I am not sure).

-Howard