Re: [Boost-users] [bind] Doubt regarding the use of shared_ptr with boost.bind

Subject: Re: [Boost-users] [bind] Doubt regarding the use of shared_ptr with boost.bind
From: Marshall Clow (mclow.lists_at_[hidden])
Date: 2010-09-23 13:37:38

On Sep 23, 2010, at 9:59 AM, Shiou Ming Lee wrote:
> 
> 
> On Thu, Sep 23, 2010 at 9:25 AM, Igor R <boost.lists_at_[hidden]> wrote:
> > m_ioService.post(m_strand.wrap(
> >                             boost::bind(&LogWriter::append,
> > (*logWritersItr), _logMessage, _logLevelString)
> >                          ));
> >
> > The second param of Bind, (*logWritersItr), is a shared_ptr of LogWriter's
> > subclass. The signature of LogWriter::append() is as following:
> >
> > virtual void append(const std::string& _logMessage, const std::string&
> > _logLevelString) = 0;
> >
> > Base on document at
> > http://www.boost.org/doc/libs/1_44_0/libs/bind/bind.html, the way I use bind
> > will duplicate a copy of that shared_ptr. Any idea when will the duplicated
> > shared_ptr become invalid so that the reference count decreases by one?
> 
> bind() takes your shared_ptr by value, so it won't become "invalid".
> After io_service executes your functor, it will be destroyed, so all
> its internal copies of that shared_ptr will be destroyed as well.
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> 
> I think I don't quite completely understand functor.., is it by C++ that a functor will be destroyed automatically once executed? Or is this a feature from boost.bind?

I think that you're thinking too hard about this.
It's not a feature from boost.bind, it's just standard C++ lifetime rules.

1) The shared_ptr is copied into the bind object. (use count for the shared_ptr ++)
2) The bind object is passed (by value) to m_ioService.post.  (use count for the shared_ptr++)
*) The original bind object gets destroyed (goes out of scope, whatever)  (use count for the shared_ptr--)
*) When m_ioService.post returns, the copy of the bind object that was passed to it is destroyed (use count for the shared_ptr--)

the last two are '*'s rather than 3/4 since I don't remember the order (and it doesn't really matter, either). ;-)

If m_ioService.post makes copies of the functor and/or the shared pointer internally, then the use count for the shared pointer will be incremented for each copy, and decremented when a copy is destroyed.

-- Marshall