Re: [Boost-users] [bind] Help with functionalcompositionofshared_ptr::use_count in std::map

From: Hughes, James (jhughes_at_[hidden])
Date: 2007-06-11 09:26:20

> -----Original Message-----
> From: boost-users-bounces_at_[hidden] 
> [mailto:boost-users-bounces_at_[hidden]] On Behalf Of Peter Dimov
> Sent: 11 June 2007 14:11
> To: boost-users_at_[hidden]
> Subject: Re: [Boost-users] [bind] Help with 
> functionalcompositionofshared_ptr::use_count in std::map
> 
> Hughes, James wrote:
> 
> > RemoveIf(Fred.begin(),
> >         Fred.end(),
> >    boost::bind(&map<string, shared_ptr<data>
> >    >::value_type::second::unique, _1)
> > )
> >
> > Still get same compile fault however.
> 
> It would be nice if this could work with a single bind, but 
> unfortunately you need two:
> 
> boost::bind( &shared_ptr<data>::unique,
>     boost::bind( &map<string, shared_ptr<data> 
> >::value_type::second, _1 )
> )
> 
> There is an additional pitfall here; depending on the version 
> of boost::bind and the compiler, the inner bind may return a 
> shared_ptr by value, making the use_count at least 2. You 
> should probably just fall back to the old-fashioned
> 
> bool second_unique( map<string, shared_ptr<data> >::value_type & v ) {
>     return v.second.unique();
> }
> 
> (I'm using a non-const reference as a parameter to guard 
> against potential silent implicit conversions which could 
> also increase the use_count.)
> 
> Removing the unique entries sounds like purging a cache, 
> maybe weak_ptr will work better in your map? 
> 
> 

Thanks Peter - you are right - it's a cache!

James

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