Re: [Boost-users] multi_index and shared_ptrs

Subject: Re: [Boost-users] multi_index and shared_ptrs
From: Joaquin M Lopez Munoz (joaquin_at_[hidden])
Date: 2009-11-05 14:19:15

Daniel Burrows <dburrows <at> algebraicthunk.net> writes:

> 
>   I'm having trouble getting multi_index's key extractors to play nicely
> with shared_ptr objects.  I'm trying to eliminate my custom hash and
> equality objects in this code:
> 
> class screenshot_cache_entry : /* ... */
> {
>   /* ... */
> 
> public:
>   const screenshot_key &get_key() const;
> 
>   /* ... */
> };
> 
> [...]
>
> typedef multi_index_container<
>   boost::shared_ptr<screenshot_cache_entry>,
>   indexed_by<
>     hashed_unique<tag<by_screenshot_tag>,
>                   identity<boost::shared_ptr<screenshot_cache_entry> >,
>                   hash_cache_entry,
>                   cache_entry_equal>,
>     sequenced<tag<ordered_tag> > >
>   > cache_map;
> 
> [...]
> 
> typedef multi_index_container<
>   boost::shared_ptr<screenshot_cache_entry>,
>     indexed_by<
>       hashed_unique<tag<by_screenshot_tag>,
>                     const_mem_fun<
>                       screenshot_cache_entry, const screenshot_key &,
>                       &screenshot_cache_entry::get_key> >,
>       sequenced<tag<ordered_tag> > >
>   > cache_map;
> 
> [...]
>
> screenshot_cache.cc:526:   instantiated from here
> /usr/include/boost/multi_index/hashed_index.hpp:439: error: no match
> for call to ‘(const boost::hash<const aptitude::screenshot_key>)
> (const boost::shared_ptr<gui::<unnamed>::screenshot_cache_entry>&)’

Probably, you have code similar to the following:

  cache_map m;
  ...
  m.find(shared_ptr<gui::<unnamed>::screenshot_cache_entry>(...));

Am I right? Now that you've passed from a hashed index where
the key was boost::shared_ptr<screenshot_cache_entry> to one
where the key is screenshot_cache_entry, lookup ops have to
be passed values convertible to screenshot_cache_entry, not
to boost::shared_ptr<screenshot_cache_entry> as was your
original situation.

Please report back whether this solves your problem.

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo