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

From: Küppers, Ben (ben.kuppers_at_[hidden])
Date: 2007-06-11 08:17:19

> map<string, shared_ptr<data> > Fred;
> 
> RemoveIf(Fred.begin(),
>          Fred.end(),
>          boost::bind(&map<string, shared_ptr<data>
> >::value_type::second::unique, _1)
> 	)

James,

remove_if requires an assignment operator on the operated type but the value_type of a map is defined as std::pair< const T1, T2 >. The const on the first type will cause any assignment to fail. In other words, you can't use remove_if on a map.

That said, in order to fix your bind you would need two functors. The type passed to the bind is the value_type of the map which is by definition a pair. You need to first invoke a functor on the pair to extract the second member and then call the member unique on the resulting smart pointer. In order to do so you'll need to write your own extractor, because the STL does not provide a functor to obtain the "second" field from a pair (I haven't found one in boost either).
 
template< class Pair >
struct second_of : std::unary_function< Pair, typename Pair:second_type > {
   typename Pair::second_type opertator()( const Pair& the_pair ) const {
      return the_pair.second;
   }
};

std::for_each(
   Fred.begin(),
   Fred.end(),
   boost::bind(
      boost::shared_ptr< data >::unique,
      boost::bind(
         second_of< std::map< std::string, boost::shared_ptr< data > >::value_type >(), _1 ) ) );

Note that you can call for_each as it does not require the assignment operator.

Hope this helps,

Ben

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