Re: [Boost-users] [ptr_container] ptr_map predicates

From: Thorsten Ottosen (tottosen_at_[hidden])
Date: 2006-02-09 05:09:18

Bill Buklis wrote:
> How would I accomplish the following using ptr_map instead of std::map?
> 
> typedef std::map< std::string, int* > stlmap;
> 
> bool predicate( stlmap::value_type& mapitem )
> {
>     // return true if key value has at least 4 characters
>     return( mapitem.first.size() > 3 );
> }
> 
> void sample( stlmap& themap )
> {
>     std::find_if( themap.begin(), themap.end(), predicate );
> }
> 
> 
> The predicate for std::find_if requires *iterator for the parameter.
> However, with ptr_map *iterator returns a reference to the element value
> instead of the key/value pair. Normally this is fine (in general code I like
> this feature), but in this case I need access to the key in the predicate.
> How do I get at it?

this is an interesting use-case that I haven't thought of.

> ptr_map version of above:
> 
> typedef boost::ptr_map< std::string, int > bpmap;
> 
> bool predicate( ??? )
> {
>   ???
> }

bool predicate( const std::pair<const std::string,void*>& p )
{
   return p.first.size() > 3;
}

> void sample( bpmap& themap )
> {
>     std::find_if( ??? );
> }

void sample( bpmap& m )
{
   std::find_if( m.begin().base(), m.end().base(), predicate() );
}

iterator.base() should return an iterator to the underlying void-ptr-map.

I hope it works

-Thorsten