Re: [Boost-bugs] [Boost C++ Libraries] #924: Convenience function for const map read

Subject: Re: [Boost-bugs] [Boost C++ Libraries] #924: Convenience function for const map read
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2007-07-03 15:15:39

#924: Convenience function for const map read
-------------------------------+--------------------------------------------
  Reporter:  nobody            |       Owner:  nobody     
      Type:  Feature Requests  |      Status:  assigned   
 Milestone:                    |   Component:  None       
   Version:  None              |    Severity:  Showstopper
Resolution:  None              |    Keywords:             
-------------------------------+--------------------------------------------
Changes (by dave):

  * severity:  => Showstopper

Old description:

> {{{
> It's really awkward to use an STL map when you have
> only a const reference to it, because you cannot use
> operator[] -- you have to use find, check if the value
> was found, etc.  (The reason being that operator[]
> creates a new map entry initialized with the null value
> if it doesn't find the key.)  It would be much more
> convenient to have
> a lookup convenience function that either throws an
> exception if the key doesn't exist, or returns a
> default-initialized value.  The version that returns a
> default value if the key doesn't exist might look
> something like this:
>
> template <class Map>
> typename Map::data_type const &
> map_get(Map const & m, typename Map::key_type key)
> {
>   static typename Map::data_type default_value;
>   typename Map::const_iterator it = m.find(key);
>   return (it == m.end() ? default_value : it->second);
> }
>
> }}}

New description:

 It's really awkward to use an STL map when you have
 only a const reference to it, because you cannot use
 operator[] -- you have to use find, check if the value
 was found, etc.  (The reason being that operator[]
 creates a new map entry initialized with the null value
 if it doesn't find the key.)  It would be much more
 convenient to have
 a lookup convenience function that either throws an
 exception if the key doesn't exist, or returns a
 default-initialized value.  The version that returns a
 default value if the key doesn't exist might look
 something like this:

 {{{
 template <class Map>
 typename Map::data_type const &
 map_get(Map const & m, typename Map::key_type key)
 {
   static typename Map::data_type default_value;
   typename Map::const_iterator it = m.find(key);
   return (it == m.end() ? default_value : it->second);
 }

 }}}

-- 
Ticket URL: <http://svn.boost.org/trac/boost/ticket/924#comment:1>
Boost C++ Libraries <http://www.boost.org/>
Boost provides free peer-reviewed portable C++ source libraries.