Re: How to make insert-and-overwrite into a map

From:

Carl Barron <cbarron413@adelphia.net>

Newsgroups:

comp.lang.c++.moderated

Date:

Mon, 1 Oct 2007 15:46:49 CST

Message-ID:

<011020071355004795%cbarron413@adelphia.net>

In article <1191237032.879861.126100@w3g2000hsg.googlegroups.com>,
<yuval.ronen@gmail.com> wrote:

Hi.
Since std::map::insert doesn't overwrite the mapped value of an
already-existing key, I wonder how to accomplish an overwriting
insert. Using operator[] is not a good idea because it enforces the
mapped value to be default constructible. Checking the boolean value
returned by insert() and if necessary, calling find(), and assign to
the found value, is also not a good idea because it means searching
the map happens twice - both by insert() and find() - which is
inefficient.

Is there another way, or was it just forgotten by the map interface
designers?

there is
std::pair<iterator,bool> insert(value_type const &);

returns an pair the iterator points to the item with the key
that is in the map, and the bool is true if item was inserted,
so something like:

template <class Map>
void always_insert(Map &m,typename Map::value_type const &v)
{
   std::pair<typename Map::iterator,bool> res = m.insert(v);
   if(!res.second) // key was already in map change it.
         *res.first = v;
}

--
      [ See http://www.gotw.ca/resources/clcm.htm for info about ]
      [ comp.lang.c++.moderated. First time posters: Do this! ]