Re: STL and finding objects by name

From:
Alberto Ganesh Barbati <AlbertoBarbati@libero.it>
Newsgroups:
comp.lang.c++.moderated
Date:
Fri, 4 Apr 2008 13:33:35 CST
Message-ID:
<kfsJj.50877$FR.228650@twister1.libero.it>
Matthew Bucknall ha scritto:

class Thing
{
    public:

    Thing(const std::string& name):
        m_name(name)
    {}

    std::string get_name() const { return m_name; }

    bool operator< (const Thing& rhs) const
    {
        return m_name < rhs.m_name;
    }

    private:

    const std::string m_name;
};

std::set<Thing> things;

std::set<Thing>::iterator find_thing(const std::string& name)
{
    // this won't work of course, but this hopefully illustrates
    // what I want to do

    return things.find(name);
}

My question is, how can named objects (such as Thing) be stored in an
STL container such that they can then be efficiently found by name?
Note, I want named objects to have direct access to their name so
storing objects in a std::map<std::string, Thing> is no good IMHO
because items contained in the map don't have access to their keys.


Unfortunately, std::set is not very friendly in this scenario. The only
way to do proper lookup is to create an instance of Thing, for example
like this:

   std::set<Thing>::iterator find_thing(const std::string& name)
   {
      return things.find(Thing(name));
   }

If Thing is an heavy weight object, you could add a function set_name()
to change an object name, making it private to avoid involuntarily
breaking the std::set invariant, but making it a friend of find_thing so
that it can re-use one Thing instance:

   std::set<Thing>::iterator find_thing(const std::string& name)
   {
      static Thing key;
      key.set_name(name);
      return things.find(key);
   }

(not the most elegant code, I know.)

As a last resort, you could avoid using std::set entirely and rely on
smarter containers, for example Boost.MultiIndex
http://www.boost.org/doc/libs/1_35_0/libs/multi_index/doc/index.html

HTH,

Ganesh

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