I'm trying to use a Boost MultiIndex container in my simulation. My knowledge of C++ syntax is very weak, and I'm concerned I'm not properly removing an element from the container or deleting it from memory. I also need to modify elements, and I was hoping to confirm the syntax and basic philosophy here too.

// main.cpp
...
#include <boost/multi_index_container.hpp>
#include <boost/multi_index/hashed_index.hpp>
#include <boost/multi_index/member.hpp>
#include <boost/multi_index/ordered_index.hpp>
#include <boost/multi_index/mem_fun.hpp>
#include <boost/tokenizer.hpp>
#include <boost/shared_ptr.hpp>
...
#include "Host.h" // class Host, all members private, using get fxns to access

using boost::multi_index_container;
using namespace boost::multi_index;

typedef multi_index_container<
  boost::shared_ptr< Host >,
  indexed_by< 
    hashed_unique< const_mem_fun<Host,int,&Host::getID> >
    //   ordered_non_unique< BOOST_MULTI_INDEX_MEM_FUN(Host,int,&Host::getAge) >
    > // end indexed_by
  > HostContainer;

typedef HostContainer::nth_index<0>::type HostsByID;

int main() {
   ...
   HostContainer allHosts;
   Host * newHostPtr;
   newHostPtr = new Host( t, DOB, idCtr, 0, currentEvents );
   allHosts.insert( boost::shared_ptr<Host>(newHostPtr) );
   // allHosts gets filled up

   int randomHostID = 4;
   int newAge = 50;
   modifyHost( randomHostID, allHosts, newAge );
   killHost( randomHostID, allHosts );
}

void killHost( int id, HostContainer & hmap ){
  HostsByID::iterator it = hmap.find( id );
  cout << "Found host id " << (*it)->getID() << "Attempting to kill. hmap.size() before is " << hmap.size() << " and ";
  hmap.erase( it ); // Is this really erasing (freeing from mem) the underlying Host object?
  cout << hmap.size() << " after." << endl;
}

void modifyHost( int id, HostContainer & hmap, int newAge ){
  HostsByID::iterator it = hmap.find( id );
  (*it) -> setAge( newAge ); // Not actually the "modify" function for MultiIndex...
}

My questions are

1. In the MultiIndex container allHosts of shared_ptrs to Host objects, is calling allHosts.erase( it ) on an iterator to the object's shared_ptr enough to delete the object permanently and free it from memory? It appears to be removing the shared_ptr from the container.
2. The allhosts container currently has one functioning index that relies on the host's ID. If I introduce an ordered second index that calls on a member function (Host::getAge()), where the age changes over the course of the simulation, is the index always going to be updated when I refer to it?
3. What is the difference between using the MultiIndex's modify to modify the age of the underlying object versus the approach I show above?
4. I'm vaguely confused about what is assumed/required to be constant in MultiIndex.

Thanks in advance.

Update

Here's my attempt to get the modify syntax working, based on what I see in a related Boost example.

struct update_age {
  update_age():(){} // have no idea what this really does... elicits error
  void operator() (boost::shared_ptr<Host> ptr) {
    ptr->incrementAge(); // incrementAge() is a member function of class Host
  }
};

and then in modifyHost, I'd have hmap.modify(it,update_age). Even if by some miracle this turns out to be right, I'd love some kind of explanation of what's going on.