Erasing and modifying elements in Boost MultiIndex Container
- by Sarah
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
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.
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?
What is the difference between using the MultiIndex's modify to modify the age of the underlying object versus the approach I show above?
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.
