Is it safe to get an object in std::map by reference?

Question

I have a map like this

map<int,object> objmap;
object& obj = objmap.find(num)->second;
object& obj2 = objmap[num];

Whatever changes I make in the object have to be reflected on the map. Similar thing cant be done in a vector as it changes location of objects when it wants more space. Is it safe to do it in a std::map? and is it advisable? The second version gives an error as my object doesn't have an empty constructor. If I declare an empty constructor doing nothing, will the two lines will work the same way?

Answer 1

So long as the object in question isn't removed from the map, then yes it is safe. Once inserted into a map objects don't move around even if other elements are added or removed.

object& obj = objmap.find(num)->second;

This is potentially dangerous unless you are sure that an element with key num actually exists in the map. If you are not sure, you could use the overload of insert that returns an iterator and a bool which indicates whether a new element was inserted or an element with the given key was already present in the map.

E.g.

object& obj = objmap.insert( std::make_pair(num, object(arg1, arg2, argN)) ).first->second;