Question

I have a private field

std::map<std::string, std::multiset<GraphObject*>>* the_Map;

How can I allocate memory for it and insert a GraphObject?
Do I have to use the new operator?

the_Map = new map<string,multiset<GraphObject*>>;

And how can I insert a new GraphObject?
It’s part of a datastructure and I really need a pointer to a map.

Answer 1

How can I allocate memory for it and insert a GraphObject?

It doesn’t want to be a pointer at all; just make the map itself a member of the class and memory allocation will happen automatically.

Correctly inserting an object is rather fiddly, since you’re storing pointers there too. If it doesn’t need to be a pointer, then storing objects would make your life much easier. If it really does have to be a pointer (e.g. because GraphObject is a polymorphic base class), I would recommend storing smart pointers: std::unique_ptr, or std::tr1::shared_ptr or boost::shared_ptr if you’re stuck in the past.

If you really, really need to use raw pointers for some insane reason, then the closest you can get to an exception-safe insertion is probably:

GraphObject * object = new Whatever(...);
try {
    the_Map[key].insert(object);
} catch(...) {
    delete object;
    throw;
}

or if you don’t care about the possiblity of memory leaks on insertion failure:

the_Map[key].insert(new Whatever(...));

Also don’t forget to delete each object when you remove it; that won’t happen automatically.

I really need a pointer to a map.

No you don’t. But if you really believe you do, and want to ignore everyone’s advice not to, then you’ll need an actual map to point to. I would recommend that you make this map a member of the class, so that its lifetime is managed automatically.

If you really want to make life difficult for whoever maintains the code, then I suppose you could allocate one with new. In that case, remember to delete it when you’ve finished with it; probably in the class destructor. And if you do that, remember the Rule of Three and implement or delete the copy constructor and copy-assignment operator, since the default implementations will do the wrong thing.