Basic question in assigning value to unordered_map

Question

I have an unordered_map which stores the int as key an pointer as value. I need to check the existence of the key. If the key is not available, I need to insert the key and value. Which one is the better approach?

Thanks.

unordered_map<int, classA*>testMap;
classA* ptr = testMap[1];
if(ptr == NULL)
   testMap[1] = new classA;


OR

unordered_map<int, classA*>::iterator it = testMap.find(1);
if(it == testMap.end())
{
  testMap.insert(make_pair(1, new classA));
}

Answer 1

Neither method is good because both use two queries into the map where one would be sufficient.

A better method is retrieving a reference to the element, and if that reference is a null pointer, assign to it:

classA*& ptr = testMap[1];
if (ptr == 0)
    ptr = new classA;

This works because querying a non-existing element in a map automatically inserts it (default-constructed, thus a null pointer will be inserted), and operator[] returns a reference to that element (be it newly created or already existing).

But notice that the semantics between this method (or your first method) and your second method subtly differ: your second method only inserts the element if the key doesn’t exist in the map. My method (and your first method) also create a new element if the key in fact already existed, but its value was a null pointer.

Answer 2

The second is the better approach. In the first, when you do classA* ptr = testMap[1], you are creating an item in the hash with the default value of NULL.

If you were to change the map value to something other than a pointer (maybe a vector for example) then you might not have a suitable default value to test against. Also, in the future, NULL might be a valid value for your map, so your default value test would be meaningless.