The  operator adds a blank entry to the map if it does not exist. If the find() function does not find the entry it simply points to the end of the map. (the map is not modified in any way)
Not sure if that answered your question or not. I don't know if cout << foo; is undefined behavior or if that is valid code. i.e.
map<int, int> foo;
foo = 5; // Add an entry with key 5 and value 5.
int a = foo; // Retrieve value at key 5 (which is 5)
map<int, int>::iterator f = foo.find(5); // Returns a valid iterator as key 5 exists.
map<int, int>::iterator g = foo.find(6); // Returns an "invalid" iterator as key 6 does not exist. No modifications to the map are made.
cout << foo; // Output value at key #6.. wait key #6 does not exist.. Adds key #6 to the map and sets to zero and outputs zero. So now the map has grown.
map<int. foo> mapOne;
mapOne; // Add a new entry with key 5. Does the value struct foo get initialized to zero? I didn't bother checking.