Using The [] Operator Efficiently With C++ unordered_map

Question

Firstly could someone clarify whether in C++ the use of the [] operator in conjunction with an unordered_map for lookups wraps a call to the find() method, or is using the [] op

Answer 1

You can both check if an element exists, and insert a new element if it does not exist, with the special insert function that returns a pair in which the boolean value tells you if the value has been actually inserted. For example, the code here:

  unordered_map mymap;
  pair::iterator,bool> ret;

  // first insert function version (single parameter):;
  mymap.insert ( pair('z',200) );
  ret=mymap.insert (pair('z',500) ); 
  if (ret.second==false)
  {
    cout << "element 'z' already existed";
    cout << " with a value of " << ret.first->second << endl;
  }

The code here inserts the pair <'z',200> into the map if it does not exist. It returns the iterator where it is inserted if the value of the second element of the pair returned is true, or, it returns the iterator where the element actually was, if the second element of the pair is false.