The C++ course FAQ

List of questions

Questions and answers

I didn't find any information about iterators. Can you explain?

I daresay you did not really look well enough. I just ran "c++ iterator" on Google and found many interesting results.

In any case, an iterator is the STL-equivalent to a pointer used to "travel along" an array. If you use a "pure C" array like

 char array[10] = "wow";
you can loop on it by saying
 for(char* ptr = array; *ptr != '\0'; ptr++) {
   cout << *ptr;
If you use a generic STL container, like e.g. vector<>, you say
 vector myVector;
 // fill the vector
 for(vector::iterator vi = myVector.begin(); vi != myVector.end(); vi++){
   cout << *vi;

More precisely, iterators are objects for which the unary operator * and the increment and decrement operators ++ and -- have been overloaded, so that they behave like pointers. An iterator does NOT contain, like a pointer does, simply a memory address. But an iterator will contain some private data which allows it to behave like a pointer (i.e. one of its fields contains a memory address). The overloaded unary operator * will return the object pointed to by the private data contained within the iterator, and the overloaded ++ (respectively --) operator will change the private data within the iterator so that the iterator "points" to the next (respectively previous) element in the STL container.

One important caveat: whereas pointers obey a linear ordering, iterators do not necessarily do so. In other words, if two pointers

  char* p1;
  char* p2;
are such that p1 < p2, then you may be sure that by repeatedly issuing the statement
you will eventually make the condition
  p1 == p2
true. This is not necessarily true of iterators (at least not in all STL releases or implementations). The upshot of this is: the loop
 for(vector::iterator vi = myVector.begin(); vi < myVector.end(); vi++){
   cout << *vi;
(where the important issue is that the termination conditions is vi < myVector.end() instead of vi != myVector.end()) may never terminate!

How can I transform an STL std::string to a pointer of char, i.e. char *?
 string myString = "wow";
 char* myArray = myString.c_str();
Do NOT delete myArray, its allocation and deallocation is handled directly by the string class.