1.5.2 Iterator

• There are 5 classification iterators: input, output, forward, bi-direction, random. You need to know these five are not real class, they are just conception, not real implementation.
• As mentioned earlier, each container class defines a class scope typedef name called iterator. So the vector<int> class has iterators of type vector<int>::iterator. The document will tell you vector iterators is random access iterators.
• In practical point of view: There are four points you need to know:
  1. Five classification and associated supported operation.
  2. Common container’s iterator classification.
  3. Common algorithm can accept what kind of iterator. (See next chapter)
  4. Use typedef simply define container iterator.(typedef is good friend when you use stl more and more);
• Now, five classification iterators play important role here: From this figure, you can know what operation each iterator supports.
  

  

• Basic container iterator is below:
  

  

• In each container, iterator of container is implemented by itself. There is no iterator class inheritance hierarchical structure. I means that in vector, there is vectIter: Iterator (vectIter inheriate from base iterator class) , then you define ++ operator in class vectIter class. It’s totally wrong. In fact, In vector, Maybe iterator is defined by:
  template<type T> 
  vector{ 
  .... 
  typedef T* iterator; 
  }; 
   
  vector<int>::iterator ip; //use it outside. 
  // ++ and -- is done by pointer automatically.

  and All the member function of vector know the iterator very well, inside, it will use T* directly,

  push_back(T x){ 
  *end++ = x; 
  }  // you can see there is no iterator 
     // when you implement container implementation.
• When you see the algorithm, you will see the algorithm is based on template, not inheritance. (You don’t need to make type can be cast or not. ). Algorithm don’t care what iterator really is, It just make sure each iterator can support what operation.
  template <class InputIterator, class OutputIterator> 
    OutputIterator copy (InputIterator first, InputIterator last, 
              OutputIterator result);
• Common four iterator errors.
  1. Valid values: Is the iterator dereferenceable? For example, writing "*e.end()" is always a programming error.
  2. Valid lifetimes: Is the iterator still valid when it’s being used? Or has it been invalidated by some operation since we obtained it?
  3. Valid ranges: Is a pair of iterators a valid range? Is first really before (or equal to) last? Do both really point into the same container?
  4. Illegal builtin manipulation: For example, is the code trying to modify a temporary of builtin type, as in "–e.end()" above? (Fortunately, the compiler can often catch this kind of mistake for you, and for iterators of class type, the library author will often choose to allow this sort of thing for syntactic convenience.)
  int main(){ 
  vector<Date> e; 
  copy( istream_iterator<Date>( cin ), istream_iterator<Date>(), back_inserter( e ) ); 
  vector<Date>::iterator first = find( e.begin(), e.end(), "01/01/95" ); 
  vector<Date>::iterator last = find( e.begin(), e.end(), "12/31/95" ); 
  *last = "12/30/95"; 
  copy( first, last, ostream_iterator<Date>( cout, "\n" ) ); 
  e.insert( --e.end(), TodaysDate() ); 
  copy( first, last, ostream_iterator<Date>( cout, "\n" ) ); 
  }
• You can inheriate you own iterator from std::iterator: yes, that’s what it’s for.If you mean anything else: no, because none of the STL iterators have virtual destructors. They’re not meant for inheritance and a class inheriting from them might not clean up properly. a Good example can be seen link: http://www.cplusplus.com/reference/iterator/iterator/
• two common iterator operation: advance() and distance().
  std::list<int>::iterator first = mylist.begin(); 
  std::list<int>::iterator last = mylist.end(); 
   
  std::advance(first, 3) // 
  std::cout  << std::distance(first,last)
• istream_iterator and istreambuf_iterator topic:
   std::ostream_iterator<int> out_it (std::cout,",␣"); 
   std::copy ( myvector.begin(), myvector.end(), out_it ); 
   
   ifstream inputFile("aa.txt"); 
   string fileData((istreambuf_iterator<char>(inputFile)) , 
    istreambuf_iterator<char>()); 
  // It will just read all the character.(including white character) 
  // I don’t need format data, 
   
   ifstream inputFile("aa.dat"); 
   list<int> data((istreambuf_iterator<char>(inputFile)) , 
    istreambuf_iterator<char>()); 
   // It wil  read format data, and use white space as delimiter.