Iterator – behavioral design pattern – library MetaTrader 5

//+------------------------------------------------------------------+
//|                                                201021_113117.mq5 |
//|                                    2019-2020, dimitri pecheritsa |
//|                                                 792112@gmail.com |
//+------------------------------------------------------------------+
//   from: design patterns: elements of reusable object-oriented software
//   by: gof: erich gamma, richard helm, ralph johnson, john vlissides
//   published in 1994
//+------------------------------------------------------------------+
//| iterator - behavioral design pattern                             |
//+------------------------------------------------------------------+
//   provide a way to access the elements of an aggregate object
// sequentially without exposing its underlying representation
//+------------------------------------------------------------------+
//| applicability                                                    |
//+------------------------------------------------------------------+
//   access an aggregate object's contents without exposing its
// internal representation
//   support multiple traversals of aggregate objects
//   provide a uniform interface for traversing different aggregate
// structures (that is, to support polymorphic iteration)
//+------------------------------------------------------------------+
//| structure                                                        |
//+------------------------------------------------------------------+
//
//         |    Aggregate   |<------|Client|------->|   Iterator  |
//         |----------------|                       |-------------|
//         |CreateIterator()|                       |First()      |
//                 ^                                |Next()       |
//                 |                                |IsDone()     |
//                 |                                |CurrentItem()|
//                 |                                       ^
//                 |                                       |
//|             Concrete             |- - - - - - - ->|Concrete|
//|            Aggregate             |<---------------|Iterator|
//|----------------------------------|
//|CreateIterator()                  |
//| return new ConcreteIterator(this)|
//
//+------------------------------------------------------------------+
//| participants                                                     |
//+------------------------------------------------------------------+
//   iterator
//      defines an interface for accessing and traversing elements
//   aggregate
//      defines an interface for creating an iterator object
//   concrete iterator
//      implements the iterator interface
//      keeps track of the current position in the traversal of the
// aggregate
//   concrete aggregate
//      implements the iterator creation interface to return an
// instance of the proper concrete iterator
//+------------------------------------------------------------------+
//| collaborations                                                   |
//+------------------------------------------------------------------+
//   a concrete iterator keeps track of the current object in the
// aggregate and can compute the succeeding object in the traversal                                       |
//---iterator participants
#include <Mqh201021_104101.mqh> //iterator
#include <Mqh201021_104329.mqh> //aggregate
#include <Mqh201021_104505.mqh> //concrete iterator
#include <Mqh201021_104755.mqh> //concrete aggregate
//---
void OnStart()
  {
   Aggregate<string>* aggregate=new ConcreteAggregate;
   aggregate+="element a";
   aggregate+="element b";
   aggregate+="element c";
   aggregate+="element d";
//---
   Iterator<string>* iterator=aggregate.CreateIterator();
   for(iterator.First(); !iterator.IsDone(); iterator.Next())
     {
      Print(iterator.CurrentItem());
     }
//---
   delete aggregate;
   delete iterator;
  }
//+------------------------------------------------------------------+
//| output                                                           |
//+------------------------------------------------------------------+
//   first iteration of aggregate 
//   element a
//   next iteration of aggregate 
//   element b
//   next iteration of aggregate 
//   element c
//   next iteration of aggregate 
//   element d
//+------------------------------------------------------------------+
//| consequences                                                     |
//+------------------------------------------------------------------+
//   it supports variations in the traversal of an aggregate
//      to change  the traversal algorithm just replace the iterator 
// instance with a different one
//      you can also define iterator subclasses to support new traversals
//   iterators simplify the aggregate interface
//   more than one traversal can be pending on an aggregate
//+------------------------------------------------------------------+
//| implementation                                                   |
//+------------------------------------------------------------------+
//   who controls the iteration? 
//   who defines the traversal algorithm? 
//   how robust is the iterator? 
//   additional iterator operations
//   using polymorphic iterators in c++
//   iterators may have privileged access
//   iterators for composites
//   null iterators
//+------------------------------------------------------------------+