Memento – behavioral design pattern – library MetaTrader 5

//+------------------------------------------------------------------+
//|                                                201104_114239.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
//+------------------------------------------------------------------+
//| memento - behavioral design pattern                              |
//+------------------------------------------------------------------+
//   without violating encapsulation, capture and externalize an object's
//internal state so that the object can be restored to this state later
//+------------------------------------------------------------------+
//| applicability                                                    |
//+------------------------------------------------------------------+
//   a snapshot of (some portion of) an object's state must be saved so
//that it can be restored to that state later, and
//   a direct interface to obtaining the state would expose implementation
//details and break the object's encapsulation
//+------------------------------------------------------------------+
//| structure                                                        |
//+------------------------------------------------------------------+
//                                                memento
// |        Originator        |- - ->| Memento  |<-------o|Caretaker|
// |--------------------------|      |----------|
// |SetMemento(Memento m)     |      |GetState()|
// | state=m.GetState()       |      |SetState()|
// |CreateMemento()           |      |----------|
// | return new Memento(state)|      |state     |
// |--------------------------|
// |state                     |
//
//+------------------------------------------------------------------+
//| participants                                                     |
//+------------------------------------------------------------------+
//   memento
//      stores internal state of the originator object. the memento may
//store as much or as little of the originator's internal state as necessary
//at its originator's discretion
//      protects against access by objects other than the originator.
//mementos have effectively two interfaces. caretaker sees a narrow interface
//to the memento — it can only pass the memento to other objects. originator,
//in contrast, sees a wide interface, one that lets it access all the data
//necessary to restore itself to its previous state. ideally, only the
//originator that produced the memento would be permitted to access the
//memento's internal state
//   originator
//      creates a memento containing a snapshot of its current internal state
//      uses the memento to restore its internal state
//   caretaker
//      is responsible for the memento's safekeeping
//      never operates on or examines the contents of a memento
//+------------------------------------------------------------------+
//| collaborations                                                   |
//+------------------------------------------------------------------+
#include <Mqh201104_114823.mqh> //memento
#include <Mqh201104_115143.mqh> //originator
#include <Mqh201104_115621.mqh> //caretaker
//   a caretaker requests a memento from an originator, holds it for a
//time, and passes it back to the originator:
//
//   aCaretaker              anOriginator              aMemento
//       |                        |                        |
//      | |CreateMemento()        |
//      | |--------------------->| |new Memento            |
//      | |                      | |- - - - - - - - - - ->| |
//      | |                      | |SetState()             |
//      | |                      | |--------------------->| |
//       |                        |                        |
//       /                        /                        /
//       /                        /                        /
//       |                        |                        |
//      | |SetMemento()           |                        |
//      | |--------------------->| |GetState()             |
//      | |                      | |--------------------->| |
//       |                        |                        |
//
//   sometimes the caretaker won't pass the memento back to the originator,
//because the originator might never need to revert to an earlier state
//   mementos are passive. only the originator that created a memento will
//assign or retrieve its state
//---
void OnStart()
  {
   Originator originator;
   Caretaker caretaker;
//---
   originator.State("on");
   caretaker.memento=originator.CreateMemento();
   originator.State("off");
   originator.SetMemento(caretaker.memento);
  }
//+------------------------------------------------------------------+
//| output                                                           |
//+------------------------------------------------------------------+
//   changing originator state to on
//   creating memento with current state on
//   changing originator state to off
//   restoring originator state from memento to on
//+------------------------------------------------------------------+
//| consequences                                                     |
//+------------------------------------------------------------------+
//   preserving encapsulation boundaries
//   it simplifies originator
//   using mementos might be expensive
//   defining narrow and wide interfaces
//   hidden costs in caring for mementos
//+------------------------------------------------------------------+
//| implementation                                                   |
//+------------------------------------------------------------------+
//   language support
//   storing incremental changes
//+------------------------------------------------------------------+
//| related patterns                                                 |
//+------------------------------------------------------------------+
//   command: commands can use mementos to maintain state for undoable operations
//   iterator: mementos can be used for iteration as described earlier
//+------------------------------------------------------------------+