59. Memento Implementation

class Memento {
public:
    virtual ~Memento();

private:
    friend class Item;
    Memento();
    //...
};//class Memento

class Item {
public:
    Memento* create_memento();
    void restore_memento( Memento* );
    //...
};//class Item

Class Item is the originator with some internal states.

The user of Item::create_memento( ) and Item::restore_memento( ) is commonly referred to as a caretaker.

We usually want Memento objects be created only by Item::create_memento( ).

We might also want to allow incremental restoration via a stack of Mementos.

It is possible to implement Memento file I/O (or database) interface to add persistence of states.