Сlass duo_list it's a doubly-linked list, each element include one object.

Object can be three types: "whole number", "float number" and "string".

All of these classes are the inheritors of from the abstract base class which pointer contain elements of a doubly linked list.

Classes "whole number", "float number" contain int and float respectively.

In the class of "string" is contained the object data_string - is encapsulated char array with the methods of access and manipulate data.

The entire hierarchy of content objects supports output to the console.

duo_list has method to add, delete elements from begin and end of it.

Add, delete elements, using his iterator.

Outputs his elements into console.