The Modern C++ Challenge

71. Observable vector container

Write a class template that behaves like a vector but can notify registered parties of internal state changes. The class must provide at least the following operations:

• Various constructors for creating new instances of the class
• operator= to assign values to the container
• push_back() to add a new element at the end of the container
• pop_back() to remove the last element from the container
• clear() to remove all the elements from the container
• size() to return the number of elements from the container
• empty() to indicate whether the container is empty or has elements

operator=, push_back(), pop_back(), and clear() must notify others of the state changes. The notification should include the type of the change, ...