What is an iterator in c plus plus?

C++ allows programmers to express ideas in code. Classes are the principal method we use to express those ideas, classifying both data and functions to produce objects that behave in an obvious, intuitive and consistent manner.

If were to define a 3D drawing system we might start with some primitive objects such as spheres, pyramids and cuboids. We could simply define three separate classes for these objects but then we would miss a fundamental aspect: all three share something in common. They are all shapes. While spheres, pyramids and cuboids are all examples of real-world objects, a shape is not a real world object at all, it is merely the concept of an object. Concepts are abstract classes -- we cannot construct objects from them. They don't exist in the real-world but we can certainly imagine they exist and we can easily say that a sphere is a type of shape. If we can imagine it exists then we can express that idea in our code. Our shape class simply describes everything that is common to all shape objects, whether spheres, pyramids, cuboids or any other type of real-world shape. This means we can place all types of shape in containers and treat them as a single entity -- a collection of shape. Each type of shape has its own specialised properties but they also share a common interface: they can all be drawn, erased, moved, rotated, coloured and so on.

Thus we can use the shape class to declare the common interface while each specific type of shape provides the actual implementation. In this way our code does not need to know what specific type of shape it is working with, it is enough to know that it is a shape (of some kind) and we can invoke its draw, erase, move, rotate or colour methods and let the object itself decide how to implement that method.

In short, a concept is an abstraction that allows us to express something that is common between objects that would otherwise be treated separately. It allows us to generalise our code in such a way that we can cater for new objects that do not yet exist. So long as they are conceptually the same, they can be treated the same, without having to alter the code that provides the treatment in any way.