C++ :: Design ID To Store Vectors For CAD Program?

Let's say we have entities like lines and circles. And each entity can have a pen attached. And a pen is a color and line width. All entities needs to be manipulated through a operations interface. All entities added to the program needs to be added through using a factory pattern.

So we end up something like (pseudo code)

class Line // for immutable objects {
..
..
} class MutableLine extends Line //for mutable lines {
} MutableLine line=factory->newLine(20,20,50,50); // Create a new line entity

[code]....

So, essentially I want no code to be able to operate directory on entities, unless during creation of the object (color, layer, line width etc...) so I am planning to create Mutable versions of all entities besides the immutable versions.

When requested for selected entities or all entities, I am planning to return a list of immutable objects and in fact I am planning to return a new copy so it's not possible to operate on anything directly.

The reason is that I am planning to create different storage backends swell, so I can operate on entities in a database, or shared memory... stuff like that. At least to hide the internals and provide a stable API.

My questions are:

How can I make sure that people don't do 'tricks' casts for example to a mutable version to change objects directly?