The opaque pointer (pimpl) idiom has been inherited from C language where it is used to encapsulate implementation details. However, both old-school and "modern" C++ dispence you from writing some ugly code, and allow to use interfaces with object factories.
The example of implementation with both pimpl and interface approach is as follows.
Do not use static_cast when you cast from a basic class to a derived one. This may lead to undefined behavior. To handle the polymorphism, a virtual inheritance or a multiple inheritance case always use dynamic_cast instead.
The following short example shows the undefined behavior cases. This works with GCC, VC and online C++ shells: