C++ Class forward declaration drawbacks?

Question

I want to use forward declaration of a class in my software, so I can have typedefs
and use them inside the class full declaration.

Smth like this:

Answer 1

From the C++ standard:

5.3.5/5:

"If the object being deleted has incomplete class type at the point of deletion and the complete class has a non-trivial destructor or a deallocation function, the behavior is undefined."