The use of incomplete class declarations (also known as "forward" declarations) is common. While it is possible to declare pointers and references to incomplete classes, because the class definition is not available it's not possible to access a member of the class, determine the size of the class object, and so on. However, it is possible to cast and delete a pointer to an incomplete class, but this is never a good idea.
Non-Compliant Code Example (deleting)
Consider a common application of a handle/body idiom that implements an abstract data type by a handle that contains a pointer to an implementation class.
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Note that we used a shared_ptr to refer to the Body. Other common smart pointers, including std::auto_ptr, will still produce undefined behavior.
Non-Compliant Code Example (casting)
Similarly, while it is possible to cast a pointer or reference to an incomplete class, it is never a good idea to do so. Casting a class address often involves an adjustment of the address by a fixed amount that can only be determined after the layout and inheritance structure of the class is known, and this information is not available in the case of an incomplete class.
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