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15.8. Handle Classes and Inheritance

Unfortunately, using pointers or references puts a burden on the users of our classes. We saw one such burden in the previous section that discussed the inter-actions between objects of inherited types and containers.

A  common technique in C++ is to define a so-called cover or handle class  .  The handle class stores and manages a pointer to the base class. The type of the object to which that pointer points will vary; it can point at either a base- or a derived-type object.Users access the operations of the inheritance hierarchy through the handle. Because the handle uses its pointer to execute those operations, the behavior of virtual members will vary at run time depending on the kind of object to which the handle is actually bound. Users of the handle thus obtain dynamic behavior but do not themselves have to worry about managing the pointer.

Handles that cover an inheritance hierarchy have two important design considerations: 

As with any class that holds a pointer (Section 13.5, p. 492), we must decide what to do about copy control. Handles that cover an inheritance hierarchy typically behave like either a  smart pointer (Section 13.5.1, p. 495) or a value (Section 13.5.2, p. 499). 

The handle class determines whether the handle interface will hide the inheritance hierarchy or expose it.If the hierarchy is not hidden, users must know about and use objects in the underlying hierarchy.

There is no one right choice among these options; the decisions depend on the details of the hierarchy and how the class designer wants programmers to interact with those class(es).

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