FAQ 27.09 Are there cases where dynamic type checking is necessary?

Yes, particularly with persistent heterogeneous objects.

A program can't have static knowledge about things that existed before the execution of the program. If objects from several classes were previously stored in a database, the program that peels the objects off the disk drive's platter (or, equivalently, slurps them from a coaxial cable) cannot know the types of the objects because it didn't create them.

In these cases, the objects may need to be queried about their types, especially if the persistent objects are highly heterogeneous. To whatever extent possible, use the maxim "Ask once, then remember." In other words, try to avoid asking an object its type (or its capabilities) every time it is used. This is especially true if the queries require reasoning about the objects in a nonextensible manner (that is, control flow logic that uses code to find code; see FAQ 27.04).

Note that it is normally possible to avoid the type queries if the objects are known to be of the same class (homogeneous) or at least known to be derived from some common ABC that has a fairly rich set of member functions.