When is dynamic checking necessary in c plus plus programming?

Answer 1

Dynamic checking is necessary when we need to explicitly determine the actual type of a derived class of object knowing only its base class. Normally it is not necessary to know the actual type of a derived class as we can make use of the virtual interface provided by the base class. However, derived objects may (and often do) provide interfaces that are not accessible through the virtual call mechanism and, in order to invoke those specific interfaces, we need to know the actual type -- even if only to determine if those interfaces exist.

Note that the base class virtual interface should be viewed as being the common interface for all derived objects. Interfaces that are not common to all derived objects should ideally be placed in those specific derived classes to which they apply (or to a common derived class). But in order to access these interfaces we need to know if those interfaces really exist or not and we can only achieve that with dynamic checking.

The virtual call mechanism is itself a form of dynamic checking and, because it is generally the most efficient form, it is tempting to use it for all dynamic testing. However we must resist this because base class interfaces that are not common to all derivatives of that class results in code that is more difficult to maintain, is confusing to users of those interfaces and increases code size unnecessarily.

To use a real-world example, a bat is a mammal that can fly, but not all mammals can fly. Thus it would be inappropriate for the mammal class to provide a "fly" interface because that interface only applies to a specific class of mammals; those mammals that can fly. But if we have a container full of mammals and wish to separate those that can fly from those that cannot, we need to know which have the fly interface, and we can only do that by dynamically testing each mammal to determine its actual type.