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Rule:  OOP#007

Synopsis:Selection statements (if-else and switch) should be used when the control flow depends on an object's value; dynamic binding should be used when the control flow depends on the object's type
Language:C++
Severity Level:9
Category:Object Oriented Programming


Description:

Heavy use of the selection statements if / else and switch might be an indication of a poor design. Selection statements should mostly be used when the flow of control depends on the value of an object.

Selection statements are not the best choice if the flow of control depends on the type of an object. If you want to have an extensible set of types that you operate upon, code that uses objects of different types will be difficult and costly to maintain. Each time you need to add a new type, each selection statement must be updated with a new branch. It is best to localize selection statements to a few places in the code. This however requires that you use inheritance and virtual member functions.

Suppose you have a class that is a public base class. It is possible to operate on objects of derived classes without knowing their type if you only call virtual member functions. Such member function calls are dynamically bound, i.e. the function to call is chosen at run-time. Dynamic binding is an essential component of object-oriented programming and we cannot overemphasize the importance that you understand this part of C++. You should try to use dynamic binding instead of selection statements as much as possible. It gives you a more flexible design since you can add classes without rewriting code that only depends on the base class interface.



Literature References:
ISC++ Rec 10.3