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Expertise: Intermediate
Language: C++

February 20, 1998

IsA or HasA?

When designing a class hierarchy, you may face a decision between inheritance (aka IsA ) vs. containment (aka HasA) relation. For instance, if you are designing a

 Radio 

class, and you already have the following classes implemented for you in some library:

 Dial, ElectricAppliance. 

It is quite obvious that your Radio should be derived from

 ElectricAppliance. 

However, it is not so obvious that

 Radio

should also be derived from

 Dial

. How to decide? You can check whether there is always a 1:1 relation between the two, e.g., do all radios have one and only one dial? You may realize that the answer is “no”: a radio can have no dial at all (a transmitter/receiver adjusted to a fixed frequency) or may have more than one (both an FM dial and AM dial). Hence, your

 Radio

class should be designed to have Dial(s) instead of being derived from it. Note that the relation between

 Radio 

and

  ElectricAppliance 

is always 1:1 and corroborates the decision to derive

 Radio 

from

 ElectricAppliance

.