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CS253 Inheritance Vs Composition

Is-a vs. Has-a

It’s all about:

Inheritance (Is-a)

vs.

Composition (Has-a)

Is-a

• Is-a is about what you are.
  • A dog is a mammal.
  • A real number is a scalar.
  • The U.S. president is a natural born citizen.
  • The U.S. president is a person born before 1990.
  • A car is a vehicle.
  • Beyoncé is a human being.
  • Beyoncé is a singer.
  • Beyoncé is a Grammy winner.
  • Beyoncé is a n air-breather.
  • Beyoncé is a vertebrate.
  • Beyoncé is a taxpayer.

Has-a

• Has-a is about what you have.
  • A car has a drive shaft.
  • A car has has a set of four tires.
  • The Secretary of State has an administrative staff.
  • A complex number has a real part.
  • A complex number has an imaginary part.
  • My wife has a laptop.
  • My wife has a winter coat.
  • My wife has a degree in nursing.
  • My wife has a degree in horticulture.
  • My wife has a middle name.
  • My wife has a husband.
  • My wife has a social security number.
  • My wife has a sweet smile.

O-O relevance

In object-oriented (o-o) programming:

• Is-a refers to class inheritance (superclass/subclass, base/derived classes).
• Has-a refers to composition (member variables).

Example

class Base {
  public:
    int beta;
};

class Derived : public Base {
  public:
    int gamma;
};

• Derived is-a Base.
• Base has-a beta.
• Derived has-a gamma.
• Derived has-a beta.

Another Example

class RapSheet {
  ...
};

class Inmate {
  private:
    unsigned long number;
    RapSheet offenses;
};

Of course, is-a is restricted to class objects, whereas has-a can apply to objects or built-in types.

• Inmate has-a number
• Inmate has-a RapSheet