CPSC 427: Object-Oriented Programming

Michael J. Fischer

Lecture 16
March 29, 2016

Polymorphic Derivation

Some uses for derived classes.

Code reuse. A base class can contain one copy of code that is be used by several derived variants through inheritance.
Modularity. The functionality provided by a base class can be extended in a derived class. Example: BSquare extends Square by adding board coordinates and clusters.
Generic programming and isolation. Demo 17-Craps-extended contains a simulator for the gambling game “craps” that can use different dice implementations.
Polymorphic collections. A company has different kinds of employees with different rules for calculating their pay, each represented by a derived class with its own calculatePay function appropriate to that kind of employee.

Type Hierarchies

Consider following simple type hierarchy:

    class B     { public: int f(); ... };
    class U : B { int f(); ... };
    class V : B { int f(); ... };

We have a base class B and derived classes U and V.

A different method f() is defined in each.

Relationships: A U is a B (and more). A V is a B (and more).

A U can be used wherever a B is expected.

Example: Definition f(B& x) ... ; call U z; f(z);

Inside of f(), only the B-part of z is visible. This is called slicing.

Pointers and slicing

Declare B* bp; U* up = new U; V* vp = new V.

Can write bp = up; or bp = vp;.

Why does this make sense?

*up has an embedded instance of B.
*vp has an embedded instance of B.

If bp = up, then bp points to the embedded B-instance of object *up. The rest of *up is inaccessible because of object slicing.

Ordinary derivation

In our previous example

    class B     { public: int f(); ... };
    class U : B { int f(); ... };
    class V : B { int f(); ... };
    B* bp;

bp can point to objects of type B, type U, or type V.

Want bp->f() to refer to U::f() if bp points to a U object.

Want bp->f() to refer to V::f() if bp points to a V object.

However, with ordinary derivation, bp->f() always refers to B::f().

Polymorphic derivation

The keyword virtual allows for polymorphic derivation.

    class B     { public: virtual int f(); ... };
    class U : B { virtual int f(); ... };
    class V : B { virtual int f(); ... };
    B* bp;

A virtual function is dispatched at run time to the class of the actual object.

bp->f() refers to U::f() if bp points to a U.

bp->f() refers to V::f() if bp points to a V.

bp->f() refers to B::f() if bp points to a B.

Here, the type refers to the allocation type.

Unions and type tags

We can regard bp as a pointer to the union of types B, U and V.

To know which of B::f(), U::f() or V::f() to use for the call bp->f() requires runtime type tags.

If a class has virtual functions, the compiler adds a type tag field to each object.

This takes space at run time.

The compiler also generates a vtable to use in dispatching calls on virtual functions.

Virtual destructors

Consider delete bp;, where bp points to a U but has type B*.

The U destructor will not be called unless destructor B::~B() is declared to be virtual.

Note: The base class destructor is always called, whether or not it is virtual.

In this way, destructors are different from other member methods.

Conclusion: If a derived class has a non-empty destructor, the base class destructor should be declared virtual.

Uses of polymorphism

Some uses of polymorphism:

To define an extensible set of representations for a class.
To allow containers to store mixtures of different but related types of objects.
To support run-time variability of within a restricted set of related types.

Multiple representations

Might want different representations for an object.

Example: A point in the plane can be represented by either Cartesian or Polar coordinates.

A Point base class can provide abstract operations on points.

E.g., virtual int quadrant() const returns the quadrant of *this.

For Cartesian coordinates, quadrant is determined by the signs of the x and y coordinates of the point.

For polar coordinates, quadrant is determined by the angle θ.

Both Cartesian and Polar derived classes should contain a method for int quadrant() const.

Heterogeneous containers

One might wish to have a stack of Point objects.

The element type of the stack would be Point*.

The actual values would have type either Cartesian* or Polar*.

The automatically generated type tags and dynamic dispatching obviates the need to cast the result of pop() to the correct type.

Example:
Stack st; Point* p;

p = st.pop(); // no need to cast result

p->quadrant(); // automatic dispatch

Run-time variability

Two types are closely related; differ only slightly.

Example: Company has several different kinds of employees.

Employee base class has a large and complicated payroll function.
Payroll is same for all kinds of employees except for a function pay() that computes the actual weekly pay.
Each employee kind has its own pay() function.
Big payroll function is in base class.
It calls pay() to get the actual pay for this Employee.

Pure virtual functions

Suppose we don’t want B::f() and never create instances of B.

We make B::f() into a pure virtual function by writing =0.

    class B     { public: virtual int f()=0; ... };
    class U : B { virtual int f(); ... };
    class V : B { virtual int f(); ... };
    B* bp;

A pure virtual function is sometimes called a promise.

It tells the compiler that a construct like bp->f() is legal.

The compiler requires every derived class to contain a method f().

Abstract classes

An abstract class is a class with one or more pure virtual functions.

An abstract class cannot be instantiated.

It can only be used as the base for another class.

The destructor can never be a pure virtual function but will generally be virtual.

A pure abstract class is one where all member functions are pure virtual (except for the destructor) and there are no data members,

Pure abstract classes define an interface à la Java.

An interface allows user-supplied code to integrate into a large system.