CS 4448 - Fall 1998
Object-Oriented Programming and Design
Talk 9.3
by
Patrick Link

C++ Interfaces

About Interfaces

Interfaces "provide a convenient means of resolving the tension that sometimes occurs between what a class is and what a class can do"
interfaces are not inheritance-based polymorphism; interfaces allow the designer to express situations that apply polymorphism to objects that do not share a common base class

Interface Design Goals

there is a clear distinction between capability classifications and property-based classifications
example: making french fries with both a Machine class and a Person class (Java implementation Listing One)
Comparing Interfaces to C++ Abstract Classes: Inheritance is the only mechanism for providing a shared implementation for an interface. Pitfall: relying on inheritance to achieve an interface when it is not always an appropriate solution.

Ways to Achieve Interface Designs In C++: Inheritance-Based

Using multiple-inheritance to achieve a "dual aspect inheritance scheme"; primary superclass provides a shared implementation, additional lines of inheritance mix in purely abstract classes
Purely abstract classes "effectively impose an interface requirement" (example: Listing Two)
"there really is no difference between the mechanism of applying purely abstract superclasses and Java interfaces"

Ways to Achieve Interface Designs In C++: Noninheritance-Based

Anonymous Interfaces Using Templates: templates specify an interface that must be implemented by a class to use a function template (example: Listing Three)
interface requirements for a template are enforced by the compiler: a version of the function matching the type of the argument is generated provided the interface is compatible
templates require interface generality and consistency
Adapter Classes: proxy to resolve interface mismatches
Adapter class meets interface requirements by providing its own compliant interface
Adapter then mediates between the compliant interface and a target object
Level of resolution varies from simple pass-through mechanisms to a complete transformation of an object so as to make it look like a different type of object (example: Listing Four)
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Revised: November 1, 1998