C++ tips - Separating interface and implementation, abstract base classes

[ben2ong2]

[ Taken from the C++ FAQ. -adc ]

Q70: What's the big deal of separating interface from implementation?

A: Separating interface from implementation is a key to reusable software.
Interfaces are a company's most valuable resources.  Designing an interface
takes longer than whipping together a concrete class which fulfills that
interface.  Furthermore interfaces require the resources of more expensive
people (for better and worse, most companies separate `designers' from
`coders').  Since they're so valuable, they should be protected from being
tarnished by data structures and other artifacts of the implementation (any
data structures you put in a class can never be `revoked' by a derived class,
which is why you want to `separate' the interface from the implementation).


Q71: How do I separate interface from implementation in C++ (like Modula-2)?

A: Short answer: use an ABC (see next question for what an ABC is).


Q72: What is an ABC (`abstract base class')?

A: An ABC corresponds to an abstract concept.  If you asked a Mechanic if he
repaired Vehicles, he'd probably wonder what *kind* of Vehicle you had in mind.
Chances are he doesn't repair space shuttles, ocean liners, bicycles, and
volkswaggon beetles too.  The problem is that the term `Vehicle' is an abstract
concept; you can't build one until you know what kind of vehicle to build.  In
C++, you'd make Vehicle be an ABC, with Bicycle, SpaceShuttle, etc, being
subclasses (an OceanLiner is-a-kind-of-a Vehicle).

In real-world OOP, ABCs show up all over the place.  Technically, an ABC is a
class that has one or more pure virtual member functions (see next question).
You cannot make an object (instance) of an ABC.

[ ... to be continued ]