Tools.h++ Manual

104011 Tandem Computers Incorporated 13-1

Templates 13

13.1 Introduction

Ever since Version 2.0, Tools.h++ has traditionally offered two types of

collection classes: the so-called "generic" collection classes and the Smalltalk-

like collection classes. Each had its advantages and disadvantages. The

Smalltalk-like classes offer a pleasant programming interface, code reuse, and a

consistent interface. However, they actually collect pointers to a common base

class (

RWCollectable

), requiring that all collected objects inherit from this

class—the actual type of the class is unknown at runtime. This requires the

programmer to rely on some other kind of information, either the

isA()

function or some logical inference, to decide whether it is safe to "downcast"

the pointer to an

RWCollectable

to a pointer to the derived class. A recent

language extension

holds the promise that such downcasting will at least be

standardized by the language. However, the programmer will always have to

remember to perform the check. The check may also fail: an object's

inheritance hierarchy may make the type cast impossible and the prudent

programmer must be prepared for this.

1. See, for example, Josée Lajoie’s article “The new language extensions” in the July-August 1993 issue of The C++

Report.