Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ames!apple!rutgers!att!ulysses!andante!alice!bs
From: bs@alice.UUCP (Bjarne Stroustrup)
Newsgroups: comp.lang.c++
Subject: Re: C++ design
Summary: type system
Keywords: information
Message-ID: <9478@alice.UUCP>
Date: 13 Jun 89 19:44:05 GMT
References: <9474@alice.UUCP>
Organization: AT&T Bell Laboratories, Murray Hill NJ
Lines: 24



C++ is not simply a set of features grafted onto C. The C++ type system
provides coherent checking of all expressions. This system can - when
necessary - be broken by explicit use of casts or explicit use of the
ellipsis (...) in a function declaration. The simple system of built-in
types and standard onversions can be augmented by user-defined types (classes)
and user-defined conversions so that new types with essentially the same
status as the built-in ones can be provided. The general idea of these
mechanisms (classes and operator overloading) is to make C++ extensible
but not mutable. It is not possible to, say, redefine + to mean subtraction
for integers, though a programmer (with a warped sense of humor) can
define his/her own INT class where + means subtraction.

Where user-defined types are used ambiguity control is used to make sure
that ambiguities are either resolved or reported as errors at compile time.
Similar ambiguity control is used for name lookup where multiple inheritance
is used.

A C++ program consistes of many separately compiled parts. Consistency of
the type and name usage is ensured by the use of header files containing
the sets of declarations specifying interfaces. In release 1.*, of C++
there was no check on the consistency of the use of these interfaces.
In 2.0, consistency is checked at link time.