Re: inheritance is not for code-reuse (??)

On May 3, 10:52 pm, Bart Simpson <123evergr...@terrace.com> wrote:


The object models of Smalltalk and C++ have one big difference.
Smalltalk uses something called operational polymorphism, often these
days called duck typing. C++ on the other hand uses inclusional
polymorphism which is a form of type constraint.

This basic difference in the type models/message dispatchers in the
two languages makes a huge difference in how classes are used in
practice, even though they can both be shown to be equivalent (like a
Turing machine being provably equivalent to Church's lambda calculus -
they're the same in one sense, but the way a problem is approached in
each is still completely different).

I think the FAQ is only half right. Most people most of the time use
classes in the way that the FAQ describes in the two languages, but
this is really habitual rather than imposed by the object models of
the languages.

You do get code re-use for free with inheritance, but to get the sort
of re-use you get with Smalltalk in C++ you have to use templates. The
STL makes use of re-use in (pretty much) the same way that the
Smalltalk containers do.

In C++ though a class hierarchy is most often used to constrain the
types that a function will use (and Java uses hierarchy in this way
too). This constraint cannot be used in Smalltalk whose message
dispatcher ignores the actual type of the receiver (well, of course it
doesn't exactly, but compared to C++ it does).

I've not really read through the C++ FAQ's comparison with Smalltalk
before and I'm not entirely sure that it is explained as well as it
could be. Something I'll have to study and think about.

I'm not sure if this really answers your question or not either. The
two languages are quite different in the ways that they are normally
used, but each language can "meet the other" if the context is right.

K