protocols, inheritance and polymorphism

Jacek Generowicz jacek.generowicz at cern.ch
Thu Nov 25 09:44:32 CET 2004


"Dan Perl" <danperl at rogers.com> writes:

> Can you elaborate on problems that static languages have with OOP?

They make dynamic polymorphism impossible.

Which is why most object-oriented C++ programs are dynamically typed;
only the programmer is burdened with the work that the mostly-absent
dynamic type system should be doing.

Dynamic polymorphism crucially requires knowledge of the *run-time*
(dynamic) type of objects, in order to be able to dispatch to the
correct method. In C++ you turn your classes into dynamically typed
ones with the "virtual" keyword, which introduces a vtable and
run-time type identification (RTTI). As long as you are only
interested in calling methods which are declared in some base class,
this dynamic type systems looks almost satisfactory. As soon as you
want to use a method present in the subclass but not in the
superclass, it is up to you to faff around with dynamic_cast and
checking of the resulting pointer; all stuff which a dynamic type
system should be doing for you, but C++ makes you do yourself because
it pretends that your programs are statically typed when, in fact,
they are dynamically typed.



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