<s>
In	O
computing	O
,	O
the	O
System	B-Device
Object	I-Device
Model	I-Device
(	O
SOM	O
)	O
is	O
an	O
object-oriented	B-Language
shared	O
library	O
system	O
developed	O
by	O
IBM	O
.	O
</s>
<s>
DSOM	O
,	O
a	O
distributed	O
version	O
based	O
on	O
CORBA	B-Architecture
,	O
allowed	O
objects	O
on	O
different	O
computers	O
to	O
communicate	O
.	O
</s>
<s>
This	O
is	O
not	O
the	O
case	O
for	O
all	O
other	O
C++	B-Language
libraries	O
,	O
which	O
in	O
some	O
cases	O
require	O
recompiling	O
all	O
programs	O
that	O
use	O
them	O
whenever	O
the	O
libraries	O
are	O
changed	O
,	O
known	O
as	O
the	O
fragile	B-Error_Name
binary	I-Error_Name
interface	I-Error_Name
problem	I-Error_Name
.	O
</s>
<s>
SOM	O
provides	O
an	O
application	B-Application
programming	I-Application
interface	I-Application
(	O
API	B-Application
)	O
that	O
gives	O
programs	O
access	O
to	O
information	O
about	O
a	O
SOM	O
class	O
or	O
SOM	O
object	O
.	O
</s>
<s>
SOM	O
was	O
intended	O
to	O
be	O
used	O
universally	O
from	O
IBM	O
's	O
mainframe	B-Architecture
computers	I-Architecture
right	O
down	O
to	O
the	O
desktop	O
in	O
OS/2	B-Application
,	O
allowing	O
programs	O
to	O
be	O
written	O
that	O
would	O
run	O
on	O
the	O
desktop	O
but	O
use	O
mainframes	B-Architecture
for	O
processing	O
and	O
data	O
storage	O
.	O
</s>
<s>
IBM	O
produced	O
versions	O
of	O
SOM/DSOM	O
for	O
OS/2	B-Application
,	O
Microsoft	B-Application
Windows	I-Application
and	O
various	O
Unix	B-Application
flavours	O
(	O
notably	O
IBM	O
's	O
own	O
AIX	B-Application
)	O
.	O
</s>
<s>
It	O
was	O
most	O
widely	O
used	O
in	O
their	O
OpenDoc	B-Application
framework	O
,	O
but	O
saw	O
limited	O
use	O
in	O
other	O
roles	O
as	O
well	O
.	O
</s>
<s>
Perhaps	O
the	O
most	O
widespread	O
uses	O
of	O
SOM	O
within	O
IBM	O
were	O
in	O
later	O
versions	O
of	O
OS/2	B-Application
,	O
which	O
used	O
it	O
for	O
most	O
code	O
,	O
including	O
the	O
Workplace	B-Device
Shell	I-Device
.	O
</s>
<s>
Object	B-Application
REXX	I-Application
for	O
OS/2	B-Application
is	O
able	O
to	O
deal	O
with	O
SOM	O
classes	O
and	O
objects	O
including	O
WPS	O
.	O
</s>
<s>
They	O
were	O
ported	O
to	O
OS/390	B-Application
,	O
and	O
are	O
still	O
available	O
on	O
this	O
OS	O
.	O
</s>
<s>
NonStop	B-Architecture
DOM	O
and	O
some	O
other	O
technologies	O
eventually	O
merged	O
into	O
NonStop	B-Architecture
CORBA	B-Architecture
,	O
but	O
current	O
documentation	O
of	O
NonStop	B-Architecture
products	O
does	O
not	O
contain	O
signs	O
of	O
SOM	O
technology	O
still	O
powering	O
NonStop	B-Architecture
products	O
.	O
</s>
<s>
With	O
the	O
"	O
death	O
"	O
of	O
OS/2	B-Application
in	O
the	O
mid-1990s	O
,	O
the	O
raison	O
d'être	O
for	O
SOM/DSOM	O
largely	O
disappeared	O
;	O
if	O
users	O
would	O
not	O
be	O
running	O
OS/2	B-Application
on	O
the	O
desktop	O
,	O
there	O
would	O
be	O
no	O
universal	O
object	O
library	O
anyway	O
.	O
</s>
<s>
In	O
1997	O
,	O
when	O
Steve	O
Jobs	O
returned	O
to	O
Apple	O
and	O
ended	O
many	O
development	O
efforts	O
including	O
Copland	B-Application
and	O
OpenDoc	B-Application
,	O
SOM	O
was	O
replaced	O
with	O
Objective-C	B-Language
already	O
being	O
in	O
use	O
in	O
OPENSTEP	B-Operating_System
(	O
to	O
become	O
Mac	B-Application
OS	I-Application
X	O
later	O
)	O
.	O
</s>
<s>
SOM/DSOM	O
development	O
faded	O
,	O
and	O
is	O
no	O
longer	O
actively	O
developed	O
,	O
although	O
it	O
continues	O
to	O
be	O
included	O
and	O
used	O
in	O
OS/2	B-Application
-based	O
systems	O
such	O
as	O
ArcaOS	B-Device
.	O
</s>
<s>
Despite	O
effective	O
death	O
of	O
OS/2	B-Application
and	O
OpenDoc	B-Application
,	O
SOM	O
could	O
have	O
yet	O
another	O
niche	O
:	O
Windows	B-Application
and	O
cross-platform	B-Operating_System
development	O
.	O
</s>
<s>
The	O
first	O
version	O
of	O
VisualAge	O
C++	B-Language
for	O
Windows	B-Application
was	O
3.5	O
.	O
</s>
<s>
VisualAge	O
C++	B-Language
4.0	O
introduced	O
.icc	O
projects	O
and	O
removed	O
icc.exe	O
and	O
ilink.exe	O
command	O
line	O
compiler	O
and	O
linker	O
from	O
supply	O
.	O
</s>
<s>
VisualAge	O
C++	B-Language
comes	O
with	O
its	O
own	O
samples	O
,	O
but	O
there	O
are	O
no	O
.icc	O
SOM	O
samples	O
even	O
in	O
VAC++	O
4.0	O
for	O
OS/2	B-Application
.	O
</s>
<s>
VisualAge	O
C++	B-Language
bundled-in	O
Object	O
Component	O
Library	O
(	O
OCL	O
)	O
was	O
not	O
based	O
on	O
SOM	O
.	O
</s>
<s>
It	O
was	O
probably	O
meant	O
to	O
be	O
ported	O
to	O
SOM	O
using	O
C++	B-Language
Direct-to-SOM	O
mode	O
,	O
but	O
in	O
VAC	O
v3.6.5	O
this	O
mode	O
was	O
abandoned	O
,	O
and	O
OCL	O
has	O
no	O
SOM	O
interface	O
so	O
far	O
.	O
</s>
<s>
Finally	O
,	O
IBM	O
never	O
open-sourced	O
SOM	O
(	O
as	O
done	O
to	O
Object	B-Application
REXX	I-Application
)	O
,	O
despite	O
several	O
articles	O
and	O
petitions	O
.	O
</s>
<s>
Historically	O
,	O
SOM	O
was	O
compared	O
to	O
Microsoft	O
's	O
Component	B-Application
Object	I-Application
Model	I-Application
(	O
COM	B-Application
)	O
by	O
IBM	O
.	O
</s>
<s>
However	O
,	O
from	O
some	O
points	O
of	O
view	O
there	O
is	O
no	O
place	O
for	O
COM	B-Application
at	O
all	O
.	O
</s>
<s>
From	O
the	O
point	O
of	O
view	O
of	O
release	O
to	O
release	O
transformations	O
,	O
COM	B-Application
is	O
on	O
procedural	O
level	O
,	O
thus	O
,	O
the	O
table	O
1	O
in	O
RRBC	O
article	O
(	O
Release-to-Release	O
Binary	O
Compatibility	O
referenced	O
earlier	O
)	O
does	O
not	O
contain	O
COM	B-Application
column	O
at	O
all	O
.	O
</s>
<s>
Most	O
information	O
in	O
this	O
table	O
is	O
still	O
applicable	O
to	O
modern	O
versions	O
(	O
as	O
of	O
2015	O
)	O
,	O
except	O
Objective-C	B-Language
2.0	O
getting	O
so	O
called	O
non-fragile	O
instance	O
variables	O
.	O
</s>
<s>
Some	O
solutions	O
remained	O
experimental	O
:	O
SGI	O
Delta/C	O
++	O
or	O
Sun	O
OBI	O
.	O
</s>
<s>
For	O
instance	O
,	O
Netscape	B-Protocol
Plugin	I-Protocol
Application	I-Protocol
Programming	I-Protocol
Interface	I-Protocol
(	O
NPAPI	B-Protocol
)	O
browser	O
plugins	O
were	O
written	O
using	O
Java	B-Language
API	B-Application
initially	O
(	O
LiveConnect	O
)	O
,	O
but	O
Java	B-Language
Virtual	I-Language
Machine	I-Language
(	O
JVM	B-Language
)	O
was	O
later	O
excluded	O
from	O
the	O
chain	O
.	O
</s>
<s>
It	O
can	O
be	O
seen	O
as	O
Java	B-Language
replaced	O
with	O
Cross	B-Application
Platform	I-Application
Component	I-Application
Object	I-Application
Model	I-Application
(	O
XPCOM	B-Application
)	O
.	O
</s>
<s>
Common	B-Application
Lisp	I-Application
Object	I-Application
System	I-Application
(	O
CLOS	B-Application
)	O
and	O
Smalltalk	B-Application
are	O
not	O
known	O
as	O
being	O
chain	O
links	O
like	O
Java	B-Language
in	O
LiveConnect	O
.	O
</s>
<s>
Objective-C	B-Language
is	O
also	O
not	O
known	O
much	O
in	O
this	O
role	O
and	O
not	O
known	O
to	O
be	O
marketed	O
this	O
way	O
,	O
but	O
its	O
runtime	O
is	O
one	O
of	O
the	O
most	O
friendly	O
to	O
similar	O
use	O
cases	O
.	O
</s>
<s>
Generic	O
C++	B-Language
is	O
still	O
being	O
used	O
in	O
Qt	B-Language
and	O
the	O
K	B-Application
Desktop	I-Application
Environment	I-Application
(	O
KDE	B-Application
)	O
.	O
</s>
<s>
Qt	B-Language
and	O
KDE	B-Application
are	O
notable	O
for	O
describing	O
efforts	O
it	O
takes	O
to	O
maintain	O
binary	O
compatibility	O
without	O
special	O
support	O
in	O
development	O
tools	O
.	O
</s>
<s>
GObject	B-Language
only	O
aimed	O
to	O
avoid	O
dependence	O
on	O
C++	B-Language
compiler	O
,	O
but	O
RRBC	O
issues	O
are	O
the	O
same	O
as	O
in	O
generic	O
C++	B-Language
.	O
</s>
<s>
Without	O
special	O
runtime	O
many	O
other	O
programming	O
languages	O
will	O
have	O
the	O
same	O
issues	O
,	O
e.g.	O
,	O
Delphi	B-Language
,	O
Ada	B-Language
.	O
</s>
<s>
It	O
can	O
be	O
illustrated	O
by	O
so-called	O
unprecedented	O
approach	O
it	O
took	O
to	O
produce	O
Delphi	B-Language
2006	I-Language
binary	O
compatible	O
Delphi	B-Language
2007	O
release	O
:	O
</s>
<s>
Objective-C	B-Language
is	O
the	O
most	O
promising	O
competitor	O
to	O
SOM	O
(	O
although	O
not	O
being	O
actively	O
marketed	O
as	O
multi-language	O
platform	O
)	O
,	O
and	O
SOM	O
should	O
preferably	O
be	O
compared	O
to	O
Objective-C	B-Language
as	O
opposed	O
to	O
COM	B-Application
as	O
it	O
happened	O
historically	O
.	O
</s>
<s>
With	O
non-fragile	O
instance	O
variables	O
in	O
Objective-C	B-Language
2.0	O
it	O
is	O
the	O
best	O
alternative	O
amongst	O
actively	O
supported	O
.	O
</s>
<s>
COM	B-Application
,	O
XPCOM	B-Application
are	O
being	O
used	O
actively	O
,	O
but	O
they	O
only	O
manage	O
interfaces	O
,	O
not	O
implementations	O
,	O
and	O
thus	O
are	O
not	O
on	O
the	O
same	O
level	O
as	O
SOM	O
,	O
GObject	B-Language
and	O
Objective-C	B-Language
.	O
Windows	B-Application
Runtime	O
under	O
closer	O
look	O
behaves	O
much	O
like	O
COM	B-Application
.	O
</s>
<s>
Its	O
metadata	O
description	O
is	O
based	O
on	O
.NET	O
,	O
but	O
since	O
WinRT	O
does	O
not	O
contain	O
special	O
runtime	O
to	O
resolve	O
RRBC	O
issues	O
,	O
like	O
in	O
Objective-C	B-Language
or	O
SOM	O
,	O
several	O
restrictions	O
had	O
to	O
be	O
applied	O
that	O
limit	O
WinRT	O
on	O
procedural	O
level	O
:	O
</s>
<s>
Polymorphism	O
is	O
n’t	O
available	O
to	O
WinRT	O
types	O
,	O
and	O
the	O
closest	O
you	O
can	O
come	O
is	O
implementing	O
WinRT	O
interfaces	O
;	O
you	O
must	O
declare	O
as	O
sealed	O
any	O
classes	O
that	O
are	O
publicly	O
exposed	O
by	O
your	O
Windows	B-Application
Runtime	O
Component	O
.	O
</s>
<s>
SOM	O
is	O
similar	O
in	O
concept	O
to	O
COM	B-Application
.	O
</s>
<s>
SOM	O
can	O
be	O
considered	O
more	O
robust	O
than	O
COM	B-Application
.	O
</s>
<s>
COM	B-Application
offers	O
two	O
methods	O
of	O
accessing	O
methods	O
onto	O
an	O
object	O
,	O
and	O
an	O
object	O
can	O
implement	O
either	O
one	O
of	O
them	O
or	O
both	O
.	O
</s>
<s>
The	O
first	O
one	O
is	O
dynamic	O
and	O
late	O
binding	O
(	O
IDispatch	B-Application
)	O
,	O
and	O
is	O
language-neutral	O
similar	O
to	O
what	O
is	O
offered	O
by	O
SOM	O
.	O
</s>
<s>
The	O
second	O
,	O
called	O
a	O
Custom	O
Interface	O
,	O
is	O
using	O
a	O
function	O
table	O
which	O
can	O
be	O
built	O
in	O
C	O
but	O
is	O
also	O
directly	O
compatible	O
with	O
the	O
binary	O
layout	O
of	O
the	O
virtual	O
table	O
of	O
C++	B-Language
objects	O
in	O
Microsoft	O
's	O
C++	B-Language
compiler	O
.	O
</s>
<s>
With	O
compatible	O
C++	B-Language
compilers	O
,	O
Custom	O
Interfaces	O
can	O
therefore	O
be	O
defined	O
directly	O
as	O
pure	O
virtual	O
C++	B-Language
classes	O
.	O
</s>
<s>
This	O
is	O
an	O
example	O
of	O
the	O
fragile	B-Application
base	I-Application
class	I-Application
problem	I-Application
,	O
which	O
can	O
lead	O
to	O
DLL	O
hell	O
,	O
as	O
a	O
new	O
version	O
of	O
a	O
shared	O
library	O
is	O
installed	O
and	O
all	O
programs	O
based	O
on	O
the	O
older	O
version	O
can	O
stop	O
functioning	O
properly	O
.	O
</s>
<s>
To	O
prevent	O
this	O
problem	O
,	O
COM	B-Application
developers	O
must	O
remember	O
to	O
never	O
change	O
an	O
interface	O
once	O
it	O
is	O
published	O
,	O
and	O
new	O
interfaces	O
need	O
to	O
be	O
defined	O
if	O
new	O
methods	O
or	O
other	O
changes	O
are	O
required	O
.	O
</s>
<s>
Whereas	O
basic	O
COM	B-Application
essentially	O
defines	O
a	O
cut-down	O
version	O
of	O
C++	B-Language
to	O
program	O
to	O
,	O
SOM	O
supports	O
almost	O
all	O
common	O
features	O
and	O
even	O
some	O
more	O
esoteric	O
ones	O
.	O
</s>
<s>
For	O
instance	O
SOM	O
supports	O
multiple	B-Application
inheritance	I-Application
,	O
metaclasses	B-Language
and	O
dynamic	O
dispatching	O
.	O
</s>
<s>
Some	O
of	O
these	O
features	O
are	O
not	O
found	O
in	O
most	O
languages	O
,	O
which	O
had	O
led	O
most	O
SOM/COM	O
-like	O
systems	O
to	O
be	O
simpler	O
at	O
the	O
cost	O
of	O
supporting	O
fewer	O
languages	O
.	O
</s>
<s>
The	O
full	O
flexibility	O
of	O
multi-language	O
support	O
was	O
important	O
to	O
IBM	O
,	O
however	O
,	O
as	O
they	O
had	O
a	O
major	O
effort	O
underway	O
to	O
support	O
both	O
Smalltalk	B-Application
(	O
single	O
inheritance	O
and	O
dynamic	O
dispatch	O
)	O
with	O
C++	B-Language
(	O
multiple	B-Application
inheritance	I-Application
and	O
fixed	O
dispatch	O
)	O
.	O
</s>
<s>
The	O
most	O
notable	O
difference	O
between	O
SOM	O
and	O
COM	B-Application
is	O
support	O
for	O
inheritance	O
—	O
COM	B-Application
does	O
not	O
have	O
any	O
.	O
</s>
<s>
COM	B-Application
demands	O
that	O
the	O
programmer	O
specify	O
the	O
exact	O
base	O
class	O
at	O
compile	O
time	O
,	O
making	O
it	O
impossible	O
to	O
insert	O
other	O
derived	O
classes	O
in	O
the	O
middle	O
(	O
at	O
least	O
in	O
other	O
COM	B-Application
libraries	O
)	O
.	O
</s>
<s>
The	O
downside	O
to	O
this	O
approach	O
is	O
that	O
it	O
is	O
possible	O
that	O
new	O
versions	O
of	O
this	O
base	O
class	O
may	O
no	O
longer	O
work	O
even	O
if	O
the	O
API	B-Application
remains	O
the	O
same	O
.	O
</s>
<s>
While	O
SOM	O
and	O
COM	B-Application
were	O
contrapositioned	O
by	O
IBM	O
,	O
they	O
were	O
not	O
mutually	O
exclusive	O
.	O
</s>
<s>
In	O
1995	O
Novell	O
contributed	O
ComponentGlue	O
technology	O
to	O
OpenDoc	B-Application
for	O
Windows	B-Application
.	O
</s>
<s>
This	O
technology	O
provided	O
different	O
means	O
to	O
integrate	O
between	O
COM	B-Application
-	O
and	O
SOM-based	O
components	O
.	O
</s>
<s>
In	O
particular	O
,	O
SOM	O
objects	O
can	O
be	O
made	O
available	O
to	O
OLE2	O
applications	O
by	O
either	O
late	O
binding	O
bridge	O
(	O
based	O
on	O
IDispatch	B-Application
)	O
or	O
COM	B-Application
interfaces	O
having	O
higher	O
performance	O
.	O
</s>
<s>
In	O
essence	O
,	O
SOM	O
classes	O
are	O
implementing	O
COM	B-Application
interfaces	O
this	O
way	O
.	O
</s>
<s>
The	O
flexibility	O
offered	O
by	O
SOM	O
was	O
considered	O
worth	O
the	O
trouble	O
by	O
almost	O
all	O
,	O
but	O
similar	O
systems	O
,	O
such	O
as	O
Sun	O
Microsystems	O
 '	O
Distributed	B-Application
Objects	I-Application
Everywhere	I-Application
,	O
also	O
supported	O
full	O
inheritance	O
.	O
</s>
<s>
NeXT	O
's	O
Portable	B-Application
Distributed	I-Application
Objects	I-Application
avoided	O
these	O
issues	O
via	O
a	O
strong	O
versioning	O
system	O
,	O
allowing	O
library	O
authors	O
to	O
ship	O
new	O
versions	O
along	O
with	O
the	O
old	O
,	O
thereby	O
guaranteeing	O
backward	B-General_Concept
compatibility	I-General_Concept
for	O
the	O
small	O
cost	O
of	O
disk	O
space	O
.	O
</s>
