For the similarly named executable file format in the HP-UX operating system, see System Object Model (file format)

System Object Model (file format)

In computing, the System Object Model is a proprietary executable file format developed by Hewlett-Packard for its HP-UX and MPE/ix operating systems....

In computing, the System Object Model (SOM) is an object-oriented shared library system developed by IBM

IBM

International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...

. DSOM, a distributed version based on CORBA

Çorba

Chorba , ciorbă , shurpa , shorpo , or sorpa is one of various kinds of soup or stew found in national cuisines across Middle East...

, allowed objects on different computers to communicate.

Applications

SOM was intended to be used universally from IBM's mainframe

Mainframe computer

Mainframes are powerful computers used primarily by corporate and governmental organizations for critical applications, bulk data processing such as census, industry and consumer statistics, enterprise resource planning, and financial transaction processing.The term originally referred to the...

 computers right down to the desktop in OS/2

OS/2

OS/2 is a computer operating system, initially created by Microsoft and IBM, then later developed by IBM exclusively. The name stands for "Operating System/2," because it was introduced as part of the same generation change release as IBM's "Personal System/2 " line of second-generation personal...

, allowing programs to be written that would run on the desktop but use mainframes for processing and data storage. IBM produced versions of SOM/DSOM for OS/2, Microsoft Windows

Microsoft Windows

Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...

 and various Unix

Unix

Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...

 flavours (notably IBM's own AIX

AIX operating system

AIX AIX AIX (Advanced Interactive eXecutive, pronounced "a i ex" is a series of proprietary Unix operating systems developed and sold by IBM for several of its computer platforms...

). SOM/DSOM was also an important part of the later versions of the VisualAge

VisualAge

VisualAge was the name of a family of computer integrated development environments from IBM, which included support for multiple programming languages. VisualAge was first released in the 1980s and is still available in 2011...

 development platform, allowing different languages to call code in a standardized format.

Perhaps the most widespread uses of SOM within IBM were in later versions of OS/2, which used it for most code, including the Workplace Shell

Workplace Shell

The Workplace Shell is a object-oriented desktop shell produced by IBM's Boca Raton development lab for OS/2 2.0. It is based on Common User Access and made a radical shift away from the Program Manager type interface that earlier versions of OS/2 shared with Windows 3.x or the...

. With the "death" of OS/2 in the mid-1990s, the raison d'être for SOM/DSOM largely disappeared; if users would not be running OS/2 on the desktop, there would be no universal object library anyway. SOM/DSOM development faded, and is no longer actively developed.

For some time after the formation of the AIM alliance

AIM alliance

The AIM alliance was an alliance formed on October 2, 1991, between Apple Inc. , IBM, and Motorola to create a new computing standard based on the PowerPC architecture. The stated goal of the alliance was to challenge the dominant Wintel computing platform with a new computer design and a...

, SOM/DSOM was also used by Apple Computer

Apple Computer

Apple Inc. is an American multinational corporation that designs and markets consumer electronics, computer software, and personal computers. The company's best-known hardware products include the Macintosh line of computers, the iPod, the iPhone and the iPad...

 for similar purposes. It was most widely used in their OpenDoc

OpenDoc

OpenDoc was a multi-platform software componentry framework standard for compound documents, intended as an alternative to Microsoft's Object Linking and Embedding ....

 framework, but saw limited use in other roles as well. Most of these technologies were dropped in 1997 when Steve Jobs

Steve Jobs

Steven Paul Jobs was an American businessman and inventor widely recognized as a charismatic pioneer of the personal computer revolution. He was co-founder, chairman, and chief executive officer of Apple Inc...

 returned to Apple and ended many development efforts.

Comparison to other object models

SOM is similar in concept to Microsoft's Component Object Model

Component Object Model

Component Object Model is a binary-interface standard for software componentry introduced by Microsoft in 1993. It is used to enable interprocess communication and dynamic object creation in a large range of programming languages...

. Both systems address the problem of producing a standard library format that can be called from more than one language. SOM can be considered more robust than COM. COM offers two methods of accessing methods onto an object, and an object can implement either one of them or both. The first one is dynamic and late binding

Late binding

Late binding is a computer programming mechanism in which the method being called upon an object is looked up by name at runtime. This is informally known as duck typing or name binding....

 (IDispatch

IDispatch

IDispatch is the interface that exposes the OLE Automation protocol. It is one of the standard interfaces that can be exposed by COM objects. The I in IDispatch refers to interface...

), and is language-neutral similar to what is offered by SOM. The second one, called a Custom Interface, is using a function table which can be built in C but is also directly compatible with the binary layout of the virtual table of C++ objects in Microsoft's C++ compiler. With compatible C++ compilers, Custom Interfaces can therefore be defined directly as pure virtual C++ classes. The resulting interface can then be called by languages that can call C functions through pointers. Custom Interfaces trade robustness for performance. Once an interface is published in a released product, it can not be changed, because client applications of this interface were compiled against a specific binary layout of this interface. This is an example of the fragile base class

Fragile base class

The fragile base class problem is a fundamental architectural problem of object-oriented programming systems where base classes are considered "fragile" because seemingly safe modifications to a base class, when inherited by the derived classes, may cause the derived classes to malfunction...

 problem, which can lead to DLL hell

DLL hell

In computing, DLL Hell is a term for the complications that arise when working with dynamic link libraries used with Microsoft Windows operating systems, particularly legacy 16-bit editions which all run in a single memory space....

, as a new version of a shared library is installed and all programs based on the older version can stop functioning properly. To prevent this problem, COM developers must remember to never change an interface once it is published, and new interfaces need to be defined if new methods or other changes are required.

SOM prevents these issues by providing only late binding, to allow the run-time linker to re-build the table on the fly. This way, changes to the underlying libraries are resolved when they are loaded into programs, although there is a performance cost.

SOM is also much more robust in terms of fully supporting a wide variety of OO languages. Whereas basic COM essentially defines a cut-down version of C++ to program to, SOM supports almost all common features and even some more esoteric ones. For instance SOM supports multiple inheritance

Multiple inheritance

Multiple inheritance is a feature of some object-oriented computer programming languages in which a class can inherit behaviors and features from more than one superclass....

, metaclass

Metaclass

In object-oriented programming, a metaclass is a class whose instances are classes. Just as an ordinary class defines the behavior of certain objects, a metaclass defines the behavior of certain classes and their instances. Not all object-oriented programming languages support metaclasses...

es and dynamic dispatch

Dynamic dispatch

In computer science, dynamic dispatch is the process of mapping a message to a specific sequence of code at runtime. This is done to support the cases where the appropriate method can't be determined at compile-time...

ing. Some of these features are not found in most languages, which had led most SOM/COM-like systems to be simpler at the cost of supporting fewer languages. The full flexibility of multi-language support was important to IBM, however, as they had a major effort underway to support both Smalltalk

Smalltalk

Smalltalk is an object-oriented, dynamically typed, reflective programming language. Smalltalk was created as the language to underpin the "new world" of computing exemplified by "human–computer symbiosis." It was designed and created in part for educational use, more so for constructionist...

 (single inheritance and dynamic dispatch

Dynamic dispatch

In computer science, dynamic dispatch is the process of mapping a message to a specific sequence of code at runtime. This is done to support the cases where the appropriate method can't be determined at compile-time...

) with C++

C++

C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell...

 (multiple inheritance

Multiple inheritance

Multiple inheritance is a feature of some object-oriented computer programming languages in which a class can inherit behaviors and features from more than one superclass....

 and fixed dispatch).

The most notable difference between SOM and COM is support for inheritance—COM does not have any. It might seem odd that Microsoft produced an object library system that could not support one of the most fundamental concepts of OO programming; the main reason for this is that it is difficult to know where a base class exists in a system where libraries are loaded in a potentially random order. COM demands that the programmer specify the exact base class at compile time, making it impossible to insert other derived classes in the middle (at least in other COM libraries).

SOM instead uses a simple algorithm, looking for potential base classes by following the inheritance tree and stopping at the first one that matches; this is the basic idea behind inheritance in most cases. The downside to this approach is that it is possible that new versions of this base class may no longer work even if the API

Application programming interface

An application programming interface is a source code based specification intended to be used as an interface by software components to communicate with each other...

 remains the same. This possibility exists in any program, not only those using a shared library, but a problem can become very difficult to track down if it exists in someone else's code. In SOM, the only solution is extensive testing of new versions of libraries, which is not always easy.

The flexibility offered by SOM was considered worth the trouble by almost all , but similar systems, such as Sun Microsystems

Sun Microsystems

Sun Microsystems, Inc. was a company that sold :computers, computer components, :computer software, and :information technology services. Sun was founded on February 24, 1982...

' Distributed Objects Everywhere

Distributed Objects Everywhere

Distributed Objects Everywhere was a long-running Sun Microsystems project to build a distributed computing environment based on the CORBA system in the 'back end' and OpenStep as the user interface. First started in 1990 and announced soon thereafter, it remained vaporware for many years before...

, also supported full inheritance. NeXT

NeXT

Next, Inc. was an American computer company headquartered in Redwood City, California, that developed and manufactured a series of computer workstations intended for the higher education and business markets...

's Portable Distributed Objects

Portable Distributed Objects

Portable Distributed Objects, or PDO, is a programming API for creating object-oriented code that can be executed remotely on a network of computers. It was created by NeXT Computer, Inc. using their OpenStep system, whose use of Objective-C made the package very easy to write...

 avoided these issues via a strong versioning system, allowing library authors to ship new versions along with the old, thereby guaranteeing backward compatibility

Backward compatibility

In the context of telecommunications and computing, a device or technology is said to be backward or downward compatible if it can work with input generated by an older device...

for the small cost of disk space.

External links

• System Object Model introduction
• Learning To Love SOM
• To Inherit or Not to Inherit? Compares MS's and IBM's approaches to the object library system in terms of inheritance.