TweetFollow Us on Twitter

Learning to Love SOM
Volume Number:11
Issue Number:1
Column Tag:Foundation Technology

Learning To Love SOM

Choosing it for OpenDoc was a no-brainer

By Jens Peter Alfke, Apple Computer, Inc.

The System Object Model (SOM) provides the object-oriented substrate used by OpenDoc and by future versions of the Macintosh Toolbox. SOM is fairly complex, relatively new on the Mac, and competes against other proprietary object models. It’s not surprising, then, that there is some degree of apprehension and misinformation surrounding it. In this article, I will give you the straight scoop on SOM.

Frankly, my own initial feelings toward SOM were not warm and fuzzy. After we on the OpenDoc team decided to make the move to SOM, and I began reading about it and thinking about the task of converting our existing code base, I was heard to ask Kurt Piersol: “We’re all going to die, aren’t we?” Fortunately, I was wrong; and since then I’ve come to like and respect SOM.

Why Is SOM Necessary?

Object-based shared libraries are a holy grail of software engineering, since they turn code into black-box components that can be plugged together by programmers. Unfortunately, existing object models have been either too slow and too hard to use from existing code (e.g. Smalltalk), or insufficiently robust (e.g. C++.)

Fragile Base Classes

What do I mean by “insufficiently robust”? The technical term is fragile base-classes, and what it means is that changes to a base class - such as adding methods or instance variables - often break code that uses the base class, or makes subclasses of it, until that client code is recompiled.

In other words, if I update my JensTools C++ class library from version 1.0 to 1.1, and users install the new version, all their apps that use it may crash until they get new releases from the developers. Needless to say, this has impeded the use of object-based shared libraries.

There are two ways around this. The first is to abandon key features of object-oriented programming like inheritance and polymorphism. This is what Microsoft’s Component Object Model (COM) does. What you’re left with is a system of procedural dispatch tables that I find quite reminiscent of the Component Manager. I’ve used the Component Manager extensively as part of developing AppleScript, and let me say that it is not my favorite shared library solution. (For a much more detailed look at how SOM and COM differ, read “SOM vs. COM” in the documentation folder on the CD.)

The better solution is to attack the problem head on and fix it, which is what researchers at IBM did in producing SOM. I won’t go into the gory details here, but by abstracting the way object instantiation and method dispatch work, they were able to produce an object model that is extremely robust and still efficient and fully object-oriented.

In other words, by basing my JensTools library on SOM (though it’s still implemented in C++) I am able to add new methods and/or change its internal implementation while retaining full binary compatibility: when users install the new version, their existing apps continue to work perfectly, while new clients can take advantage of the new features.

Language and Compiler Neutrality

A frequently-mentioned advantage of SOM is that it can be used with any programming language, once the appropriate interfaces and glue code are written. This is because the SOM kernel isn’t based on any single object model, and has robust enough support for almost any language (for instance, it supports metaclasses and name-based dispatching, features found in Smalltalk but not in C++.) C and C++ support currently exist, with Smalltalk in the works.

Language neutrality is also sometimes disparaged; some see it as nothing more than unnecessary support for exotic languages that no one uses. Setting aside the fact that many large businesses are using Smalltalk, and that it would be damn cool to be able to write OpenDoc parts in Dylan, what these critics don’t realize or admit is that different C++ compilers might as well be different languages when it comes to their runtime object model. Due to differences in vtable layout and parameter passing, objects created by one C++ compiler generally cannot be used by client code compiled by a different C++ compiler. On the Mac, cfront, Apple’s new Mr.C compiler, Symantec C++ and Metrowerks’ C++ are all mutually incompatible.

This incompatibility causes headaches even with COM. A frequent trick used by COM programs is to use a C++ vtable as a COM method table. By strange coincidence, this works just fine with the vtables laid out by the Microsoft C++ compiler. But it doesn’t work with most other C++ compilers, and users of those compilers are forced to lay out the method tables by hand, which turns out to be a lot more difficult than creating a SOM class (in fact, it’s a lot like the tables SOM builds internally and thankfully hides from you.)

Cross-Platform Support

Implementations of SOM currently exist for OS/2, Unix, Windows, and now the Macintosh (both 68k and native PowerPC, included on the OpenDoc CD.) Future targets include Netware, IBM’s Workplace, MVS and OS/400. IBM plans to license SOM to Component Integration Labs, so it will be available for other vendors to license.

Using SOM With C++

This adds up to a pretty compelling case for using SOM. There’s nothing else available that supports real object-oriented programming, with strong binary compatibility, language and compiler independence, that runs on all major operating systems. Choosing it for OpenDoc was a no-brainer once we’d analyzed the alternatives.

Unfortunately you do give up some things when you use SOM from C++. SOM is not tied to the C++ object model, and it doesn’t support some fancier C++ features like templates and operator overloading (although it does do things C++ doesn’t, like metaclasses and name-based dispatching.) SOM classes aren’t the same as C++ classes; this isn’t apparent on the client side since you call methods of a SOM object exactly as you would a C++ object, but a SOM class’ methods are implemented as procedural functions, which adds a certain amount of “syntactic vinegar” to your implementation (as my colleague Richard Rodseth puts it.)

Many of these drawbacks will be alleviated by direct-to-SOM C++ compilers. These are compilers with a native understanding of the SOM object model, which make creating a SOM class as simple as inheriting from SOMObject. Direct-to-SOM compilers are already available for OS/2 and Windows, and may be on the Mac soon.

Until then, there are tricks you can use if you want to do your work closer to normal C++. One that works well, and is used in the sample OpenDoc leaf part class that comes with PartMaker, is to create a normal C++ class with basically the same API as the SOM class you want to implement. Then the SOM class’ implementation simply instantiates a matching object of the C++ class, and each SOM method calls the corresponding C++ method. This incurs a little bit of extra overhead, but eases the transition to SOM for someone familiar with C++.

Cool Features

SOM supports features you don’t ordinarily get with C++. For instance, you can determine the class of an arbitrary SOM object at runtime, or check whether an object descends from a particular class or implements a particular method. You can examine all the methods defined by a particular class and send an arbitrary message to an object given a string representing the method name. You can even add new methods to a class at runtime. All this is possible because SOM supports metaclasses, a concept originating in Smalltalk which means that SOM classes are real objects.

Building A SOM Class

The full process of implementing a SOM class consists of:

• Write an IDL file: a SOM header that declares your class’ interface. The class will inherit from an existing SOM class (such as SOMObject, or ODPart for an OpenDoc part handler). In the interface you add any extra methods and instance variables that your part objects will need.

IDL stands for Interface Definition Language, a simple syntax for defining classes. It’s part of the industry-standard CORBA architecture. Fortunately, IDL looks very much like a C++ class definition, with a few extensions.

• Crank your IDL file through the SOM compiler. This translates your part’s definition from the abstract IDL syntax into a C or C++ API, plus the appropriate magic glue for the SOM runtime. The output is several binding files that you use to build your part with C or C++.

• Fill out the implementation. One of the binding files generated is a .c or .cpp file that contains a blank C or C++ implementation of your part: the method functions are all there but their bodies are empty. You fill in the bodies with the actual code for each method.

• Compile and link the implementation files. The implementation binding file and any other source files you create are linked against the SOM library, and the libraries of any other SOM classes you use or inherit from, to produce a shared library that implements your class. You can use any compiler that knows how to build CFM shared libraries, such as scpp or CodeWarrior PPC.

• Iterate. If you fix bugs or make other changes that just modify existing class methods, or non-class code, all you need to do is recompile and relink. If you need to change the class structure by adding methods or instance variables, you’ll need to run the SOM compiler again. The (blank) new methods will be appended to your implementation file without disturbing the existing C/C++ code.

Networked Objects

SOM has an extension called DSOM that supports distributed objects. It allows SOM objects on different machines on a network (or in separate address spaces on one machine) to talk to each other as though they were all running in the same process. DSOM is pretty transparent to your code; you just have to avoid pitfalls like trying to send a remote object a raw pointer to data.

DSOM already runs on OS/2 and will be ported to the Mac OS; we plan to support it in the second release of OpenDoc to allow distributed parts, documents and other services.

On a broader scale yet, DSOM is an implementation of CORBA, an industry standard for distributed objects. This means that DSOM clients can interact with non-SOM distributed applications from vendors like DEC and H/P, running on workstations or mainframes. This gives SOM-based systems like OpenDoc a well defined way to connect to large corporate databases, which may or may not excite you but makes IS managers sit up and drool.

Conclusion

SOM is one of those things where you have to look at the big picture. Yes, it’s a bit of a pain at the micro-level of individual lines of code. But alternatives like COM, which may seem simpler at first, turn out to be more complicated when used with some compilers, and too limited to support true object-oriented programming. And SOM becomes extremely cool at the larger scale of reusable and robust shared class libraries, and positively mind-bending with its prospects of distributed objects and Net-spanning applications.

 
AAPL
$102.50
Apple Inc.
+0.25
MSFT
$45.43
Microsoft Corpora
+0.55
GOOG
$571.60
Google Inc.
+2.40

MacTech Search:
Community Search:

Software Updates via MacUpdate

Skype 6.19.0.450 - Voice-over-internet p...
Skype allows you to talk to friends, family and co-workers across the Internet without the inconvenience of long distance telephone charges. Using peer-to-peer data transmission technology, Skype... Read more
VueScan 9.4.41 - Scanner software with a...
VueScan is a scanning program that works with most high-quality flatbed and film scanners to produce scans that have excellent color fidelity and color balance. VueScan is easy to use, and has... Read more
Cloud 3.0.0 - File sharing from your men...
Cloud is simple file sharing for the Mac. Drag a file from your Mac to the CloudApp icon in the menubar and we take care of the rest. A link to the file will automatically be copied to your clipboard... Read more
LibreOffice 4.3.1.2 - Free Open Source o...
LibreOffice is an office suite (word processor, spreadsheet, presentations, drawing tool) compatible with other major office suites. The Document Foundation is coordinating development and... Read more
SlingPlayer Plugin 3.3.20.505 - Browser...
SlingPlayer is the screen interface software that works hand-in-hand with the hardware inside the Slingbox to make your TV viewing experience just like that at home. It features an array of... Read more
Get Lyrical 3.8 - Auto-magically adds ly...
Get Lyrical auto-magically add lyrics to songs in iTunes. You can choose either a selection of tracks, or the current track. Or turn on "Active Tagging" to get lyrics for songs as you play them.... Read more
Viber 4.2.2 - Send messages and make cal...
Viber lets you send free messages and make free calls to other Viber users, on any device and network, in any country! Viber syncs your contacts, messages and call history with your mobile device,... Read more
Cocktail 7.6 - General maintenance and o...
Cocktail is a general purpose utility for OS X that lets you clean, repair and optimize your Mac. It is a powerful digital toolset that helps hundreds of thousands of Mac users around the world get... Read more
LaunchBar 6.1 - Powerful file/URL/email...
LaunchBar is an award-winning productivity utility that offers an amazingly intuitive and efficient way to search and access any kind of information stored on your computer or on the Web. It provides... Read more
Maya 2015 - Professional 3D modeling and...
Maya is an award-winning software and powerful, integrated 3D modeling, animation, visual effects, and rendering solution. Because Maya is based on an open architecture, all your work can be scripted... Read more

Latest Forum Discussions

See All

Rhonna Designs Magic (Photography)
Rhonna Designs Magic 1.0 Device: iOS Universal Category: Photography Price: $1.99, Version: 1.0 (iTunes) Description: Want to sprinkle *magic* on your photos? With RD Magic, you can add colors, filters, light leaks, bokeh, edges,... | Read more »
This Week at 148Apps: August 25-29, 2014
Shiny Happy App Reviews   | Read more »
Qube Kingdom – Tips, Tricks, Strategies,...
Qube Kingdom is a tower defense game from DeNA. You rally your troops – magicians, archers, knights, barbarians, and others – and fight against an evil menace looking to dominate your kingdom of tiny squares. Planning a war isn’t easy, so here are a... | Read more »
Qube Kingdom Review
Qube Kingdom Review By Nadia Oxford on August 29th, 2014 Our Rating: :: KIND OF A SQUARE KINGDOMUniversal App - Designed for iPhone and iPad Qube Kingdom has cute visuals, but it’s a pretty basic tower defense game at heart.   | Read more »
Fire in the Hole Review
Fire in the Hole Review By Rob Thomas on August 29th, 2014 Our Rating: :: WALK THE PLANKUniversal App - Designed for iPhone and iPad Seafoam’s Fire in the Hole looks like a bright, 8-bit throwback, but there’s not enough booty to... | Read more »
Alien Creeps TD is Now Available Worldwi...
Alien Creeps TD is Now Available Worldwide Posted by Ellis Spice on August 29th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Dodo Master Review
Dodo Master Review By Jordan Minor on August 29th, 2014 Our Rating: :: NEST EGGiPad Only App - Designed for the iPad Dodo Master is tough but fair, and that’s what makes it a joy to play.   | Read more »
Motorsport Manager Review
Motorsport Manager Review By Lee Hamlet on August 29th, 2014 Our Rating: :: MARVELOUS MANAGEMENTUniversal App - Designed for iPhone and iPad Despite its depth and sense of tactical freedom, Motorsport Manager is one of the most... | Read more »
Motorsport Manager – Beginner Tips, Tric...
The world of Motorsport management can be an unforgiving and merciless one, so to help with some of the stress that comes with running a successful race team, here are a few hints and tips to leave your opponents in the dust. | Read more »
CalPal Update Brings the App to 2.0, Add...
CalPal Update Brings the App to 2.0, Adds Lots of New Stuff Posted by Ellis Spice on August 29th, 2014 [ permalink ] | Read more »

Price Scanner via MacPrices.net

Apple now offering refurbished 21-inch 1.4GHz...
The Apple Store is now offering Apple Certified Refurbished 21″ 1.4GHz iMacs for $929 including free shipping plus Apple’s standard one-year warranty. Their price is $170 off the cost of new models,... Read more
Save $50 on the 2.5GHz Mac mini, on sale for...
B&H Photo has the 2.5GHz Mac mini on sale for $549.99 including free shipping. That’s $50 off MSRP, and B&H will also include a free copy of Parallels Desktop software. NY sales tax only. Read more
Save up to $300 on an iMac with Apple refurbi...
The Apple Store has Apple Certified Refurbished iMacs available for up to $300 off the cost of new models. Apple’s one-year warranty is standard, and shipping is free. These are the best prices on... Read more
The Rise of Phablets
Carlisle & Gallagher Consulting Group, a businesses and technology consulting firm focused solely on the financial services industry, has released an infographic depicting the convergence of... Read more
Bad Driver Database App Allows Good Drivers t...
Bad Driver Database 1.4 by Facile Group is a new iOS and Android app that lets users instantly input and see how many times a careless, reckless or just plain stupid driver has been added to the... Read more
Eddy – Cloud Music Player for iPhone/iPad Fre...
Ukraine based CapableBits announces the release of Eddy, its tiny, but smart and powerful cloud music player for iPhone and iPad that allows users to stream or download music directly from cloud... Read more
A&D Medical Launches Its WellnessConnecte...
For consumers and the healthcare providers and loved ones who care for them, A&D Medical, a leader in connected health and biometric measurement devices and services, has launched its... Read more
Anand Lal Shimpi Retires From AnandTech
Anand Lal Shimpi, whose AnandTech Website is famous for its meticulously detailed and thoroughgoing reviews and analysis, is packing it in. Lal Shimpi, who founded the tech site at age 14 in 1997,... Read more
2.5GHz Mac mini, Apple refurbished, in stock...
The Apple Store has Apple Certified Refurbished 2.5GHz Mac minis available for $509, $90 off MSRP. Apple’s one-year warranty is included, and shipping is free. Read more
13-inch 2.5GHz MacBook Pro on sale for $999,...
B&H Photo has the 13″ 2.5GHz MacBook Pro on sale for $999.99 including free shipping plus NY sales tax only. Their price is $100 off MSRP. Read more

Jobs Board

*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
Senior Event Manager, *Apple* Retail Market...
…This senior level position is responsible for leading and imagining the Apple Retail Team's global event strategy. Delivering an overarching brand story; in-store, Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.