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.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Civilization VI 1.0.6 - Next iteration o...
Sid Meier’s Civilization VI is the next entry in the popular Civilization franchise. Originally created by legendary game designer Sid Meier, Civilization is a strategy game in which you attempt to... Read more
Civilization VI 1.0.6 - Next iteration o...
Sid Meier’s Civilization VI is the next entry in the popular Civilization franchise. Originally created by legendary game designer Sid Meier, Civilization is a strategy game in which you attempt to... Read more
djay Pro 2.0.1 - Transform your Mac into...
djay Pro provides a complete toolkit for performing DJs. Its unique modern interface is built around a sophisticated integration with iTunes and Spotify, giving you instant access to millions of... Read more
Microsoft OneNote 15.41 - Free digital n...
OneNote is your very own digital notebook. With OneNote, you can capture that flash of genius, that moment of inspiration, or that list of errands that's too important to forget. Whether you're at... Read more
TechTool Pro 9.6 - Hard drive and system...
TechTool Pro has long been one of the foremost utilities for keeping your Mac running smoothly and efficiently. With the release of version 9, it has become more proficient than ever. TechTool... Read more
Apple iOS 11.2.1 - The latest version of...
iOS 11 sets a new standard for what is already the world’s most advanced mobile operating system. It makes iPhone better than before. It makes iPad more capable than ever. And now it opens up both to... Read more
Things 3.3 - Elegant personal task manag...
Things is a task management solution that helps to organize your tasks in an elegant and intuitive way. Things combines powerful features with simplicity through the use of tags and its intelligent... Read more
RapidWeaver 7.5.5 - Create template-base...
RapidWeaver is a next-generation Web design application to help you easily create professional-looking Web sites in minutes. No knowledge of complex code is required, RapidWeaver will take care of... Read more
Adobe Animate CC 2018 18.0.1.115 - Anima...
Animate CC 2018 is available as part of Adobe Creative Cloud for as little as $19.99/month (or $9.99/month if you're a previous Flash Professional customer). Animate CC 2018 (was Flash CC) lets you... Read more
Postbox 5.0.22 - Powerful and flexible e...
Postbox is a new email application that helps you organize your work life and get stuff done. It has all the elegance and simplicity of Apple Mail, but with more power and flexibility to manage even... Read more

Latest Forum Discussions

See All

Life Is Strange (Games)
Life Is Strange 1.1 Device: iOS Universal Category: Games Price: $2.99, Version: 1.1 (iTunes) Description: Life Is Strange is a five part episodic game that sets out to revolutionize story-based choice and consequence games by... | Read more »
Oddworld: New 'n' Tasty (Game...
Oddworld: New 'n' Tasty 1.0 Device: iOS Universal Category: Games Price: $7.99, Version: 1.0 (iTunes) Description: ** PLEASE NOTE: Requires 3.6GB free space to install. Runs at variable resolutions based on device capabilities.... | Read more »
Gorogoa (Games)
Gorogoa 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: Gorogoa is an elegant evolution of the puzzle genre, told through a beautifully hand-drawn story designed and illustrated by Jason... | Read more »
Why Guns of Boom will be big for mobile...
Earlier this week, Game Insight, the minds that brought you Guns of Boom, revealed plans for an esports mode in the popular FPS title, with big implications for the game's future. Guns of Boom has been quite popular for some time now, so it's... | Read more »
Rules of Survival guide - how to boost y...
It's not easy surviving in the "every-man-for-himself" world of Rules of Survival. You'll be facing off against many other players who might be more skilled than you, or are luckier than you. There are a lot of factors weighing against you. With... | Read more »
FEZ Pocket Edition (Games)
FEZ Pocket Edition 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: | Read more »
Amazing Katamari Damacy guide - beginner...
Amazing Katamari Damacy brings the bizarro world of the original games to mobile and shifts them into an endless format that's just as addictive as the PlayStation entries. Your goal is still to roll as much random stuff as you possibly can, though... | Read more »
Portal Knights guide - crafting tips and...
In Portal Knights, you're only as strong as the items you have at your disposal. This sandbox adventure is all about crafting and building up the next big thing. Whether you're an avid explorer or collector, crafting will likely play a large part... | Read more »
The best deals on the App Store this wee...
A new week means new discounts on the App Store. This week's deals run the gamut of action-adventure titles, puzzle games, and one of the best narrative adventure series out there. If you're looking to fill out your mobile gaming library on a... | Read more »
What you need to know about Animal Cross...
We hope you've been hard at work on collecting all of those holiday items in Animal Crossing: Pocket Camp, because you're about to get a whole new list of fun things to do as the game receives its first big update sometime soon. There are a lot of... | Read more »

Price Scanner via MacPrices.net

Beats Holiday sale at B&H, headphones and...
B&H Photo has Beats by Dr. Dre headphones, earphones, and speakers on sale for up to $80 off MSRP as part of their Holiday sale. Expedited shipping is free, and B&H charges sales tax to NY... Read more
Holiday sale: Apple resellers offer 2017 15″...
MacMall has 15″ MacBook Pros on sale for $220-$300 off MSRP, each including free shipping: – 15″ 2.8GHz MacBook Pro Space Gray (MPTR2LL/A): $2179, $220 off MSRP – 15″ 2.8GHz MacBook Pro Silver (... Read more
Holiday sale: Apple resellers offer 13″ MacBo...
B&H Photo has 13″ MacBook Pros on sale for up to $150 off MSRP. Shipping is free, and B&H charges sales tax for NY & NJ residents only: – 13-inch 2.3GHz/128GB Space Gray MacBook Pro (... Read more
Apple Watch Series 2, Certified Refurbished,...
Apple has Certified Refurbished Apple Watch Nike+ Series 2s, 42mm Space Gray Aluminum Case with Anthracite/Black Nike Sport Bands, available for $249 (38mm) or $279 (42mm). The 38mm model was out of... Read more
Apple offers Certified Refurbished 2016 12″ R...
Apple has Certified Refurbished 2016 12″ Retina MacBooks available starting at $949. Apple will include a standard one-year warranty with each MacBook, and shipping is free. The following... Read more
B&H drops price on 13″ 256GB MacBook Air...
B&H has the 13″ 1.8GHz/256GB Apple MacBook Air (MQD42LL/A) now on sale for $1079 including free shipping plus NY & NJ sales tax only. Their price is $120 off MSRP, and it’s the lowest price... Read more
Holiday sale: 9″ iPads starting at $299, take...
MacMall has 9″ WiFi iPads on sale for $30 off including free shipping: – 9″ 32GB WiFi iPad: $299 – 9″ 128GB WiFi iPad: $399 Read more
Green Monday deal: 15″ 2.8GHz MacBook Pro on...
B&H Photo has the 15″ 2.8GHz Space Gray MacBook Pro on sale for $250 off MSRP for today only as part of their Green Monday/Holiday sale. Shipping is free, and B&H charges sales tax for NY... Read more
Green Monday sale: B&H offers 12″ Apple i...
B&H Photo has 12″ iPad Pros on sale for up to $150 off MSRP as part of their Green Monday/Holiday sale. Shipping is free, and B&H charges sales tax in NY & NJ only: – 12″ 64GB WiFi iPad... Read more
Holiday deal: 21″ and 27″ Apple iMacs on sale...
MacMall has 2017 21″ and 27″ Apple iMacs on sale for up to $200 off MSRP. Shipping is free: – 21″ 2.3GHz iMac: $999 $100 off MSRP – 21″ 3.0GHz iMac: $1199 $100 off MSRP – 21″ 3.4GHz iMac: $1379 $120... Read more

Jobs Board

*Apple* Solutions Consultant - Apple (United...
# Apple Solutions Consultant Job Number: 113124408 Waterford, CT, Connecticut, United States Posted: 17-Oct-2017 Weekly Hours: 40.00 **Job Summary** Are you Read more
QA Automation Engineer, *Apple* Pay - Apple...
# QA Automation Engineer, Apple Pay Job Number: 113202642 Santa Clara Valley, California, United States Posted: 11-Dec-2017 Weekly Hours: 40.00 **Job Summary** At Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description:SalesSpecialist - Retail Customer Service and SalesTransform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description:SalesSpecialist - Retail Customer Service and SalesTransform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.