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

iShowU Instant 1.3.2 - Full-featured scr...
iShowU Instant gives you real-time screen recording like you've never seen before! It is the fastest, most feature-filled real-time screen capture tool from shinywhitebox yet. All of the features you... Read more
NeoFinder 7.5.1 - Catalog your external...
NeoFinder (formerly CDFinder) rapidly organizes your data, either on external or internal disks, or any other volumes. It catalogs and manages all your data, so you stay in control of your data... Read more
App Tamer 2.5 - Efficiently manage your...
App Tamer tames your processor-monopolizing apps and keeps them from chewing up excessive CPU time and battery life. Powered by a unique AutoStop feature, App Tamer stops each application when you... Read more
MainStage 3 3.4.4 - Live performance too...
Apple MainStage makes it easy to bring to the stage all the same instruments and effects that you love in your recording. Everything from the Sound Library and Smart Controls you're familiar with... Read more
iTubeDownloader 6.5.13 - Easily download...
iTubeDownloader is a powerful-yet-simple YouTube downloader for the masses. Because it contains a proprietary browser, you can browse YouTube like you normally would. When you see something you want... Read more
FileZilla 3.47.0 - Fast and reliable FTP...
FileZilla (ported from Windows) is a fast and reliable FTP client and server with lots of useful features and an intuitive interface. Version 3.47.0: Fixed regression loading advanced site... Read more
Transmit 5.6.3 - Excellent FTP/SFTP clie...
Transmit is an excellent FTP (file transfer protocol), SFTP, S3 (Amazon.com file hosting) and iDisk/WebDAV client that allows you to upload, download, and delete files over the internet. With the... Read more
Doomsday 2.2.2 - Play classic Doom on mo...
id Software's Doom pioneered the modern first-person shooter genre. Released in 1993, it was a quantum leap in game engine technology with fluid and - at the time - incredibly realistic 3D graphics.... Read more
Ableton Live 10.1.9 - Record music using...
Ableton Live lets you create and record music on your Mac. Use digital instruments, pre-recorded sounds, and sampled loops to arrange, produce, and perform your music like never before. Ableton Live... Read more
Maintenance 2.6.5 - System maintenance u...
Maintenance is a system maintenance and cleaning utility. It allows you to run miscellaneous tasks of system maintenance: Check the the structure of the disk Repair permissions Run periodic scripts... Read more

Latest Forum Discussions

See All

Creepy Little Monsters is a cute, monste...
Creepy Little Monsters is a retro throwback that sees you traversing tricky puzzle-platformer levels as a one-eyed monster. It aims to offer a fresh take on 80s and 90s classics of the genre, and it's out right now for iOS and Android. [Read more... | Read more »
Tyrant's Arena delivers intense her...
Tyrant's Arena is an intense midcore multiplayer actioner where you'll compete in tricky 3v3 matches to crush your opponents and earn neat rewards. It comes to us from developer Kroy Games, and it's now available for pre-registration on iOS and... | Read more »
Mobile Games Starter Kit
Over here at 148Apps, we regularly dive deep into the latest and greatest mobile games hitting the App Store, but that’s not always what people are looking for when searching for a new mobile game. Some folks just want to dip their toes into... | Read more »
Unresolved is a hard-hitting narrative a...
Ghofran Akil's Unresolved in an upcoming text-based adventure game that sees you playing as a mother attempting to find her disappeared husband during the Lebanese Civil War. [Read more] | Read more »
Marvel Strike Force introduces new brawl...
FoxNext's squad-based RPG Marvel Strike Force is set to receive some fresh characters from the X-Men and Iron Man series. They'll arrive as part of the game's latest update, which follows a sizable spending boycott on the title due to complaints... | Read more »
Speed Dating for Ghosts is a narrative a...
Speed Dating for Ghosts originally released on Steam back 2018, since then it has received honourable mentions for narrative during the Independent Games Festival. Now it's made its way over to iOS devices where it's available as a premium title... | Read more »
Fast-paced multiplayer title Tennis Star...
Tennis Stars: Ultimate Clash is the latest free-to-play tennis title to hit iOS and Android. It's said to be a fairly casual experience, offering easy-to-learn controls and fast-paced, mobile-friendly matches. [Read more] | Read more »
Super Mecha Champions' latest updat...
Super Mecha Champions' latest update sees the addition of a brand new character called R.E.D. Alongside that, there's news about the current season and a series of Emojis that have been added to the game. [Read more] | Read more »
Apple Arcade: Ranked - Top 50 [Updated 2...
In case you missed it, I am on a quest to rank every Apple Arcade game there is. [Read more] | Read more »
Apple Arcade: Ranked - 51+ [Updated 2.19...
This is part 2 of our Apple Arcade Ranking list. To see part 1, go here. To skip to part 3, click here. 51. Mini Motorways Description: [Read more] | Read more »

Price Scanner via MacPrices.net

B&H is again offering $100 discounts on M...
B&H Photo has 4-Core and 6-Core Mac minis on sale for $100 off Apple’s standard MSRP, with prices starting at only $699. Overnight shipping is free to many US addresses: – 3.6GHz Quad-Core mini... Read more
B&H Photo drops iMac prices, offers model...
B&H Photo has new 2019 21″ and 27″ 5K iMacs in stock today and on sale for up to $250 off Apple’s MSRP, with prices starting at only $999. These are the same iMacs sold by Apple in their retail... Read more
Flash sale! 11″ 64GB WiFi iPad Pro for $674,...
Walmart has the 11″ 64GB WiFi iPad Pro on sale on their online store today for $674. That’s $125 off Apple’s MSRP for this model and the cheapest price available from any Apple reseller. Choose free... Read more
Sale! Get the 256GB 13″ Silver MacBook Air fo...
Amazon has new 2019 13″ 1.6GHz/256GB MacBook Airs, in Silver, on sale today for only $999 shipped. Their price is $300 off Apple’s MSRP for this model, and it’s the cheapest price for a 256GB MacBook... Read more
Verizon offers free iPhone 7 to customers ope...
Verizon is offering a free 32GB iPhone 7 for new or existing customers who open a new line of service, no trade-in required. Cost of the phone is credited to your account monthly over 24 months. The... Read more
Sale! 10.5″ 256GB WiFi iPad Air for $549, $10...
Amazon has new 10.5″ 256GB WiFi iPad Airs, in Space Gray, on sale today for $549 shipped. Their price is $100 off Apple’s MSRP for this model, and it’s the cheapest price available from any Apple... Read more
Back on sale! Apple’s new Mac Pro for $5499,...
B&H Photo has the base 2019 Mac Pro (3.5GHz 8-Core Xeon, 32GB RAM, 256GB SSD) in stock today and on sale for $5499 including free overnight delivery to many addresses in the US. Their price is $... Read more
B&H offers $100 discount on base 13″ 1.4G...
B&H Photo has new 2019 13″ 1.4GHz MacBook Pros on sale for $100 off Apple’s MSRP today with prices starting at $1199. Overnight shipping is free to many addresses in the US. These are the same... Read more
Apple continues to offer Certified Refurbishe...
Apple has Certified Refurbished iPhone XS models available for up to $350 off MSRP, with prices starting at $699. Each iPhone is unlocked and comes with Apple’s standard one-year warranty and a new... Read more
Apple AirPods are on sale for $30 off today
Amazon has new 2019 Apple AirPods (non-Pro models) on sale today for $30 off MSRP, starting at $129. Shipping is free: – AirPods with Wireless Charging Case: $169 $30 off MSRP – AirPods with Charging... Read more

Jobs Board

Medical Assistant - *Apple* Valley Clinic -...
…professional, quality care to patients in the ambulatory setting at the M Health Fairview Apple Valley Clinic, located in Apple Valley, MN. Join the **M Health Read more
Geek Squad Advanced Repair *Apple* Professi...
**764652BR** **Job Title:** Geek Squad Advanced Repair Apple Professional **Job Category:** Store Associates **Store NUmber or Department:** 000245- Apple Read more
Medical Assistant - *Apple* Valley Clinic -...
…professional, quality care to patients in the ambulatory setting at the M Health Fairview Apple Valley Clinic, located in Apple Valley, MN. Join the **M Health Read more
Windows/ *Apple* Technical Support Engineer...
Windows/ Apple Technical Support Engineer McLean , VA , US Apply + Be you + Be Booz Allen + Be empowered + Learn More Job Description Location: McLean, VA, US Job Read more
Medical Assistant - *Apple* Valley Clinic -...
…professional, quality care to patients in the ambulatory setting at the M Health Fairview Apple Valley Clinic, located in Apple Valley, MN. Join the **M Health Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.