I know that the Apple community – including Mac and iPhone developers – mainly use Objective-C for their development language. But it seems that not many people use Objective-C outside of the Apple community, such as in the Windows or Linux worlds.
What are the possible reasons that Objective-C is not particularly popular outside of the Apple community?
Off the top of my head, I believe C++ is older than Objective-C, and not only for this reason has a much bigger user base. Everywhere that OC may have come, C++ was already there :)
Also, C++ has more features. Many people are impressed by lots of features. And it's had more research and development poured into it... and so forth. Essentially, momentum.
This is a complicated question; but in short; I think the answer most likely lies in the age of the operating systems, and their roots.
Windows is an old operating system; and one that is built by stacking hack upon hack going back all the way to Windows 3.1. C++ is heavily favored, and in .NET, C#.
This new influx is of course based on whatever agenda Microsoft has with that platform.
Mac OS X; on the other hand, is a (comparatively) young operating system, and it's new parts (while still quite old, being inherited from the NeXT and whatnot) are all based on Objective-C because, "Hey! Why not?".
As backwards compatibility was not among the list of priorities with Mac OS X 10.0; the C/C++-based Toolbox and Carbon got the short end of the stick, and the entire operating system was more or less made as a reskinned version of NeXTStep.
The issue with Obj-C is that the power of the language comes mostly from the sizable frameworks, the generally high level of integration into the system, and so on. It's almost impossible to get a good jive like that going without a clean break from backwards compatibility and, as such, it would never really stand a chance on any platform that didn't dare to do this. Apple, with a small (at the time) and devoted user base, dared do this, and struck gold.
Microsoft is now trying, but are, in my humble opinion, failing. ("Failing?! .NET!? HOW DARE YOU!?": With 4 major revisions in about 8 years, they are doing more growing than maturation; which may be a good thing, if they can turn it around.)
Edit: There are some projects attempting to port OpenStep to Linux, but they are a bit clunky and hard to use; there are also smaller projects on NS/OS-likes with smaller problem domains, but it's uphill work.
Objective-C is nothing but a thin layer(a bit thicker with 2.0) of syntactic sugar for message passing on top of standard C. Even the most basic object orientation is provided by the runtime library, which was proprietary for a long time. Inertia is an important factor in language use.
It shines especially on GUIs, but the only toolkits that support it are Apple's and the mostly unknown and catchup-playing GNUStep.
While there is some value for Objective-C outside of GUIs, and I think people would use the extensions were they imported to C, even in system code, there is little reason to choose it over alternatives, when little of your system is meant to work with it.
@William Totland
Sorry, but your post has many problems.
Firstly, you confuse the language of choice in which the operating system with the language of choice for developers of applications for the operating system. Sure Unix is written in C (that includes Mac OS X by the way), but a lot of other languages are well supported on it. Same applies to Linux, of course.
Windows is not an old operating system. The Windows in a modern desktop computer is not related to Windows 3.1. Windows 3.1 (and 95) were written in assembler. The Windows you have today descends from NT and was written in C (same as Unix).
Objective-C is a superset of C (much more strictly than C++, in fact) so the issue of backward compatibility does not arise. Anything you can do in C you can do in Objective-C.
The framework issue is real, however. Java, Perl and .Net also have large frameworks but those of Java and Perl have been ported to all popular platforms whereas .Net runs on the platform with more than 90% of the desktop market. The Cocoa framework used by Objective-C GUI applications is only available on Mac OS X with only a less fully featured "clone" available for other platforms. C#/.Net and Objective-C/Cocoa are both in similar positions, one being more or less Microsoft only and the other being more or less Apple only.
I was recently standing in a bookshop reading Masterminds of Programming where the creators of programming languages talk about their creations. There was one chapter about Objective-C where Tom Love (one of the creator of Objective-C, along with Brad Cox) was asked why C++ had gone so far, while Objective-C hadn't:
Why do you think that C++ was used more frequently than Objective-C?
Tom: It had the AT&T moniker behind it.
Just that?
Tom: I think so.
What do you think about Objective-C today?
Tom: It still exists. How about that?