Categories
Cox's main concern was the maintainability of large code bases. Experience from the structured programming world had shown that one of the main ways to improve code was to break it down into smaller pieces. Objective-C added the concept of Categories to help with this process.
A category collects method implementations into separate files. The programmer can place groups of related methods into a category to make them more readable. For instance, one could create a "SpellChecking" category "on" the String object, collecting all of the methods related to spell checking into a single place.
That might not sound like anything new, but one key aspect of the implementation is that the methods are added to the class at runtime. That means that the programmer can add categories to existing classes without them knowing about it. If the system you are supplied with does not contain a spell checker in its String implementation, you just add it.
Combining categories with dynamic typing produces a synergy that results in tremendous flexibility that is largely unexpected in most programming systems. Consider Java: simply allowing String to have a checkSpelling method added isn't enough on its own, because the rest of your program won't allow you to call that new method - all methods must be known at compile time as a side effect of strong typing. Instead you must use helper objects, special proxy classes, or use the terrible visitor pattern, all of which add to the complexity and ugliness of the implementation.
But the usefulness of categories goes further. Since any method can be "covered" by one in a category at runtime, you can in fact fix bugs in existing compiled binaries. Once you get used to having this, you can't live without it.
A number of other languages have attempted to add this feature in a variety of ways. TOM took the Objective-C system to its logical conclusion and allowed for the addition of variables as well. Other languages have instead used prototype oriented solutions, the most notable being Self.
Other features
ObjC in fact included a laundry-list of features that are still being added to other languages, or simply don't exist at all. These led from Cox's (and later, NeXT's) realization that there is considerably more to programming than the language. The system has to be usable and flexible as a whole in order to work in a real-world setting.
- All libraries are dynamically linked. This meant that large Objective-C programs were in fact quite small, because the library was already on the machine. Common today perhaps, but this is from the early 1980s when dynamic linking was fairly rare.
- Libraries can be supplied in multiple versions in a single file. Applications can choose to use a specific version (4.3.2), the latest version (which happens to be 5.2.1), or even the latest of a particular major release (anything from 4.x). Versioning problems like this continue to haunt other systems to this day, including Java and component object model.
- Code can be provided in multiple object formats for different platforms, in a single library. This avoids the need for a virtual machine entirely. Typical multi-platform Objective-C programs are smaller than most single-platform programs on other systems.
- Delegating methods to other objects at run-time is trivial. Simply add a category that changes the "second chance" method to forward the invocation to the delegate. Presto!
- Remote invocation is trivial. Simply add a category that changes the "second chance" method to serialize the invocation and forward it off.
- Swizzling allows for classes to change at runtime. Typically used for debugging where freed objects are swizzled into Zombies, whose only purpose is to report an error when someone calls them. Swizzling was also used in EOF to create database faults.
- Archiving. An object can be archived into a stream, such as a file, and can be read and restored on demand.
Objective-C++
Objective-C++ is a front-end to the Mac OS X version of the GNU Compiler Collection that can compile source files that use both C++ and Objective-C, with certain restrictions:
- A C++ class cannot derive from an Objective-C class and vice versa.
- C++ namespaces cannot be declared inside an Objective-C declarations.
- Objective-C classes cannot have instant variables of a C++ class type that has virtual functions, nor can C++ classes have instances of Objective-C objects.
- An Objective-C declaration cannot be within a C++ template declaration and vice versa, though Objective-C classes can be used as C++ template parameters.
