Objective-C: the More Flexible C++
There are four major Objective-C compilers: Stepstone, GNU, Apple and Portable Object Compiler (POC). Stepstone and POC are preprocessors that emit vanilla C code. GNU and Apple are "native" compilers, creating intermediate code for the GNU code generator.
To compile the previous examples, enter:
gcc -Wno-import -c List.m gcc -Wno-import -c List2.m gcc -Wno-import -c main.m gcc -o List -Wno-import List2.o List.o main.o -lobjc -lpthread
Categories compartmentalize a class definition or extend an existing one, possibly at runtime. They denote a set of method definitions that is segregated from the rest of the class definition.
Protocols allow you to organize related methods into groups. Protocol hierarchy is unrelated to class hierarchy. They are used to declare methods that others are expected to implement, declare the interface to an object while concealing its class and capture the similarities among classes that are not hierarchically related.
Protocols address the lack of multiple inheritance. They are equivalent to multiple inheritance for purely "abstract" classes. The closest you can get to multiple inheritance is to create a class with instance variables that are references to other objects. Instances can specifically redirect messages to any combination of the objects of which they are compounded. The Objective-C philosophy is that you do not need multiple inheritance because it creates more problems than benefits. Using protocols, one can have type-checking features without sacrificing dynamic binding. "Any object implementing messages in Protocol X is okay for this use" constrains the functionality and not the implementation or the inheritance.
@protocol Archiving -read: (Stream *) stream; -write: (Stream *) stream; @end
and refer to it with:
/* MyClass inherits from Object and conforms to the Archiving protocol. */ @interface MyClass: Object <Archiving> @end
Example that spots incompatibility changes:
MyClass *obj1 = [MyClass new]; // OK: obj1 conforms to the Archiving protocol. id <Archiving> obj2 = obj1; // Error: obj1 does not conform to the TargetAction protocol. id <TargetAction> obj3 = obj1;
Starting with version 0.9.0, GNUstep's libFoundation comes with two garbage-collecting mechanisms that provide you with the ability to solve memory management problems. libFoundation is a library that provides an almost complete implementation of the OpenStep specification, plus many other extensions that can be found in the newer versions of Rhapsody's (Mac OS X) Foundation.
From Unit-testing Frameworks a là extreme programming (XP) to XML parsers, one can take advantage of a large existing codebase. And if all fails, one can take advantage of the entire existing C code-base, as the GNUstep XML parser has demonstrated. It is an Objective-C wrapper around libxml.
We think that Objective-C is the better C++, which is really worth giving a try. GNUstep is extremely powerful.
An extended comparison of all the compilers discussed in this article can be found on our home page.
Object Oriented Programming: An Evolutionary Approach. Brad J. Cox, Andrew J. Novobilski. Addison-Wesley, 1991.
Objective-C: Object Oriented Programming Techniques. Lewis J. Pinson, Richard S. Wiener. Addison-Wesley, 1991.
An Introduction to Object-Oriented Programming. 3rd Edition. Timothy A. Budd. Addison-Wesley, 2002.
Developing Business Applications With Openstep. N. Gervae. Springer Verlag Telos, 1996.
Openstep for Enterprises. Nancy Craighill. John Wiley & Sons, 1996.
Learning Cocoa. Apple Computer Technical Writers. O'Reilly, 2001.