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Interacting with the Runtime System

Objective-C programs interact with the runtime system at three distinct levels:

1. Through Objective-C source code.

   For the most part, the runtime system works automatically and behind the scenes. You use it just by writing and compiling Objective-C source code.

   When you compile code containing Objective-C classes and methods, the compiler creates the data structures and function calls that implement the dynamic characteristics of the language. The data structures capture information found in class and category definitions and in protocol declarations; they include the class and protocol objects discussed in “Defining a Class” and “Protocols,” as well as method selectors, instance variable templates, and other information distilled from source code. The principal runtime function is the one that sends messages, as described in “How Messaging Works.” It’s invoked by source-code message expressions.

2. Through the methods defined in the NSObject class of the Foundation framework.

   Most objects in Cocoa are subclasses of the NSObject class, so most objects inherit the methods it defines. (The notable exception is the NSProxy class; see “Forwarding” for more information.)

   Some of the NSObject methods simply query the runtime system for information. These methods allow objects to perform introspection. Examples of such methods are the class method, which asks an object to identify its class; isKindOfClass: and isMemberOfClass:, which test an object’s position in the inheritance hierarchy; respondsToSelector:, which indicates whether an object can accept a particular message; conformsToProtocol:, which indicates whether an object claims to implement the methods defined in a specific protocol; and methodForSelector:, which provides the address of a method’s implementation. Methods like these give an object the ability to introspect about itself.

   All these methods were mentioned in previous chapters and are described in detail in the NSObject class specification in the Foundation framework reference.

3. Through direct calls to runtime functions.

   The runtime system is a dynamic shared library with a public interface consisting of a set of functions and data structures in the header files located within the directory /usr/include/objc. Many of these functions allow you to use plain C to replicate what the compiler does when you write Objective-C code. Others form the basis for functionality exported through the methods of the NSObject class. These functions make it possible to develop other interfaces to the runtime system and produce tools that augment the development environment; they’re not needed when programming in Objective-C. However, a few of the runtime functions might on occasion be useful when writing an Objective-C program. All of these functions are documented in Objective-C 2.0 Runtime Reference.

Because the NSObject class is at the root of the inheritance hierarchy of the Foundation framework, the methods it defines are usually inherited by all classes. Its methods therefore establish behaviors that are inherent to every instance and every class object. However, in a few cases, the NSObject class merely defines a template for how something should be done; it doesn’t provide all the necessary code itself.

For example, the NSObject class defines a description instance method that returns a string describing the contents of the class. This is primarily used for debugging—the GDB print-object command prints the string returned from this method. NSObject’s implementation of this method doesn’t know what the class contains, so it returns a string with the name and address of the object. Subclasses of NSObject can implement this method to return more details. For example, the Foundation class NSArray returns a list of descriptions of the objects it contains.

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