@defs ( className) Takes the name of an Objective-C class and evaluates to a sequence of type declarations that duplicate the field declarations of the class. This directive can appear only in a C structure declaration. @protocol ( ProtocolName) Evaluates to a pointer to an instance of the Protocol class. You need this because you can't get a protocol class instance by using the protocol name directly, as you can with a class object. @selector ( MethodName) Evaluates to a SEL representing the specified method. @ "string" A shorthand for creating a string literal that's an instance of a user-defined string class. Use this when you need a string object constant. The directives @encode, @defs, and @"string" deserve some additional explanation. 1.4.3.1 Using @encode The following example shows how @encode can be used to get the string that the Objective-C runtime uses to describe a type: char * itype = @encode (int ); The result of this statement will be to define itype as the one-character string "i", which is the runtime representation of an int type. The @encode directive can take any C or Objective-C type. The runtime uses the mapping between types and strings to encode the signatures of methods and associate them with selectors. You can use @encode to implement your own object storage and retrieval, or other tasks that need to describe the types of values. Table 1-1 shows the results of applying @encode to C and Objective-C types. Table 1-1. Types and their encodings Type @encode(type) char c int i short s long l long long q unsigned char C unsigned int I unsigned short S unsigned long L unsigned long long Q float f double d void v char* * An object pointer @ Class # SEL : An array of N elements of type [Ntype] A structure called name with elements t 1 , t 2 , etc. {name=t 1 t 2 } A union (type, ) A bit field of size N bN A pointer to type ^type Unknown type ? The runtime system also uses encodings for type qualifiers, shown in Table 1-2, and you may encounter them in its representation of method signatures. However, you can't get those encodings with the @encode directive. Table 1-2. Type qualifiers and their encodings Qualifier @encode(qualifier) const r in n inout N out o bycopy O byref R oneway V 1.4.3.2 Using @defs The following example shows how @defs can be used to create a C structure with fields that match the fields in a class. In this example, both the order and type of the fields of MyStruct will match those in MyClass: @interface MyClass : Object { int i ; } @end typedef struct { @defs (MyClass ) } MyStruct ; The typedef in this example is seen by the compiler as: typedef struct { id isa; int i ; } MyStruct ; Having a structure that corresponds to a class lets you bypass the normal access restrictions on an Objective-C instance. In the following example, an instance of MyClass is cast to an instance of MyStruct. Once that's done, the protected field i can be accessed with impunity: MyClass* c = [MyClass new]; MyStruct* s = (MyStruct*)c; s->i = 42; Obviously this is a facility you should use with restraint. 1.4.3.3 Using @"string" The @"string" directive creates the Objective-C version of a string literal: one that you can pass to a method expecting a string object, or use to initialize or compare with another string object. When you create a string with @"string", you get an instance of a class defined by the compiler option -fconstant-string-class. The instance is static: its contents are stored in your program's file, and the instance is created at runtime and kept around for the duration of program execution. You can't change its value, because it appears as an expression in your program, not as a variable In the GNU runtime, the default string class is NXConstantString; for Darwin it is the Cocoa class NSConstantString. 1.4.4 Preprocessor Symbols Objective-C adds one preprocessor directive and defines one symbol to the preprocessor before compiling: #import fileName Using #import has the same effect as the C directive #include, but will only include the file once. _ _OBJC_ _ When gcc is compiling an Objective-C file, it defines this symbol for the preprocessor. If your code must compile as plain C as well as Objective-C, you can test to see if this symbol is defined: #ifdef _ _OBJC_ _ // Objective-C code here #endif 1.5 Compiler Flags Compiler flags are options you give to gcc when it compiles a file or set of files. You may provide these directly on the command line, or your development tools may generate them when they invoke gcc. This section describes just the flags that are specific to Objective-C. -fconstant-string-class= ClassName Tells the compiler to create an instance of ClassName for each string literal expressed with the @"string" directive. For the GNU runtime, the default class is NXConstantString; for Cocoa it is NSConstantString. -fgnu-runtime Generate code for linking with the standard GNU Objective-C runtime. This is the default option for most gcc installations. -fnext-runtime Generate code for linking with the NeXT runtime. This is the default option for Darwin. -gen-decls Write all interface declarations the compiler sees to a file named sourcename.decl. -Wno-protocol Do not warn if methods required by a protocol are not implemented in the class adopting it. -Wselector Warn if there are methods with the same name but different signatures. 1.6 Remote Messaging Objective-C's method call model lends itself well to distributed systems, where sending messages is the fundamental unit of interaction. The key difference in distributed systems is that objects may exist in different address spaces, and so cannot call each other directly. Objects that want to receive messages from other processes must specify those messages in a formal protocol. Objective-C provides special keywords to use when writing such a formal protocol that qualify the kind of sharing you will need for that protocol's message parameters.