Comments 509 Predefined Identifiers Table B.2 lists identifiers that have special meanings in Objective-C programs. Table B.2 Identifier _cmd _ _ func_ _ BOOL Class id IMP nil Nil NO NSObject Protocol SEL self super YES Special Predefined Identifiers Meaning A local variable automatically defined in a method that contains the selector for the method A local character string variable automatically defined in a function or method containing the name of the function or method Boolean value, typically used with YES and NO Class object type Generic object type Pointer to a method returning the value of type id Null object Null class object Defined as (BOOL) 0 Root Foundation object defined in <Foundation/NSObject.h> Name of class for storing information about protocols A compiled selector A local variable automatically defined in a method that references the receiver of the message The parent of the receiver of the message Defined as (BOOL) 1 Comments There are two ways to insert comments into program.A comment can begin with the two characters // , in which case any characters that follow on the line are ignored by the compiler. A comment can also begin with the two characters /* and end when the characters */ are encountered.Any characters can be included inside the comment, which can extend over multiple lines of the program.A comment can be used anywhere in the program where a blank space is allowed. Comments, however, cannot be nested, which means that the first */ characters encountered end the comment, no matter how many /* characters you use.