Copyright ©1995 by NeXT Computer, Inc.  All Rights Reserved.

class_addMethods() See class_getInstanceMethod()

class_createInstance(), class_createInstanceFromZone()

SUMMARY Create a new instance of a class

DECLARED IN objc/objc-class.h

SYNOPSIS id class_createInstance(Class aClass, unsigned int indexedIvarBytes)

id class_createInstanceFromZone(Class aClass, unsigned int indexedIvarBytes, NXZone *zone)

DESCRIPTION These functions provide an interface to the object allocators used by the run-time system.  The default allocators, which can be changed by reassigning the _alloc and _zoneAlloc variables, create a new instance of aClass by dynamically allocating memory for it, initializing its isa instance variable to point to the class, and returning the new instance.  All other instance variables are initialized to 0.

The two functions are identical, except that class_createInstanceFromZone() allocates memory for the new object from the region specified by zone and class_createInstance() allocates memory from the default zone returned by NXDefaultMallocZone().

Object's alloc and allocFromZone: methods use class_createInstanceFromZone() to allocate memory for a new object, with alloc taking the memory from the default zone.  The new method uses class_createInstance().

The second argument to both functions, indexedIvarBytes, states the number of extra bytes required for indexed instance variables.  Normally, it's 0.

Indexed instance variables are instance variables that are not declared or accounted for in the usual way, generally because they don't have a fixed size.  Usually they're arrays whose length can't be computed at compile time.  Since the components of a C structure can't be of uncertain size, indexed instance variables can't be declared in the class interface.  The class must account for them outside the normal channels provided by the Objective C language.

All of the storage required for indexed instance variables must be allocated through one of these two functions.  The following code shows how they might be used in an instance-creating class method:

+ new:(unsigned int)numBytes
self = class_createInstance((Class)self, numBytes);
length = numBytes;
. . .

- (char *)getArray

Indexed instance variables should be avoided if at all possible.  It's a much better practice to store variable-length data outside the object and declare one real instance variable that points to it and perhaps another that records its length.  For example:

+ new:(unsigned int)numBytes
self = [super new];
data = malloc(numBytes);
length = numBytes;
. . .

- (char *)getArray
return data;

RETURN If successful, both class_createInstance() and class_createInstanceFromZone() return the new instance of aClass.  If not successful, they generate an error message and call abort().

class_createInstanceFromZone() See class_createInstance()

class_getClassMethod() See class_getInstanceMethod()

class_getInstanceMethod(), class_getClassMethod(), class_addMethods(), class_removeMethods()

SUMMARY Get, add, and remove methods

DECLARED IN objc/objc-class.h

SYNOPSIS Method class_getInstanceMethod(Class aClass, SEL aSelector)

Method class_getClassMethod(Class aClass, SEL aSelector)
void class_addMethods(Class aClass, struct objc_method_list *methodList)
void class_removeMethods(Class aClass, struct objc_method_list *methodList)

DESCRIPTION The first two functions, class_getInstanceMethod() and class_getClassMethod(), return a pointer to the class data structure that describes the aSelector method.  For class_getInstanceMethod(), aSelector must identify an instance method; for class_getClassMethod(), it must identify a class method.  Both functions return a NULL pointer if aSelector doesn't identify a method defined in or inherited by aClass.

The run-time system uses the next function, class_addMethods(), to implement Objective C categories.  Each function adds the methods in methodList to the dictionary of methods defined for aClass.  To add methods that can be used by instances of a class, aClass should be the class object.  To add methods that can be used by a class object, aClass should be the metaclass object (the isa field of the Class structure).  All the methods in methodList must be mapped to valid SEL selectors before they're added to the class.  The sel_registerName() function can be used to accomplish this.

The last function, class_removeMethods(), removes methods that were previously added using class_addMethods().  The run-time system uses it to unload categories that were dynamically loaded at an earlier point in time.  Its second argument, methodList, must be identical to a pointer previously passed to class_addMethods().  To remove instance methods, aClass should be the class object.  To remove class methods, aClass should be the isa field of the Class structure.

RETURN class_getInstanceMethod() and class_getClassMethod() return a pointer to the data structure that describes the aSelector method as implemented for aClass.  If aSelector isn't defined for aClass, they return NULL.


SUMMARY Get the class template for an instance variable

DECLARED IN objc/objc-class.h

SYNOPSIS Ivar class_getInstanceVariable(Class aClass, const char *variableName)

RETURN This function returns a pointer to the class data structure that describes the variableName instance variable.  If aClass doesn't define or inherit the instance variable, a NULL pointer is returned.

class_getVersion() See class_setVersion()


SUMMARY Pose as the superclass

DECLARED IN objc/objc-class.h

SYNOPSIS Class class_poseAs(Class theImposter, Class theSuperclass)

DESCRIPTION class_poseAs() causes one class, theImposter, to take the place of its own superclass, theSuperclass.  Messages sent to theSuperclass will actually be received by theImposter.  The posing class can't declare any new instance variables, but it can define new methods and override methods defined in the superclass.

Posing is usually done through Object's poseAs: method, which calls this function.

RETURN Normally, class_poseAs() returns its first argument, theImposter.  However, if theImposter defines instance variables or is not a subclass of (or the same as) theSuperclass, it generates an error message and aborts.

class_removeMethods() See class_getInstanceMethod()

class_setVersion(), class_getVersion()

SUMMARY Set and get the class version

DECLARED IN objc/objc-class.h

SYNOPSIS void class_setVersion(Class aClass, int versionNumber)

int class_getVersion(Class aClass)

DESCRIPTION These functions set and return the class version number.  This number is used when archiving instances of the class.

Object's setVersion: and version methods do the same work as these functions.

RETURN class_getVersion() returns the version number for aClass last set by class_setVersion(), or 0 if no version has been set.

marg_getRef() See marg_getValue()

marg_getValue(), marg_getRef(), marg_setValue()

SUMMARY Examine and alter method argument values

DECLARED IN objc/objc-class.h

SYNOPSIS type-name marg_getValue(marg_list argFrame, int offset, type-name)

type-name *marg_getRef(marg_list argFrame, int offset, type-name)
void marg_setValue(marg_list argFrame, int offset, type-name, type-name value)

DESCRIPTION These three macros get and set the values of arguments passed in a message.  They're designed to be used within implementations of the forward:: method, which is described under the Object class in Chapter 1, "Root Class."

The first argument to each macro, argFrame, is a pointer to the list of arguments passed in the message.  The run-time system passes this pointer to the forward:: method, making it available to be used in these macros.  The next two arguments--an offset into the argument list and the type of the argument at that offset--can be obtained by calling method_getArgumentInfo().

The first macro, marg_getValue, returns the argument at offset in argFrame.  The return value, like the argument, is of type type-name.  The second macro, marg_getRef, returns a reference to the argument at offset in argFrame.  The pointer returned is to an argument of the type-name type.  The third macro, marg_setValue, alters the argument at offset in argFrame by assigning it value.  The new value must be of the same type as the argument.

Because these are macros, the type-name must be written as types normally are in source code; it can't be passed as a variable. Therefore, if the type is obtained from method_getArgumentInfo(), a switch statement would be required to select the correct macro call from a list of predetermined choices.  method_getArgumentInfo() encodes the argument type according to the conventions of the @encode() compiler directive.

RETURN marg_getValue returns a type-name argument value.  marg_getRef returns a pointer to a type-name argument value.

marg_setValue() See marg_getValue()

method_getArgumentInfo() See method_getNumberOfArguments()

method_getNumberOfArguments(), method_getSizeOfArguments(), method_getArgumentInfo()

SUMMARY Get information about a method

DECLARED IN objc/objc-class.h

SYNOPSIS unsigned int method_getNumberOfArguments(Method aMethod)

unsigned int method_getSizeOfArguments(Method aMethod)
unsigned int method_getArgumentInfo(Method aMethod, int index, const char **type, int *offset)

DESCRIPTION The three functions described here all provide information about the argument structure of a particular method.  They take as their first argument the method's data structure, aMethod, which can be obtained by calling class_getInstanceMethod() or class_getClassMethod().

The first function, method_getNumberOfArguments(), returns the number of arguments that aMethod takes.  This will be at least two, since it includes the "hidden" arguments, self and _cmd, which are the first two arguments passed to every method implementation.

The second function, method_getSizeOfArguments(), returns the number of bytes that all of aMethod's arguments, taken together, would occupy on the stack.  This information is required by objc_msgSendv().

The third function, method_getArgumentInfo(), takes an index into aMethod's argument list and returns, by reference, the type of the argument and the offset to the location of that argument in the list.  Indices begin with 0.  The "hidden" arguments self and _cmd are indexed at 0 and 1; method-specific arguments begin at index 2.  If index is too large for the actual number of arguments, the type aand offset pointers are set to NULL.  Otherwise, the offset is measured in bytes; it depends entirely on the size of arguments preceding the one at index.  The type is encoded according to the conventions of the @encode() compiler directive.

The information obtained from method_getArgumentInfo() can be used in the marg_getValue, marg_getRef, and marg_setValue macros to examine and alter the values of an argument on the stack after aMethod has been called.  The offset can be passed directly to these macros, but the type must first be decoded to a full type name.

RETURN method_getNumberOfArguments() returns how many arguments the implementation of aMethod takes, and method_getSizeOfArguments() returns how many bytes the arguments take up on the stack.  method_getArgumentInfo() returns the index it is passed.

method_getSizeOfArguments() See method_getNumberOfArguments()

objc_addClass() See objc_getClass()

objc_getClass(), objc_lookUpClass(), objc_getMetaClass(), objc_getClasses(), objc_addClass(), objc_getModules()

SUMMARY Manage run-time structures

DECLARED IN objc/objc-runtime.h

SYNOPSIS id objc_getClass(const char *aClassName)

id objc_lookUpClass(const char *aClassName)
id objc_getMetaClass(const char *aClassName)
NXHashTable *objc_getClasses(void)
void objc_addClass(Class aClass)
Module *objc_getModules(void)

DESCRIPTION These functions return and modify the principal data structures used by the run-time system.

The first two functions, objc_getClass() and objc_lookUpClass(), both return the id of the class object for the aClassName class.  However, if the aClassName class isn't known to the run-time system, objc_getClass() prints a message to the standard error stream and causes the process to abort, while objc_lookUpClass() merely returns nil.

The third function, objc_getMetaClass(), returns the id of the metaclass object for the aClassName class.  The metaclass object holds information used by the class object, just as the class object holds information used by instances of the class.  Like objc_getClass(), it prints a message to the standard error stream and causes the process to abort if aClassName isn't a valid class.

objc_getClasses() returns a pointer to the hash table containing all the Objective C classes that are currently known to the run-time system.  You can examine the table using the common hashing functions.  In the following example, NXNextHashState() gets each class from the table in turn, and object_getClassName() asks for their names:

NXHashTable *classes = objc_getClasses();
NXHashState state = NXInitHashState(classes);
Class thisClass;

while ( NXNextHashState(classes, &state, (void **)&thisClass) )
fprintf(stderr, "%s\n", object_getClassName((id)thisClass));

The NXHashTable type returned by objc_getClasses() is defined in the objc/hashtable.h header file and is documented in Chapter 3, "Common Classes and Functions."  This data structure can be read, as illustrated in the example above, but it should not be modified or freed.

objc_addClass() adds aClass to the list of classes known to the run-time system.  (The class is added to the hash table that objc_getClasses() returns.)

The compiler creates a Module data structure for each file it compiles.  The objc_getModules() function returns a pointer to the run-time system's list of all current modules, except those that were dynamically loaded.  Module structures are described under "Supporting Header Files" later in this chapter.

RETURN objc_lookUpClass() returns the class object for aClassName, or nil if there is no such class.  objc_getClass() and objc_getMetaClass()  return the class and metaclass objects for aClassName, if such a class exists, and abort otherwise. objc_getClasses() returns a pointer to a hash table of all current classes, and objc_getModules() returns a pointer to all current modules.

objc_getClasses() See objc_getClass()

objc_getMetaClass() See objc_getClass()

objc_getModules() See objc_getClass()

objc_loadModules(), objc_unloadModules()

SUMMARY Dynamically load and unload classes

DECLARED IN objc/objc-load.h

SYNOPSIS long objc_loadModules(char *files[], NXStream *stream, void (*callback)(Class, Category), struct mach_header **header, char *debugFilename)

long objc_unloadModules(NXStream *stream, void (*callback)(Class, Category))

DESCRIPTION objc_loadModules() dynamically loads object files containing Objective C class and category definitions into a running program.  Its first argument, files, is a list of null-terminated pathnames for the object files containing the classes and categories that are to be loaded.  They can be full paths or paths relative to the current working directory.  The second argument, stream, is a pointer to an NXStream where any error messages produced by the loader will be written.  It can be NULL, in which case no messages will be written.

The third argument, callback, allows you to specify a function that will be called immediately after each class or category is loaded.  When a category is loaded, the function is passed both the Category structure and the Class structure for that category. When a class is loaded, it's passed only the Class structure.  Like stream, callback can be NULL.

The fourth argument, header, is used to get a pointer to the mach_header structure for the loaded modules.  It, too, can be NULL.  All the modules in files are grouped under the same header.

The final argument, which also can be NULL, is the pathname for a file that the loader will create and initialize with a copy of the loaded modules.  This file can be passed to the debugger and added to the list of files being debugged.  For example:

(gdb) add-file debugFilename

obj_unloadModules() unloads all the modules loaded by objc_loadModules(), that is, all the modules from the files list.  Each time it's called, it unloads another set of modules, working its way back from the modules loaded by the most recent call to objc_loadModules() to those loaded by the next most recent call, and so on.

The first argument to obj_unloadModules(), stream, is a pointer to an NXStream where error messages will be written.  Its second argument, callback, allows you to specify a function that will be called immediately before each class or category is unloaded.  Both arguments can be NULL.

Note:  The NXBundle class, documented in Chapter 3, "Common Classes and Functions," provides a simpler and preferred way to dynamically load classes.  NXBundle integrates dynamic loading with localization (using language-specific resources such as strings, images, and sounds).

RETURN Both functions return 0 if the modules are successfully loaded or unloaded and 1 if they're not.

objc_lookUpClass() See objc_getClass()

objc_msgSend(), objc_msgSendSuper(), objc_msgSendv()

SUMMARY Send messages at run time

DECLARED IN objc/objc-runtime.h

SYNOPSIS id objc_msgSend(id theReceiver, SEL theSelector, ...)

id objc_msgSendSuper(struct objc_super *superContext, SEL theSelector, ...)
id objc_msgSendv(id theReceiver, SEL theSelector, unsigned int argSize, marg_list argFrame)

DESCRIPTION The compiler converts every message expression into a call on one of the first two of these three functions.  Messages to super are converted to calls on objc_msgSendSuper(); all others are converted to calls on objc_msgSend().

Both functions find the implementation of the theSelector method that's appropriate for the receiver of the message.  For objc_msgSend(), theReceiver is passed explicitly as an argument.  For objc_msgSendSuper(), superContext defines the context in which the message was sent, including who the receiver is.

Arguments that are included in the aSelector message are passed directly as additional arguments to both functions.

Calls to objc_msgSend() and objc_msgSendSuper() should be generated only by the compiler.  You shouldn't call them directly in the Objective C code you write.  You can however use the Object instance method performv:: to send an arbitrary message to an object..

The third function, objc_msgSendv(), is an alternative to objc_msgSend() that's designed to be used within class-specific implementations of the forward:: method.  Instead of being passed each of the arguments to the aSelector message, it takes a pointer to the arguments list, argFrame, and the size of the list in bytes, argSizeargSize can be obtained by calling method_getSizeOfArguments(); argFrame is passed as the second argument to the forward:: method.

objc_msgSendv() parses the argument list based on information stored for aSelector and the class of the receiver.  Because of this additional work, it's more expensive than objc_msgSend().

RETURN Each method passes on the value returned by the aSelector method.

objc_msgSendSuper() See objc_msgSend()

objc_msgSendv() See objc_msgSend()


SUMMARY Make the run-time system thread safe

DECLARED IN objc/objc-runtime.h

SYNOPSIS void objc_setMultithreaded(BOOL flag)

DESCRIPTION When flag is YES, this function ensures that two or more threads of the same task can safely use the run-time system for Objective C.  To guarantee correct run-time behavior, it should be called immediately before starting up a new thread.

Because of the additional checking required to ensure thread-safe behavior, messaging will be slower than normal.  Therefore, flag should be reset to the default NO when there is only one thread using Objective C.

This function cannot guarantee that all parts of the run-time system are absolutely thread-safe.  In particular, if one thread is in the middle of dynamically loading or unloading a class (using objc_loadModules() or objc_unloadModules()) while another thread is using the class, the second thread might find the class in an inconsistent state.  Similarly, a thread that gets the class hash table (using objc_getClasses()) cannot be sure that another thread won't be modifying it at the same time.

objc_unloadModules() See objc_loadModules()

object_copy() See object_dispose()

object_copyFromZone() See object_dispose()

object_dispose(), object_copy(), object_copyFromZone(),
object_realloc(), object_reallocFromZone()

SUMMARY Manage object memory

DECLARED IN objc/Object.h

SYNOPSIS id object_dispose(Object *anObject)

id object_copy(Object *anObject, unsigned int indexedIvarBytes)
id object_copyFromZone(Object *anObject, unsigned int indexedIvarBytes, NXZone *zone)
id object_realloc(Object *anObject, unsigned int numBytes)
id object_reallocFromZone(Object *anObject, unsigned int numBytes, NXZone *zone)

DESCRIPTION These five functions, along with class_createInstance() and class_createInstanceFromZone(), manage the dynamic allocation of memory for objects.  Like those two functions, each of them is simply a "cover" for--a way of calling--another, private function.

object_dispose() frees the memory occupied by anObject after setting its isa instance variable to nil, and returns nil.  The function it calls to do this work can be changed by reassigning the _dealloc variable.

object_copy() and object_copyFromZone() create a new object that's an exact copy of anObject and return the new object. object_copy() allocates memory for the copy from the same zone as the original; object_copyFromZone() places the copy in zone.  The second argument to both functions, indexedIvarBytes, specifies the number of additional bytes that should be allocated to accommodate indexed instance variables; it serves the same purpose as the second argument to class_createInstance().

The functions that object_copy() and object_copyFromZone() call to do their work can be changed by reassigning the _copy and _zoneCopy variables.

object_realloc() and object_reallocFromZone() reallocate storage for anObject, adding numBytes if possible.  The memory previously occupied by anObject is freed if it can't be reused, and a pointer to the new location of anObject is returned. object_realloc() allocates memory for the object from the same zone that it originally occupied; object_reallocFromZone() locates the object in zone.

The functions that object_realloc() and object_reallocFromZone() call to do their work can be changed by reassigning the _realloc and _zoneRealloc variables.

RETURN object_dispose() returns nil, object_copy() and object_copyFromZone() return the copy, and object_realloc() and object_reallocFromZone() return the reallocated object.  If the attempt to copy or reallocate the object fails, an error message is generated and abort() is called.


SUMMARY Return the class name

DECLARED IN objc/objc.h

SYNOPSIS const char *object_getClassName(id anObject)

DESCRIPTION This function returns the name of anObject's class, or the string "nil" if anObject is nilanObject can be either an instance or a class object.


SUMMARY Return a pointer to an object's extra memory

DECLARED IN objc/objc.h

SYNOPSIS void *object_getIndexedIvars(id anObject)

This function returns a pointer to the first indexed instance variable of anObject, if anObject has indexed instance variables.  If not, the pointer returned won't be valid and should not be used.

SEE ALSO class_createInstance()

object_getInstanceVariable() See object_setInstanceVariable()

object_realloc() See object_dispose()

object_reallocFromZone() See object_dispose()

object_setInstanceVariable(), object_getInstanceVariable()

SUMMARY Set and get instance variables

DECLARED IN objc/Object.h

SYNOPSIS Ivar object_setInstanceVariable(id anObject, const char *variableName, void *value)

Ivar object_getInstanceVariable(id anObject, const char *variableName, void **value)

DESCRIPTION object_setInstanceVariable() assigns a new value to the variableName instance variable belonging to anObject.  The instance variable must be one that's declared as a pointer; typically it's an id.  The new value of the pointer is passed in the third argument, value.  (Note that the pointer value is passed directly, not by reference.)

object_getInstanceVariable() gets the value of the pointer stored as anObject's variableName instance variable.  The pointer is returned by reference through the third argument, value.  For example:

int *i;
Ivar var = object_getInstanceVariable(anObject, "num", (void **)&i);

These functions provide a way of setting and getting instance variables that are declared as pointers, without having to implement methods for that purpose.  For example, Interface Builder calls object_setInstanceVariable() to initialize programmer-defined "outlet" instance variables.

These functions cannot reliably be used to set and get instance variables that are not pointers.

RETURN Both functions return a pointer to the class template that describes the variableName instance variable.  A NULL pointer is returned if anObject has no instance variable with that name.

The returned template has a field describing the data type of the instance variable.  You can check it to be sure that the value set is of the correct type.

sel_getName() See sel_getUid()

sel_getUid(), sel_getName()

SUMMARY Match method names with method selectors

DECLARED IN objc/objc.h

SYNOPSIS SEL sel_getUid(const char *aName)

const char *sel_getName(SEL aSelector)

DESCRIPTION The first function, sel_getUid(), returns the unique identifier that represents the aName method at run time.  The identifier is a selector (type SEL) and is used in place of the method name in compiled code; methods with the same name have the same selector.  Whenever possible, you should use the @selector() directive to ask the compiler to provide the selector for a method. This function asks the run-time system for the selector and should be used only if the name isn't known at compile time.

The second function, sel_getName(), is the inverse of the first.  It returns the name that was mapped to aSelector.

RETURN sel_getUid() returns the selector for the aName method, or 0 if there is no known method with that name.  sel_getName() returns a character string with the name of the method identified by the aSelector selector.  If aSelector isn't a valid selector, a NULL pointer is returned.


SUMMARY Determine whether a selector is valid

DECLARED IN objc/objc.h

SYNOPSIS BOOL sel_isMapped(SEL aSelector)

RETURN sel_isMapped() returns YES if aSelector is a valid selector (is currently mapped to a method implementation) or could possibly be one (because it lies within the same range as valid selectors); otherwise it returns NO.

Because all of a program's selectors are guaranteed to be mapped at start-up, this function has little real use.  It's included here for reasons of backward compatibility only.


SUMMARY Register a method name

DECLARED IN objc/objc.h

SYNOPSIS SEL sel_registerName(const char *aName)

DESCRIPTION This function registers aName as a method name and causes it to be mapped to a SEL selector, which it returns.

No check is made to see if aName is already a valid method name.  If it is, the same name will be mapped to more than one selector.  When the run-time system needs to match a selector to the name, it's indeterminant which one it will find.

RETURN sel_registerName() returns the selector it maps to the aString method.