Objective-C Runtime Reference

Declared in	IONDRVLibraries.h NSObjCRuntime.h wintypes.h
Companion guides	Objective-C Runtime Programming Guide The Objective-C Programming Language

Overview

This document describes the OS X Objective-C 2.0 runtime library support functions and data structures. The functions are implemented in the shared library found at /usr/lib/libobjc.A.dylib. This shared library provides support for the dynamic properties of the Objective-C language, and as such is linked to by all Objective-C applications.

This reference is useful primarily for developing bridge layers between Objective-C and other languages, or for low-level debugging. You typically do not need to use the Objective-C runtime library directly when programming in Objective-C.

The OS X implementation of the Objective-C runtime library is unique to the Mac. For other platforms, the GNU Compiler Collection provides a different implementation with a similar API. This document covers only the OS X implementation.

The low-level Objective-C runtime API is significantly updated in OS X version 10.5. Many functions and all existing data structures are replaced with new functions. The old functions and structures are deprecated in 32-bit and absent in 64-bit mode. The API constrains several values to 32-bit ints even in 64-bit mode—class count, protocol count, methods per class, ivars per class, arguments per method, sizeof(all arguments) per method, and class version number. In addition, the new Objective-C ABI (not described here) further constrains sizeof(anInstance) to 32 bits, and three other values to 24 bits—methods per class, ivars per class, and sizeof(a single ivar). Finally, the obsolete NXHashTable and NXMapTable are limited to 4 billion items.

“Deprecated” below means “deprecated in OS X version 10.5 for 32-bit code, and disallowed for 64-bit code.”

Who Should Read This Document

The document is intended for readers who might be interested in learning about the Objective-C runtime.

Because this isn’t a document about C, it assumes some prior acquaintance with that language. However, it doesn’t have to be an extensive acquaintance.

Functions by Task

Working with Classes

Adding Classes

Instantiating Classes

Working with Instances

Obtaining Class Definitions

Working with Instance Variables

Associative References

Sending Messages

When it encounters a method invocation, the compiler might generate a call to any of several functions to perform the actual message dispatch, depending on the receiver, the return value, and the arguments. You can use these functions to dynamically invoke methods from your own plain C code, or to use argument forms not permitted by NSObject’s perform... methods. These functions are declared in /usr/include/objc/objc-runtime.h.

objc_msgSend sends a message with a simple return value to an instance of a class.
objc_msgSend_stret sends a message with a data-structure return value to an instance of a class.
objc_msgSendSuper sends a message with a simple return value to the superclass of an instance of a class.
objc_msgSendSuper_stret sends a message with a data-structure return value to the superclass of an instance of a class.

Working with Methods

Working with Selectors

Working with Protocols

Working with Properties

Functions

class_addIvar

Adds a new instance variable to a class.

BOOL class_addIvar(Class cls, const char *name, size_t size, uint8_t alignment, const char *types)

Return Value

YES if the instance variable was added successfully, otherwise NO (for example, the class already contains an instance variable with that name).

Discussion

This function may only be called after objc_allocateClassPair and before objc_registerClassPair. Adding an instance variable to an existing class is not supported.

The class must not be a metaclass. Adding an instance variable to a metaclass is not supported.

The instance variable's minimum alignment in bytes is 1<<align. The minimum alignment of an instance variable depends on the ivar's type and the machine architecture. For variables of any pointer type, pass log2(sizeof(pointer_type)).

Declared In

runtime.h

class_addMethod

Adds a new method to a class with a given name and implementation.

BOOL class_addMethod(Class cls, SEL name, IMP imp, const char *types)

Parameters

cls: The class to which to add a method.
name: A selector that specifies the name of the method being added.
imp: A function which is the implementation of the new method. The function must take at least two arguments—self and _cmd.
types: An array of characters that describe the types of the arguments to the method. For possible values, see Objective-C Runtime Programming Guide > “Type Encodings”. Since the function must take at least two arguments—self and _cmd, the second and third characters must be “@:” (the first character is the return type).

Return Value

YES if the method was added successfully, otherwise NO (for example, the class already contains a method implementation with that name).

Discussion

class_addMethod will add an override of a superclass's implementation, but will not replace an existing implementation in this class. To change an existing implementation, use method_setImplementation.

An Objective-C method is simply a C function that take at least two arguments—self and _cmd. For example, given the following function:

void myMethodIMP(id self, SEL _cmd)

    // implementation ....

you can dynamically add it to a class as a method (called resolveThisMethodDynamically) like this:

class_addMethod([self class], @selector(resolveThisMethodDynamically), (IMP) myMethodIMP, "v@:");

Declared In

runtime.h

class_addProtocol

Adds a protocol to a class.

BOOL class_addProtocol(Class cls, Protocol *protocol)

Parameters

cls: The class to modify.
outCount: The protocol to add to cls.

Return Value

YES if the method was added successfully, otherwise NO (for example, the class already conforms to that protocol).

Declared In

runtime.h

class_conformsToProtocol

Returns a Boolean value that indicates whether a class conforms to a given protocol.

BOOL class_conformsToProtocol(Class cls, Protocol *protocol)

Parameters

cls: The class you want to inspect.
protocol: A protocol.

Return Value

YES if cls conforms to protocol, otherwise NO.

Discussion

You should usually use NSObject‘s conformsToProtocol: method instead of this function.

Declared In

runtime.h

class_copyIvarList

Describes the instance variables declared by a class.

Ivar * class_copyIvarList(Class cls, unsigned int *outCount)

Parameters

cls: The class to inspect.
outCount: On return, contains the length of the returned array. If outCount is NULL, the length is not returned.

Return Value

An array of pointers of type Ivar describing the instance variables declared by the class. Any instance variables declared by superclasses are not included. The array contains *outCount pointers followed by a NULL terminator. You must free the array with free().

If the class declares no instance variables, or cls is Nil, NULL is returned and *outCount is 0.

Declared In

runtime.h

class_copyMethodList

Describes the instance methods implemented by a class.

Method * class_copyMethodList(Class cls, unsigned int *outCount)

Parameters

cls: The class you want to inspect.
outCount: On return, contains the length of the returned array. If outCount is NULL, the length is not returned.

Return Value

An array of pointers of type Method describing the instance methods implemented by the class—any instance methods implemented by superclasses are not included. The array contains *outCount pointers followed by a NULL terminator. You must free the array with free().

If cls implements no instance methods, or cls is Nil, returns NULL and *outCount is 0.

Discussion

To get the class methods of a class, use class_copyMethodList(object_getClass(cls), &count).

To get the implementations of methods that may be implemented by superclasses, use class_getInstanceMethod or class_getClassMethod.

Declared In

runtime.h

class_copyPropertyList

Describes the properties declared by a class.

objc_property_t * class_copyPropertyList(Class cls, unsigned int *outCount)

Parameters

cls: The class you want to inspect.
outCount: On return, contains the length of the returned array. If outCount is NULL, the length is not returned.

Return Value

An array of pointers of type objc_property_t describing the properties declared by the class. Any properties declared by superclasses are not included. The array contains *outCount pointers followed by a NULL terminator. You must free the array with free().

If cls declares no properties, or cls is Nil, returns NULL and *outCount is 0.

Declared In

runtime.h

class_copyProtocolList

Describes the protocols adopted by a class.

Protocol ** class_copyProtocolList(Class cls, unsigned int *outCount)

Parameters

cls: The class you want to inspect.
outCount: On return, contains the length of the returned array. If outCount is NULL, the length is not returned.

Return Value

An array of pointers of type Protocol* describing the protocols adopted by the class. Any protocols adopted by superclasses or other protocols are not included. The array contains *outCount pointers followed by a NULL terminator. You must free the array with free().

If cls adopts no protocols, or cls is Nil, returns NULL and *outCount is 0.

Declared In

runtime.h

class_createInstance

Creates an instance of a class, allocating memory for the class in the default malloc memory zone.

id class_createInstance(Class cls, size_t extraBytes)

Parameters

cls: The class that you wish to allocate an instance of.
extraBytes: An integer indicating the number of extra bytes to allocate. The additional bytes can be used to store additional instance variables beyond those defined in the class definition.

Return Value

An instance of the class cls.

Declared In

runtime.h

class_getClassMethod

Returns a pointer to the data structure describing a given class method for a given class.

Method class_getClassMethod(Class aClass, SEL aSelector)

Parameters

aClass: A pointer to a class definition. Pass the class that contains the method you want to retrieve.
aSelector: A pointer of type SEL. Pass the selector of the method you want to retrieve.

Return Value

A pointer to the Method data structure that corresponds to the implementation of the selector specified by aSelector for the class specified by aClass, or NULL if the specified class or its superclasses do not contain a class method with the specified selector.

Discussion

Note that this function searches superclasses for implementations, whereas class_copyMethodList does not.

Declared In

runtime.h

class_getClassVariable

Returns the Ivar for a specified class variable of a given class.

Ivar class_getClassVariable(Class cls, const char* name)

Parameters

cls: The class definition whose class variable you wish to obtain.
name: The name of the class variable definition to obtain.

Return Value

A pointer to an Ivar data structure containing information about the class variable specified by name.

Declared In

runtime.h

class_getInstanceMethod

Returns a specified instance method for a given class.

Method class_getInstanceMethod(Class aClass, SEL aSelector)

Parameters

aClass: The class you want to inspect.
aSelector: The selector of the method you want to retrieve.

Return Value

The method that corresponds to the implementation of the selector specified by aSelector for the class specified by aClass, or NULL if the specified class or its superclasses do not contain an instance method with the specified selector.

Discussion

Note that this function searches superclasses for implementations, whereas class_copyMethodList does not.

Declared In

runtime.h

class_getInstanceSize

Returns the size of instances of a class.

size_t class_getInstanceSize(Class cls)

Parameters

cls: A class object.

Return Value

The size in bytes of instances of the class cls, or 0 if cls is Nil.

Declared In

runtime.h

class_getInstanceVariable

Returns the Ivar for a specified instance variable of a given class.

Ivar class_getInstanceVariable(Class cls, const char* name)

Parameters

cls: The class whose instance variable you wish to obtain.
name: The name of the instance variable definition to obtain.

Return Value

A pointer to an Ivar data structure containing information about the instance variable specified by name.

Declared In

runtime.h

class_getIvarLayout

Returns a description of the Ivar layout for a given class.

const char *class_getIvarLayout(Class cls)

Parameters

cls: The class to inspect.

Return Value

A description of the Ivar layout for cls.

Declared In

runtime.h

class_getMethodImplementation

Returns the function pointer that would be called if a particular message were sent to an instance of a class.

IMP class_getMethodImplementation(Class cls, SEL name)

Parameters

cls: The class you want to inspect.
name: A selector.

Return Value

The function pointer that would be called if [object name] were called with an instance of the class, or NULL if cls is Nil.

Discussion

class_getMethodImplementation may be faster than method_getImplementation(class_getInstanceMethod(cls, name)).

The function pointer returned may be a function internal to the runtime instead of an actual method implementation. For example, if instances of the class do not respond to the selector, the function pointer returned will be part of the runtime's message forwarding machinery.

Declared In

runtime.h

class_getMethodImplementation_stret

Returns the function pointer that would be called if a particular message were sent to an instance of a class.

IMP class_getMethodImplementation_stret(Class cls, SEL name)

Parameters

cls: The class you want to inspect.
name: A selector.

Return Value

The function pointer that would be called if [object name] were called with an instance of the class, or NULL if cls is Nil.

Declared In

runtime.h

class_getName

Returns the name of a class.

const char * class_getName(Class cls)

Parameters

cls: A class object.

Return Value

The name of the class, or the empty string if cls is Nil.

Declared In

runtime.h

class_getProperty

Returns a property with a given name of a given class.

objc_property_t class_getProperty(Class cls, const char *name)

Return Value

A pointer of type objc_property_t describing the property, or NULL if the class does not declare a property with that name, or NULL if cls is Nil.

Declared In

runtime.h

class_getSuperclass

Returns the superclass of a class.

Class class_getSuperclass(Class cls)

Parameters

cls: A class object.

Return Value

The superclass of the class, or Nil if cls is a root class, or Nil if cls is Nil.

Discussion

You should usually use NSObject‘s superclass method instead of this function.

Declared In

runtime.h

class_getVersion

Returns the version number of a class definition.

int class_getVersion(Class theClass)

Parameters

theClass: A pointer to an Class data structure. Pass the class definition for which you wish to obtain the version.

Return Value

An integer indicating the version number of the class definition.

Discussion

You can use the version number of the class definition to provide versioning of the interface that your class represents to other classes. This is especially useful for object serialization (that is, archiving of the object in a flattened form), where it is important to recognize changes to the layout of the instance variables in different class-definition versions.

Classes derived from the Foundation framework NSObject class can obtain the class-definition version number using the getVersion class method, which is implemented using the class_getVersion function.

Declared In

runtime.h

class_getWeakIvarLayout

Returns a description of the layout of weak Ivars for a given class.

const char *class_getWeakIvarLayout(Class cls)

Parameters

cls: The class to inspect.

Return Value

A description of the layout of the weak Ivars for cls.

Declared In

runtime.h

class_isMetaClass

Returns a Boolean value that indicates whether a class object is a metaclass.

BOOL class_isMetaClass(Class cls)

Parameters

cls: A class object.

Return Value

YES if cls is a metaclass, NO if cls is a non-meta class, NO if cls is Nil.

Declared In

runtime.h

class_replaceMethod

Replaces the implementation of a method for a given class.

IMP class_replaceMethod(Class cls, SEL name, IMP imp, const char *types)

Parameters

cls: The class you want to modify.
name: A selector that identifies the method whose implementation you want to replace.
imp: The new implementation for the method identified by name for the class identified by cls.
types: An array of characters that describe the types of the arguments to the method. For possible values, see Objective-C Runtime Programming Guide > “Type Encodings”. Since the function must take at least two arguments—self and _cmd, the second and third characters must be “@:” (the first character is the return type).

Return Value

The previous implementation of the method identified by name for the class identified by cls.

Discussion

This function behaves in two different ways:

If the method identified by name does not yet exist, it is added as if class_addMethod were called. The type encoding specified by types is used as given.
If the method identified by name does exist, its IMP is replaced as if method_setImplementation were called. The type encoding specified by types is ignored.

Declared In

runtime.h

class_respondsToSelector

Returns a Boolean value that indicates whether instances of a class respond to a particular selector.

BOOL class_respondsToSelector(Class cls, SEL sel)

Parameters

cls: The class you want to inspect.
sel: A selector.

Return Value

YES if instances of the class respond to the selector, otherwise NO.

Discussion

You should usually use NSObject's respondsToSelector: or instancesRespondToSelector: methods instead of this function.

Declared In

runtime.h

class_setIvarLayout

Sets the Ivar layout for a given class.

void class_setIvarLayout(Class cls, const char *layout)

Parameters

cls: The class to modify.
layout: The layout of the Ivars for cls.

Declared In

runtime.h

class_setSuperclass

Sets the superclass of a given class.

Class class_setSuperclass(Class cls, Class newSuper)

Parameters

cls: The class whose superclass you want to set.
newSuper: The new superclass for cls.

Return Value

The old superclass for cls.

Special Considerations

You should not use this function.

Declared In

runtime.h

class_setVersion

Sets the version number of a class definition.

void class_setVersion(Class theClass, int version)

Parameters

theClass: A pointer to an Class data structure. Pass the class definition for which you wish to set the version.
version: An integer. Pass the new version number of the class definition.

Discussion

Classes derived from the Foundation framework NSObject class can set the class-definition version number using the setVersion: class method, which is implemented using the class_setVersion function.

Declared In

runtime.h

class_setWeakIvarLayout

Sets the layout for weak Ivars for a given class.

void class_setWeakIvarLayout(Class cls, const char *layout)

Parameters

cls: The class to modify.
layout: The layout of the weak Ivars for cls.

Declared In

runtime.h

ivar_getName

Returns the name of an instance variable.

const char * ivar_getName(Ivar ivar)

Return Value

A C string containing the instance variable's name.

Declared In

runtime.h

ivar_getOffset

Returns the offset of an instance variable.

ptrdiff_t ivar_getOffset(Ivar ivar)

Discussion

For instance variables of type id or other object types, call object_getIvar and object_setIvar instead of using this offset to access the instance variable data directly.

Declared In

runtime.h

ivar_getTypeEncoding

Returns the type string of an instance variable.

const char * ivar_getTypeEncoding(Ivar ivar)

Return Value

A C string containing the instance variable's type encoding.

Discussion

For possible values, see Objective-C Runtime Programming Guide > “Type Encodings”.

Declared In

runtime.h

method_copyArgumentType

Returns a string describing a single parameter type of a method.

char * method_copyArgumentType(Method method, unsigned int index)

Parameters

method: The method to inspect.
index: The index of the parameter to inspect.

Return Value

A C string describing the type of the parameter at index index, or NULL if method has no parameter index index. You must free the string with free().

Declared In

runtime.h

method_copyReturnType

Returns a string describing a method's return type.

char * method_copyReturnType(Method method)

Parameters

method: The method to inspect.

Return Value

A C string describing the return type. You must free the string with free().

Declared In

runtime.h

method_exchangeImplementations

Exchanges the implementations of two methods.

void method_exchangeImplementations(Method m1, Method m2)

Discussion

This is an atomic version of the following:

IMP imp1 = method_getImplementation(m1);

IMP imp2 = method_getImplementation(m2);

method_setImplementation(m1, imp2);

method_setImplementation(m2, imp1);

Declared In

runtime.h

method_getArgumentType

Returns by reference a string describing a single parameter type of a method.

void method_getArgumentType(Method method, unsigned int index, char *dst, size_t dst_len)

Discussion

The parameter type string is copied to dst. dst is filled as if strncpy(dst, parameter_type, dst_len) were called. If the method contains no parameter with that index, dst is filled as if strncpy(dst, "", dst_len) were called.

Declared In

runtime.h

method_getImplementation

Returns the implementation of a method.

IMP method_getImplementation(Method method)

Parameters

method: The method to inspect.

Return Value

A function pointer of type IMP.

Declared In

runtime.h

method_getName

Returns the name of a method.

SEL method_getName(Method method)

Parameters

method: The method to inspect.

Return Value

A pointer of type SEL.

Discussion

To get the method name as a C string, call sel_getName(method_getName(method)).

Declared In

runtime.h

method_getNumberOfArguments

Returns the number of arguments accepted by a method.

unsigned method_getNumberOfArguments(Method method)

Parameters

method: A pointer to a Method data structure. Pass the method in question.

Return Value

An integer containing the number of arguments accepted by the given method.

method_getReturnType

Returns by reference a string describing a method's return type.

void method_getReturnType(Method method, char *dst, size_t dst_len)

Discussion

The method's return type string is copied to dst. dst is filled as if strncpy(dst, parameter_type, dst_len) were called.

Declared In

runtime.h

method_getTypeEncoding

Returns a string describing a method's parameter and return types.

const char * method_getTypeEncoding(Method method)

Parameters

method: The method to inspect.

Return Value

A C string. The string may be NULL.

Declared In

runtime.h

method_setImplementation

Sets the implementation of a method.

IMP method_setImplementation(Method method, IMP imp)

Return Value

The previous implementation of the method.

Declared In

runtime.h

objc_allocateClassPair

Creates a new class and metaclass.

objc_allocateClassPair(Class superclass, const char *name, size_t extraBytes)

Parameters

superclass: The class to use as the new class's superclass, or Nil to create a new root class.
name: The string to use as the new class's name. The string will be copied.
extraBytes: The number of bytes to allocate for indexed ivars at the end of the class and metaclass objects. This should usually be 0.

Return Value

The new class, or Nil if the class could not be created (for example, the desired name is already in use).

Discussion

You can get a pointer to the new metaclass by calling object_getClass(newClass).

To create a new class, start by calling objc_allocateClassPair. Then set the class's attributes with functions like class_addMethod and class_addIvar. When you are done building the class, call objc_registerClassPair. The new class is now ready for use.

Instance methods and instance variables should be added to the class itself. Class methods should be added to the metaclass.

Declared In

runtime.h

objc_copyProtocolList

Returns an array of all the protocols known to the runtime.

Protocol **objc_copyProtocolList(unsigned int *outCount)

Parameters

outCount: Upon return, contains the number of protocols in the returned array.

Return Value

A C array of all the protocols known to the runtime. The array contains *outCount pointers followed by a NULL terminator. You must free the list with free().

Discussion

This function acquires the runtime lock.

Declared In

runtime.h

objc_duplicateClass

Used by Foundation's Key-Value Observing.

objc_duplicateClass

Special Considerations

Do not call this function yourself.

Declared In

runtime.h

objc_getAssociatedObject

Returns the value associated with a given object for a given key.

id objc_getAssociatedObject(id object, void *key)

Parameters

object: The source object for the association.
key: The key for the association.

Return Value

The value associated with the key key for object.

objc_getClass

Returns the class definition of a specified class.

id objc_getClass(const char *name)

Parameters

name: The name of the class to look up.

Return Value

The Class object for the named class, or nil if the class is not registered with the Objective-C runtime.

Discussion

objc_getClass is different from objc_lookUpClass in that if the class is not registered, objc_getClass calls the class handler callback and then checks a second time to see whether the class is registered. objc_lookUpClass does not call the class handler callback.

Special Considerations

Earlier implementations of this function (prior to OS X v10.0) terminate the program if the class does not exist.

objc_getClassList

Obtains the list of registered class definitions.

int objc_getClassList(Class *buffer, int bufferLen)

Parameters

buffer: An array of Class values. On output, each Class value points to one class definition, up to either bufferLen or the total number of registered classes, whichever is less. You can pass NULL to obtain the total number of registered class definitions without actually retrieving any class definitions.
bufferLen: An integer value. Pass the number of pointers for which you have allocated space in buffer. On return, this function fills in only this number of elements. If this number is less than the number of registered classes, this function returns an arbitrary subset of the registered classes.

Return Value

An integer value indicating the total number of registered classes.

Discussion

The Objective-C runtime library automatically registers all the classes defined in your source code. You can create class definitions at runtime and register them with the objc_addClass function.

Listing 1 demonstrates how to use this function to retrieve all the class definitions that have been registered with the Objective-C runtime in the current process.

Listing 1 Using objc_getClassList

int numClasses;

Class * classes = NULL;

classes = NULL;

numClasses = objc_getClassList(NULL, 0);

if (numClasses > 0 )

    classes = malloc(sizeof(Class) * numClasses);

    numClasses = objc_getClassList(classes, numClasses);

    free(classes);

Special Considerations

You cannot assume that class objects you get from this function are classes that inherit from NSObject, so you cannot safely call any methods on such classes without detecting that the method is implemented first.

objc_getFutureClass

Used by CoreFoundation's toll-free bridging.

Class objc_getFutureClass(const char *name)

Special Considerations

Do not call this function yourself.

Declared In

runtime.h

objc_getMetaClass

Returns the metaclass definition of a specified class.

id objc_getMetaClass(const char *name)

Parameters

name: The name of the class to look up.

Return Value

The Class object for the metaclass of the named class, or nil if the class is not registered with the Objective-C runtime.

Discussion

If the definition for the named class is not registered, this function calls the class handler callback and then checks a second time to see if the class is registered. However, every class definition must have a valid metaclass definition, and so the metaclass definition is always returned, whether it’s valid or not.

objc_getProtocol

Returns a specified protocol.

Protocol *objc_getProtocol(const char *name)

Parameters

name: The name of a protocol.

Return Value

The protocol named name, or NULL if no protocol named name could be found.

Discussion

This function acquires the runtime lock.

Declared In

runtime.h

objc_getRequiredClass

Returns the class definition of a specified class.

id objc_getRequiredClass(const char *name)

Parameters

name: The name of the class to look up.

Return Value

The Class object for the named class.

Discussion

This function is the same as objc_getClass, but kills the process if the class is not found.

This function is used by ZeroLink, where failing to find a class would be a compile-time link error without ZeroLink.

Declared In

runtime.h

objc_lookUpClass

Returns the class definition of a specified class.

id objc_lookUpClass(const char *name)

Parameters

name: The name of the class to look up.

Return Value

The Class object for the named class, or nil if the class is not registered with the Objective-C runtime.

Discussion

objc_getClass is different from this function in that if the class is not registered, objc_getClass calls the class handler callback and then checks a second time to see whether the class is registered. This function does not call the class handler callback.

objc_msgSend

Sends a message with a simple return value to an instance of a class.

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

Parameters

theReceiver: A pointer that points to the instance of the class that is to receive the message.
theSelector: The selector of the method that handles the message.
...: A variable argument list containing the arguments to the method.

Return Value

The return value of the method.

Discussion

When it encounters a method call, the compiler generates a call to one of the functions objc_msgSend, objc_msgSend_stret, objc_msgSendSuper, or objc_msgSendSuper_stret. Messages sent to an object’s superclass (using the super keyword) are sent using objc_msgSendSuper; other messages are sent using objc_msgSend. Methods that have data structures as return values are sent using objc_msgSendSuper_stret and objc_msgSend_stret.

objc_msgSendSuper

Sends a message with a simple return value to the superclass of an instance of a class.

id objc_msgSendSuper(struct objc_super *super, SEL op,  ...)

Parameters

super: A pointer to an objc_super data structure. Pass values identifying the context the message was sent to, including the instance of the class that is to receive the message and the superclass at which to start searching for the method implementation.
op: A pointer of type SEL. Pass the selector of the method that will handle the message.
...: A variable argument list containing the arguments to the method.

Return Value

The return value of the method identified by op.

Discussion

objc_msgSendSuper_stret

Sends a message with a data-structure return value to the superclass of an instance of a class.

void objc_msgSendSuper_stret(struct objc_super *super, SEL op, ...)

Parameters

super: A pointer to an objc_super data structure. Pass values identifying the context the message was sent to, including the instance of the class that is to receive the message and the superclass at which to start searching for the method implementation.
op: A pointer of type SEL. Pass the selector of the method.
...: A variable argument list containing the arguments to the method.

Discussion

objc_msgSend_fpret

Sends a message with a floating-point return value to an instance of a class.

double objc_msgSend_fpret(id self, SEL op, ...)

Parameters

self: A pointer that points to the instance of the class that is to receive the message.
op: The selector of the method that handles the message.
...: A variable argument list containing the arguments to the method.

Discussion

On the i386 platform, the ABI for functions returning a floating-point value is incompatible with that for functions returning an integral type. On the i386 platform, therefore, you must use objc_msgSend_fpret for functions that for functions returning non-integral type. For float or long double return types, cast the function to an appropriate function pointer type first.

This function is not used on the PPC or PPC64 platforms.

Declared In

objc-runtime.h

objc_msgSend_stret

Sends a message with a data-structure return value to an instance of a class.

void objc_msgSend_stret(void * stretAddr, id theReceiver, SEL theSelector,  ...)

Parameters

stretAddr: On input, a pointer that points to a block of memory large enough to contain the return value of the method. On output, contains the return value of the method.
theReceiver: A pointer to the instance of the class that is to receive the message.
theSelector: A pointer of type SEL. Pass the selector of the method that handles the message.
...: A variable argument list containing the arguments to the method.

Discussion

objc_registerClassPair

Registers a class that was allocated using objc_allocateClassPair.

void objc_registerClassPair(Class cls)

Parameters

cls: The class you want to register.

Declared In

runtime.h

objc_removeAssociatedObjects

Removes all associations for a given object.

void objc_removeAssociatedObjects(id object)

Parameters

object: An object that maintains associated objects.

Discussion

The main purpose of this function is to make it easy to return an object to a "pristine state”. You should not use this function for general removal of associations from objects, since it also removes associations that other clients may have added to the object. Typically you should use objc_setAssociatedObject with a nil value to clear an association.

objc_setAssociatedObject

Sets an associated value for a given object using a given key and association policy.

void objc_setAssociatedObject(id object, void *key, id value, objc_AssociationPolicy policy)

Parameters

object: The source object for the association.
key: The key for the association.
value: The value to associate with the key key for object. Pass nil to clear an existing association.
policy: The policy for the association. For possible values, see “Associative Object Behaviors.”

objc_setFutureClass

Used by CoreFoundation's toll-free bridging.

void objc_setFutureClass(Class cls, const char *name)

Special Considerations

Do not call this function yourself.

Declared In

runtime.h

object_copy

Returns a copy of a given object.

id object_copy(id obj, size_t size)

Parameters

obj: An Objective-C object.
size: The size of the object obj.

Return Value

A copy of obj.

Declared In

runtime.h

object_dispose

Frees the memory occupied by a given object.

id object_dispose(id obj)

Parameters

obj: An Objective-C object.

Return Value

nil.

Declared In

runtime.h

object_getClass

Returns the class of an object.

Class object_getClass(id object)

Parameters

object: The object you want to inspect.

Return Value

The class object of which object is an instance, or Nil if object is nil.

Declared In

runtime.h

object_getClassName

Returns the class name of a given object.

const char *object_getClassName(id obj)

Parameters

obj: An Objective-C object.

Return Value

The name of the class of which obj is an instance.

Declared In

runtime.h

object_getIndexedIvars

Returns a pointer to any extra bytes allocated with a instance given object.

OBJC_EXPORT void *object_getIndexedIvars(id obj)

Parameters

obj: An Objective-C object.

Return Value

A pointer to any extra bytes allocated with obj. If obj was not allocated with any extra bytes, then dereferencing the returned pointer is undefined.

Discussion

This function returns a pointer to any extra bytes allocated with the instance (as specified by class_createInstance with extraBytes>0). This memory follows the object's ordinary ivars, but may not be adjacent to the last ivar.

The returned pointer is guaranteed to be pointer-size aligned, even if the area following the object's last ivar is less aligned than that. Alignment greater than pointer-size is never guaranteed, even if the area following the object's last ivar is more aligned than that.

In a garbage-collected environment, the memory is scanned conservatively.

Declared In

runtime.h

object_getInstanceVariable

Obtains the value of an instance variable of a class instance.

Ivar object_getInstanceVariable(id obj, const char *name, void **outValue)

Parameters

obj: A pointer to an instance of a class. Pass the object containing the instance variable whose value you wish to obtain.
name: A C string. Pass the name of the instance variable whose value you wish to obtain.
outValue: On return, contains a pointer to the value of the instance variable.

Return Value

A pointer to the Ivar data structure that defines the type and name of the instance variable specified by name.

Declared In

runtime.h

object_getIvar

Reads the value of an instance variable in an object.

id object_getIvar(id object, Ivar ivar)

Parameters

object: The object containing the instance variable whose value you want to read.
ivar: The Ivar describing the instance variable whose value you want to read.

Return Value

The value of the instance variable specified by ivar, or nil if object is nil.

Discussion

object_getIvar is faster than object_getInstanceVariable if the Ivar for the instance variable is already known.

Declared In

runtime.h

object_setClass

Sets the class of an object.

Class object_setClass(id object, Class cls)

Parameters

object: The object to modify.
sel: A class object.

Return Value

The previous value of object‘s class, or Nil if object is nil.

Declared In

runtime.h

object_setInstanceVariable

Changes the value of an instance variable of a class instance.

Ivar object_setInstanceVariable(id obj, const char *name, void *value)

Parameters

obj: A pointer to an instance of a class. Pass the object containing the instance variable whose value you wish to modify.
name: A C string. Pass the name of the instance variable whose value you wish to modify.
value: The new value for the instance variable.

Return Value

A pointer to the Ivar data structure that defines the type and name of the instance variable specified by name.

Declared In

runtime.h

object_setIvar

Sets the value of an instance variable in an object.

void object_setIvar(id object, Ivar ivar, id value)

Parameters

object: The object containing the instance variable whose value you want to set.
ivar: The Ivar describing the instance variable whose value you want to set.
value: The new value for the instance variable.

Discussion

object_setIvar is faster than object_setInstanceVariable if the Ivar for the instance variable is already known.

Declared In

runtime.h

property_getAttributes

Returns the attribute string of an property.

const char *property_getAttributes(objc_property_t property)

Return Value

A C string containing the property's attributes.

Discussion

The format of the attribute string is described in “Declared Properties” in Objective-C Runtime Programming Guide.

Declared In

runtime.h

property_getName

Returns the name of a property.

const char *property_getName(objc_property_t property)

Return Value

A C string containing the property's name.

Declared In

runtime.h

protocol_conformsToProtocol

Returns a Boolean value that indicates whether one protocol conforms to another protocol.

BOOL protocol_conformsToProtocol(Protocol *proto, Protocol *other)

Parameters

proto: A protocol.
other: A protocol.

Return Value

YES if proto conforms to other, otherwise NO.

Discussion

One protocol can incorporate other protocols using the same syntax that classes use to adopt a protocol:

@protocol ProtocolName < protocol list >

All the protocols listed between angle brackets are considered part of the ProtocolName protocol.

Declared In

runtime.h

protocol_copyMethodDescriptionList

Returns an array of method descriptions of methods meeting a given specification for a given protocol.

struct objc_method_description *protocol_copyMethodDescriptionList(Protocol *p, BOOL isRequiredMethod, BOOL isInstanceMethod, unsigned int *outCount)

Parameters

p: A protocol.
isRequiredMethod: A Boolean value that indicates whether returned methods should be required methods (pass YES to specify required methods).
isInstanceMethod: A Boolean value that indicates whether returned methods should be instance methods (pass YES to specify instance methods).
outCount: Upon return, contains the number of method description structures in the returned array.

Return Value

A C array of objc_method_description structures containing the names and types of p’s methods specified by isRequiredMethod and isInstanceMethod. The array contains *outCount pointers followed by a NULL terminator. You must free the list with free().

If the protocol declares no methods that meet the specification, NULL is returned and *outCount is 0.

Discussion

Methods in other protocols adopted by this protocol are not included.

Declared In

runtime.h

protocol_copyPropertyList

Returns an array of the properties declared by a protocol.

objc_property_t * protocol_copyPropertyList(Protocol *protocol, unsigned int *outCount)

Parameters

proto: A protocol.
outCount: Upon return, contains the number of elements in the returned array.

Return Value

A C array of pointers of type objc_property_t describing the properties declared by proto. Any properties declared by other protocols adopted by this protocol are not included. The array contains *outCount pointers followed by a NULL terminator. You must free the array with free().

If the protocol declares no properties, NULL is returned and *outCount is 0.

Declared In

runtime.h

protocol_copyProtocolList

Returns an array of the protocols adopted by a protocol.

Protocol **protocol_copyProtocolList(Protocol *proto, unsigned int *outCount)

Parameters

proto: A protocol.
outCount: Upon return, contains the number of elements in the returned array.

Return Value

A C array of protocols adopted by proto. The array contains *outCount pointers followed by a NULL terminator. You must free the array with free().

If the protocol declares no properties, NULL is returned and *outCount is 0.

Declared In

runtime.h

protocol_getMethodDescription

Returns a method description structure for a specified method of a given protocol.

struct objc_method_description protocol_getMethodDescription(Protocol *p, SEL aSel, BOOL isRequiredMethod, BOOL isInstanceMethod)

Parameters

p: A protocol.
aSel: A selector
isRequiredMethod: A Boolean value that indicates whether aSel is a required method.
isInstanceMethod: A Boolean value that indicates whether aSel is an instance method.

Return Value

An objc_method_description structure that describes the method specified by aSel, isRequiredMethod, and isInstanceMethod for the protocol p.

If the protocol does not contain the specified method, returns an objc_method_description structure with the value {NULL, NULL}.

Discussion

Methods in other protocols adopted by this protocol are not included.

Declared In

runtime.h

protocol_getName

Returns a the name of a protocol.

const char *protocol_getName(Protocol *p)

Parameters

p: A protocol.

Return Value

The name of the protocol p as a C string.

Declared In

runtime.h

protocol_getProperty

Returns the specified property of a given protocol.

objc_property_t protocol_getProperty(Protocol *proto, const char *name, BOOL isRequiredProperty, BOOL isInstanceProperty)

Parameters

proto: A protocol.
name: The name of a property.
isRequiredProperty: A Boolean value that indicates whether name is a required property.
isInstanceProperty: A Boolean value that indicates whether name is a required property.

Return Value

The property specified by name, isRequiredProperty, and isInstanceProperty for proto, or NULL if none of proto’s properties meets the specification.

Declared In

runtime.h

protocol_isEqual

Returns a Boolean value that indicates whether two protocols are equal.

BOOL protocol_isEqual(Protocol *proto, Protocol *other)

Parameters

proto: A protocol.
other: A protocol.

Return Value

YES if proto is the same as other, otherwise NO.

Declared In

runtime.h

sel_getName

Returns the name of the method specified by a given selector.

const char* sel_getName(SEL aSelector)

Parameters

aSelector: A pointer of type SEL. Pass the selector whose name you wish to determine.

Return Value

A C string indicating the name of the selector.

Declared In

runtime.h

sel_getUid

Registers a method name with the Objective-C runtime system.

SEL sel_getUid(const char *str)

Parameters

str: A pointer to a C string. Pass the name of the method you wish to register.

Return Value

A pointer of type SEL specifying the selector for the named method.

Discussion

The implementation of this method is identical to the implementation of sel_registerName.

Version Notes

Prior to OS X version 10.0, this method tried to find the selector mapped to the given name and returned NULL if the selector was not found. This was changed for safety, because it was observed that many of the callers of this function did not check the return value for NULL.

Declared In

runtime.h

sel_isEqual

Returns a Boolean value that indicates whether two selectors are equal.

BOOL sel_isEqual(SEL lhs, SEL rhs)

Parameters

lhs: The selector to compare with rhs.
rhs: The selector to compare with lhs.

Return Value

YES if rhs and rhs are equal, otherwise NO.

Discussion

sel_isEqual is equivalent to ==.

Declared In

runtime.h

sel_registerName

Registers a method with the Objective-C runtime system, maps the method name to a selector, and returns the selector value.

SEL sel_registerName(const char *str)

Parameters

str: A pointer to a C string. Pass the name of the method you wish to register.

Return Value

A pointer of type SEL specifying the selector for the named method.

Discussion

You must register a method name with the Objective-C runtime system to obtain the method’s selector before you can add the method to a class definition. If the method name has already been registered, this function simply returns the selector.

Declared In

runtime.h

Data Types

Class-Definition Data Structures

Class

An opaque type that represents an Objective-C class.

typedef struct objc_class *Class;

Declared In

objc.h

Method

An opaque type that represents a method in a class definition.

typedef struct objc_method *Method;

Declared In

runtime.h

Ivar

An opaque type that represents an instance variable.

typedef struct objc_ivar *Ivar;

Declared In

runtime.h

objc_property_t

An opaque type that represents an Objective-C declared property.

typedef struct objc_property *objc_property_t;

Declared In

runtime.h

IMP

A pointer to the start of a method implementation.

id (*IMP)(id, SEL, ...)

Discussion

This data type is a pointer to the start of the function that implements the method. This function uses standard C calling conventions as implemented for the current CPU architecture. The first argument is a pointer to self (that is, the memory for the particular instance of this class, or, for a class method, a pointer to the metaclass). The second argument is the method selector. The method arguments follow.

SEL

Defines an opaque type that represents a method selector.

typedef struct objc_selector     *SEL;

Discussion

Method selectors are used to represent the name of a method at runtime. A method selector is a C string that has been registered (or “mapped“) with the Objective-C runtime. Selectors generated by the compiler are automatically mapped by the runtime when the class is loaded.

You can add new selectors at runtime and retrieve existing selectors using the function sel_registerName.

When using selectors, you must use the value returned from sel_registerName or the Objective-C compiler directive @selector(). You cannot simply cast a C string to SEL.

Declared In

objc.h

objc_method_list

Contains an array of method definitions.

struct objc_method_list
{
struct objc_method_list *obsolete;
int method_count;
struct objc_method method_list[1];
}

Fields

obsolete: Reserved for future use.
method_count: An integer specifying the number of methods in the method list array.
method_list: An array of Method data structures.

objc_cache

Performance optimization for method calls. Contains pointers to recently used methods.

struct objc_cache
{
unsigned int mask;
unsigned int occupied;
Method buckets[1];
};

Fields

mask: An integer specifying the total number of allocated cache buckets (minus one). During method lookup, the Objective-C runtime uses this field to determine the index at which to begin a linear search of the buckets array. A pointer to a method’s selector is masked against this field using a logical AND operation (index = (mask & selector)). This serves as a simple hashing algorithm.
occupied: An integer specifying the total number of occupied cache buckets.
buckets: An array of pointers to Method data structures. This array may contain no more than mask + 1 items. Note that pointers may be NULL, indicating that the cache bucket is unoccupied, and occupied buckets may not be contiguous. This array may grow over time.

Discussion

To limit the need to perform linear searches of method lists for the definitions of frequently accessed methods—an operation that can considerably slow down method lookup—the Objective-C runtime functions store pointers to the definitions of the most recently called method of the class in an objc_cache data structure.

objc_protocol_list

Represents a list of formal protocols.

struct objc_protocol_list
{
struct objc_protocol_list *next;
int count;
Protocol *list[1];
};

Fields

next: A pointer to another objc_protocol_list data structure.
count: The number of protocols in this list.
list: An array of pointers to Class data structures that represent protocols.

Discussion

A formal protocol is a class definition that declares a set of methods, which a class must implement. Such a class definition contains no instance variables. A class definition may promise to implement any number of formal protocols.

Instance Data Types

These are the data types that represent objects, classes, and superclasses.

id pointer to an instance of a class.
objc_object represents an instance of a class.
objc_super specifies the superclass of an instance.

id

A pointer to an instance of a class.

typedef struct objc_object {
      Class isa;
} *id;

Declared In

objc.h

objc_object

Represents an instance of a class.

struct objc_object
{
struct objc_class *isa;
/* …variable length data containing instance variable values…  */
};

Fields

isa: A pointer to the class definition of which this object is an instance.

Discussion

When you create an instance of a particular class, the allocated memory contains an objc_object data structure, which is directly followed by the data for the instance variables of the class.

The alloc and allocWithZone: methods of the Foundation framework class NSObject use the function class_createInstance to create objc_object data structures.

objc_super

Specifies the superclass of an instance.

struct objc_super
{
id receiver;
Class class;
};

Fields

receiver: A pointer of type id. Specifies an instance of a class.
class: A pointer to an Class data structure. Specifies the particular superclass of the instance to message.

Discussion

The compiler generates an objc_super data structure when it encounters the super keyword as the receiver of a message. It specifies the class definition of the particular superclass that should be messaged.

Boolean Value

BOOL

Type to represent a Boolean value.

typedef signed char BOOL;

Discussion

BOOL is explicitly signed so @encode(BOOL) is c rather than C even if -funsigned-char is used.

For values, see “Boolean Values”.

Special Considerations

Since the type of BOOL is actually char, it does not behave in the same way as a C _Bool value or a C++ bool value. For example, the conditional in the following code will be false on i386 (and true on PPC):

- (BOOL)value {

    return 256;

// then

if ([self value]) doStuff();

By contrast, the conditional in the following code will be true on all platforms (even where sizeof(bool) == 1):

- (bool)value {

    return 256;

// then

if ([self value]) doStuff();

Availability

Available in OS X v10.1 and later.

Declared In

wintypes.h

Associative References

objc_AssociationPolicy

Type to specify behavior the behavior of an association.

typedef uintptr_t objc_AssociationPolicy;

Discussion

For values, see “Associative Object Behaviors”.

Constants

Boolean Values

These macros define convenient constants to represent Boolean values.

#define YES (BOOL)1
#define NO  (BOOL)0

Constants

YES

Defines YES as 1.

Available in OS X v10.0 and later.

Declared in NSObjCRuntime.h.

NO

Defines NO as 0.

Available in OS X v10.0 and later.

Declared in NSObjCRuntime.h.

Declared In

objc.h

Null Values

These macros define null values for classes and instances.

#define nil __DARWIN_NULL
#define Nil __DARWIN_NULL

Constants

nil

Defines the id of a null instance.

Available in OS X v10.0 and later.

Declared in IONDRVLibraries.h.

Nil

Defines the id of a null class.

Available in OS X v10.0 through OS X v10.4.

Declared in NSObjCRuntime.h.

Declared In

objc.h

Associative Object Behaviors

Policies related to associative references.

enum {
   OBJC_ASSOCIATION_ASSIGN = 0,
   OBJC_ASSOCIATION_RETAIN_NONATOMIC = 1,
   OBJC_ASSOCIATION_COPY_NONATOMIC = 3,
   OBJC_ASSOCIATION_RETAIN = 01401,
   OBJC_ASSOCIATION_COPY = 01403
};

Constants

OBJC_ASSOCIATION_ASSIGN: Specifies a weak reference to the associated object.
OBJC_ASSOCIATION_RETAIN_NONATOMIC: Specifies a strong reference to the associated object, and that the association is not made atomically.
OBJC_ASSOCIATION_COPY_NONATOMIC: Specifies that the associated object is copied, and that the association is not made atomically.
OBJC_ASSOCIATION_RETAIN: Specifies a strong reference to the associated object, and that the association is made atomically.
OBJC_ASSOCIATION_COPY: Specifies that the associated object is copied, and that the association is made atomically.

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