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This document is meant to describe some of the GNU Objective-C features. It is not intended to teach you Objective-C. There are several resources on the Internet that present the language.
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This section is specific for the GNU Objective-C runtime. If you are using a different runtime, you can skip it.
The GNU Objective-C runtime provides an API that allows you to interact with the Objective-C runtime system, querying the live runtime structures and even manipulating them. This allows you for example to inspect and navigate classes, methods and protocols; to define new classes or new methods, and even to modify existing classes or protocols.
If you are using a "Foundation" library such as GNUstep-Base, this library will provide you with a rich set of functionality to do most of the inspection tasks, and you probably will only need direct access to the GNU Objective-C runtime API to define new classes or methods.
8.1.1 Modern GNU Objective-C runtime API 8.1.2 Traditional GNU Objective-C runtime API
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The GNU Objective-C runtime provides an API which is similar to the one provided by the "Objective-C 2.0" Apple/NeXT Objective-C runtime. The API is documented in the public header files of the GNU Objective-C runtime:
id
, Class
and BOOL
. You have to include this header to do almost
anything with Objective-C.
class_getName()
, declared in
`objc/runtime.h'.
@synchronized()
syntax, allowing
you to emulate an Objective-C @synchronized()
block in plain
C/C++ code.
objc_mutex_lock()
, which provide a
platform-independent set of threading functions.
The header files contain detailed documentation for each function in the GNU Objective-C runtime API.
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The GNU Objective-C runtime used to provide a different API, which we
call the "traditional" GNU Objective-C runtime API. Functions
belonging to this API are easy to recognize because they use a
different naming convention, such as class_get_super_class()
(traditional API) instead of class_getSuperclass()
(modern
API). Software using this API includes the file
`objc/objc-api.h' where it is declared.
Starting with GCC 4.7.0, the traditional GNU runtime API is no longer available.
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+load
: Executing code before main This section is specific for the GNU Objective-C runtime. If you are using a different runtime, you can skip it.
The GNU Objective-C runtime provides a way that allows you to execute
code before the execution of the program enters the main
function. The code is executed on a per-class and a per-category basis,
through a special class method +load
.
This facility is very useful if you want to initialize global variables
which can be accessed by the program directly, without sending a message
to the class first. The usual way to initialize global variables, in the
+initialize
method, might not be useful because
+initialize
is only called when the first message is sent to a
class object, which in some cases could be too late.
Suppose for example you have a FileStream
class that declares
Stdin
, Stdout
and Stderr
as global variables, like
below:
FileStream *Stdin = nil; FileStream *Stdout = nil; FileStream *Stderr = nil; @implementation FileStream + (void)initialize { Stdin = [[FileStream new] initWithFd:0]; Stdout = [[FileStream new] initWithFd:1]; Stderr = [[FileStream new] initWithFd:2]; } /* Other methods here */ @end |
In this example, the initialization of Stdin
, Stdout
and
Stderr
in +initialize
occurs too late. The programmer can
send a message to one of these objects before the variables are actually
initialized, thus sending messages to the nil
object. The
+initialize
method which actually initializes the global
variables is not invoked until the first message is sent to the class
object. The solution would require these variables to be initialized
just before entering main
.
The correct solution of the above problem is to use the +load
method instead of +initialize
:
@implementation FileStream + (void)load { Stdin = [[FileStream new] initWithFd:0]; Stdout = [[FileStream new] initWithFd:1]; Stderr = [[FileStream new] initWithFd:2]; } /* Other methods here */ @end |
The +load
is a method that is not overridden by categories. If a
class and a category of it both implement +load
, both methods are
invoked. This allows some additional initializations to be performed in
a category.
This mechanism is not intended to be a replacement for +initialize
.
You should be aware of its limitations when you decide to use it
instead of +initialize
.
8.2.1 What you can and what you cannot do in +load
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+load
+load
is to be used only as a last resort. Because it is
executed very early, most of the Objective-C runtime machinery will
not be ready when +load
is executed; hence +load
works
best for executing C code that is independent on the Objective-C
runtime.
The +load
implementation in the GNU runtime guarantees you the
following things:
+load
implementation of all super classes of a class are
executed before the +load
of that class is executed;
+load
implementation of a class is executed before the
+load
implementation of any category.
In particular, the following things, even if they can work in a particular case, are not guaranteed:
@"this is a
constant string"
);
You should make no assumptions about receiving +load
in sibling
classes when you write +load
of a class. The order in which
sibling classes receive +load
is not guaranteed.
The order in which +load
and +initialize
are called could
be problematic if this matters. If you don't allocate objects inside
+load
, it is guaranteed that +load
is called before
+initialize
. If you create an object inside +load
the
+initialize
method of object's class is invoked even if
+load
was not invoked. Note if you explicitly call +load
on a class, +initialize
will be called first. To avoid possible
problems try to implement only one of these methods.
The +load
method is also invoked when a bundle is dynamically
loaded into your running program. This happens automatically without any
intervening operation from you. When you write bundles and you need to
write +load
you can safely create and send messages to objects whose
classes already exist in the running program. The same restrictions as
above apply to classes defined in bundle.
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This is an advanced section. Type encodings are used extensively by the compiler and by the runtime, but you generally do not need to know about them to use Objective-C.
The Objective-C compiler generates type encodings for all the types. These type encodings are used at runtime to find out information about selectors and methods and about objects and classes.
The types are encoded in the following way:
_Bool |
B
|
char |
c
|
unsigned char |
C
|
short |
s
|
unsigned short |
S
|
int |
i
|
unsigned int |
I
|
long |
l
|
unsigned long |
L
|
long long |
q
|
unsigned long long |
Q
|
float |
f
|
double |
d
|
long double |
D
|
void |
v
|
id |
@
|
Class |
#
|
SEL |
:
|
char* |
*
|
enum |
an enum is encoded exactly as the integer type that the compiler uses for it, which depends on the enumeration
values. Often the compiler users unsigned int , which is then encoded as I .
|
unknown type | ?
|
Complex types | j followed by the inner type. For example _Complex double is encoded as "jd".
|
bit-fields | b followed by the starting position of the bit-field, the type of the bit-field and the size of the bit-field (the bit-fields encoding was changed from the NeXT's compiler encoding, see below)
|
The encoding of bit-fields has changed to allow bit-fields to be properly handled by the runtime functions that compute sizes and alignments of types that contain bit-fields. The previous encoding contained only the size of the bit-field. Using only this information it is not possible to reliably compute the size occupied by the bit-field. This is very important in the presence of the Boehm's garbage collector because the objects are allocated using the typed memory facility available in this collector. The typed memory allocation requires information about where the pointers are located inside the object.
The position in the bit-field is the position, counting in bits, of the bit closest to the beginning of the structure.
The non-atomic types are encoded as follows:
pointers | `^' followed by the pointed type. |
arrays | `[' followed by the number of elements in the array followed by the type of the elements followed by `]' |
structures | `{' followed by the name of the structure (or `?' if the structure is unnamed), the `=' sign, the type of the members and by `}' |
unions | `(' followed by the name of the structure (or `?' if the union is unnamed), the `=' sign, the type of the members followed by `)' |
vectors | `![' followed by the vector_size (the number of bytes composing the vector) followed by a comma, followed by the alignment (in bytes) of the vector, followed by the type of the elements followed by `]' |
Here are some types and their encodings, as they are generated by the compiler on an i386 machine:
Objective-C type | Compiler encoding | ||
int a[10]; |
[10i]
struct { int i; float f[3]; int a:3; int b:2; char c; } |
{?=i[3f]b128i3b131i2c}
int a __attribute__ ((vector_size (16))); |
![16,16i]
(alignment would depend on the machine)
In addition to the types the compiler also encodes the type specifiers. The table below describes the encoding of the current Objective-C type specifiers:
Specifier | Encoding |
const |
r
|
in |
n
|
inout |
N
|
out |
o
|
bycopy |
O
|
byref |
R
|
oneway |
V
|
The type specifiers are encoded just before the type. Unlike types however, the type specifiers are only encoded when they appear in method argument types.
Note how const
interacts with pointers:
Objective-C type | Compiler encoding | ||
const int |
ri
const int* |
^ri
int *const |
r^i
const int*
is a pointer to a const int
, and so is
encoded as ^ri
. int* const
, instead, is a const
pointer to an int
, and so is encoded as r^i
.
Finally, there is a complication when encoding const char *
versus char * const
. Because char *
is encoded as
*
and not as ^c
, there is no way to express the fact
that r
applies to the pointer or to the pointee.
Hence, it is assumed as a convention that r*
means const
char *
(since it is what is most often meant), and there is no way to
encode char *const
. char *const
would simply be encoded
as *
, and the const
is lost.
8.3.1 Legacy type encoding 8.3.2 @encode 8.3.3 Method signatures
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Unfortunately, historically GCC used to have a number of bugs in its encoding code. The NeXT runtime expects GCC to emit type encodings in this historical format (compatible with GCC-3.3), so when using the NeXT runtime, GCC will introduce on purpose a number of incorrect encodings:
enum
s are always encoded as 'i' (int) even if they are actually
unsigned or long.
In addition to that, the NeXT runtime uses a different encoding for
bitfields. It encodes them as b
followed by the size, without
a bit offset or the underlying field type.
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GNU Objective-C supports the @encode
syntax that allows you to
create a type encoding from a C/Objective-C type. For example,
@encode(int)
is compiled by the compiler into "i"
.
@encode
does not support type qualifiers other than
const
. For example, @encode(const char*)
is valid and
is compiled into "r*"
, while @encode(bycopy char *)
is
invalid and will cause a compilation error.
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This section documents the encoding of method types, which is rarely needed to use Objective-C. You should skip it at a first reading; the runtime provides functions that will work on methods and can walk through the list of parameters and interpret them for you. These functions are part of the public "API" and are the preferred way to interact with method signatures from user code.
But if you need to debug a problem with method signatures and need to know how they are implemented (i.e., the "ABI"), read on.
Methods have their "signature" encoded and made available to the runtime. The "signature" encodes all the information required to dynamically build invocations of the method at runtime: return type and arguments.
The "signature" is a null-terminated string, composed of the following:
int
would have i
here.
self
and the
method selector _cmd
).
For example, a method with no arguments and returning int
would
have the signature i8@0:4
if the size of a pointer is 4. The
signature is interpreted as follows: the i
is the return type
(an int
), the 8
is the total size of the parameters in
bytes (two pointers each of size 4), the @0
is the first
parameter (an object at byte offset 0
) and :4
is the
second parameter (a SEL
at byte offset 4
).
You can easily find more examples by running the "strings" program
on an Objective-C object file compiled by GCC. You'll see a lot of
strings that look very much like i8@0:4
. They are signatures
of Objective-C methods.
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This section is specific for the GNU Objective-C runtime. If you are using a different runtime, you can skip it.
Support for garbage collection with the GNU runtime has been added by using a powerful conservative garbage collector, known as the Boehm-Demers-Weiser conservative garbage collector.
To enable the support for it you have to configure the compiler using an additional argument, `--enable-objc-gc'. This will build the boehm-gc library, and build an additional runtime library which has several enhancements to support the garbage collector. The new library has a new name, `libobjc_gc.a' to not conflict with the non-garbage-collected library.
When the garbage collector is used, the objects are allocated using the so-called typed memory allocation mechanism available in the Boehm-Demers-Weiser collector. This mode requires precise information on where pointers are located inside objects. This information is computed once per class, immediately after the class has been initialized.
There is a new runtime function class_ivar_set_gcinvisible()
which can be used to declare a so-called weak pointer
reference. Such a pointer is basically hidden for the garbage collector;
this can be useful in certain situations, especially when you want to
keep track of the allocated objects, yet allow them to be
collected. This kind of pointers can only be members of objects, you
cannot declare a global pointer as a weak reference. Every type which is
a pointer type can be declared a weak pointer, including id
,
Class
and SEL
.
Here is an example of how to use this feature. Suppose you want to implement a class whose instances hold a weak pointer reference; the following class does this:
@interface WeakPointer : Object { const void* weakPointer; } - initWithPointer:(const void*)p; - (const void*)weakPointer; @end @implementation WeakPointer + (void)initialize { if (self == objc_lookUpClass ("WeakPointer")) class_ivar_set_gcinvisible (self, "weakPointer", YES); } - initWithPointer:(const void*)p { weakPointer = p; return self; } - (const void*)weakPointer { return weakPointer; } @end |
Weak pointers are supported through a new type character specifier
represented by the `!' character. The
class_ivar_set_gcinvisible()
function adds or removes this
specifier to the string type description of the instance variable named
as argument.
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GNU Objective-C provides constant string objects that are generated directly by the compiler. You declare a constant string object by prefixing a C constant string with the character `@':
id myString = @"this is a constant string object"; |
The constant string objects are by default instances of the
NXConstantString
class which is provided by the GNU Objective-C
runtime. To get the definition of this class you must include the
`objc/NXConstStr.h' header file.
User defined libraries may want to implement their own constant string
class. To be able to support them, the GNU Objective-C compiler provides
a new command line options `-fconstant-string-class=class-name'.
The provided class should adhere to a strict structure, the same
as NXConstantString
's structure:
@interface MyConstantStringClass { Class isa; char *c_string; unsigned int len; } @end |
NXConstantString
inherits from Object
; user class
libraries may choose to inherit the customized constant string class
from a different class than Object
. There is no requirement in
the methods the constant string class has to implement, but the final
ivar layout of the class must be the compatible with the given
structure.
When the compiler creates the statically allocated constant string
object, the c_string
field will be filled by the compiler with
the string; the length
field will be filled by the compiler with
the string length; the isa
pointer will be filled with
NULL
by the compiler, and it will later be fixed up automatically
at runtime by the GNU Objective-C runtime library to point to the class
which was set by the `-fconstant-string-class' option when the
object file is loaded (if you wonder how it works behind the scenes, the
name of the class to use, and the list of static objects to fixup, are
stored by the compiler in the object file in a place where the GNU
runtime library will find them at runtime).
As a result, when a file is compiled with the `-fconstant-string-class' option, all the constant string objects will be instances of the class specified as argument to this option. It is possible to have multiple compilation units referring to different constant string classes, neither the compiler nor the linker impose any restrictions in doing this.
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The keyword @compatibility_alias
allows you to define a class name
as equivalent to another class name. For example:
@compatibility_alias WOApplication GSWApplication; |
tells the compiler that each time it encounters WOApplication
as
a class name, it should replace it with GSWApplication
(that is,
WOApplication
is just an alias for GSWApplication
).
There are some constraints on how this can be used---
WOApplication
(the alias) must not be an existing class;
GSWApplication
(the real class) must be an existing class.
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GNU Objective-C provides exception support built into the language, as in the following example:
@try { ... @throw expr; ... } @catch (AnObjCClass *exc) { ... @throw expr; ... @throw; ... } @catch (AnotherClass *exc) { ... } @catch (id allOthers) { ... } @finally { ... @throw expr; ... } |
The @throw
statement may appear anywhere in an Objective-C or
Objective-C++ program; when used inside of a @catch
block, the
@throw
may appear without an argument (as shown above), in
which case the object caught by the @catch
will be rethrown.
Note that only (pointers to) Objective-C objects may be thrown and
caught using this scheme. When an object is thrown, it will be caught
by the nearest @catch
clause capable of handling objects of
that type, analogously to how catch
blocks work in C++ and
Java. A @catch(id ...)
clause (as shown above) may also
be provided to catch any and all Objective-C exceptions not caught by
previous @catch
clauses (if any).
The @finally
clause, if present, will be executed upon exit
from the immediately preceding @try ... @catch
section.
This will happen regardless of whether any exceptions are thrown,
caught or rethrown inside the @try ... @catch
section,
analogously to the behavior of the finally
clause in Java.
There are several caveats to using the new exception mechanism:
NS_HANDLER
-style idioms provided by the
NSException
class, the new exceptions can only be used on Mac
OS X 10.3 (Panther) and later systems, due to additional functionality
needed in the NeXT Objective-C runtime.
@throw
an exception
from Objective-C and catch
it in C++, or vice versa
(i.e., throw ... @catch
).
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GNU Objective-C provides support for synchronized blocks:
@synchronized (ObjCClass *guard) { ... } |
Upon entering the @synchronized
block, a thread of execution
shall first check whether a lock has been placed on the corresponding
guard
object by another thread. If it has, the current thread
shall wait until the other thread relinquishes its lock. Once
guard
becomes available, the current thread will place its own
lock on it, execute the code contained in the @synchronized
block, and finally relinquish the lock (thereby making guard
available to other threads).
Unlike Java, Objective-C does not allow for entire methods to be
marked @synchronized
. Note that throwing exceptions out of
@synchronized
blocks is allowed, and will cause the guarding
object to be unlocked properly.
Because of the interactions between synchronization and exception
handling, you can only use @synchronized
when compiling with
exceptions enabled, that is with the command line option
`-fobjc-exceptions'.
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8.9.1 Using fast enumeration 8.9.2 c99-like fast enumeration syntax 8.9.3 Fast enumeration details 8.9.4 Fast enumeration protocol
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GNU Objective-C provides support for the fast enumeration syntax:
id array = ...; id object; for (object in array) { /* Do something with 'object' */ } |
array
needs to be an Objective-C object (usually a collection
object, for example an array, a dictionary or a set) which implements
the "Fast Enumeration Protocol" (see below). If you are using a
Foundation library such as GNUstep Base or Apple Cocoa Foundation, all
collection objects in the library implement this protocol and can be
used in this way.
The code above would iterate over all objects in array
. For
each of them, it assigns it to object
, then executes the
Do something with 'object'
statements.
Here is a fully worked-out example using a Foundation library (which
provides the implementation of NSArray
, NSString
and
NSLog
):
NSArray *array = [NSArray arrayWithObjects: @"1", @"2", @"3", nil]; NSString *object; for (object in array) NSLog (@"Iterating over %@", object); |
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A c99-like declaration syntax is also allowed:
id array = ...; for (id object in array) { /* Do something with 'object' */ } |
this is completely equivalent to:
id array = ...; { id object; for (object in array) { /* Do something with 'object' */ } } |
but can save some typing.
Note that the option `-std=c99' is not required to allow this syntax in Objective-C.
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Here is a more technical description with the gory details. Consider the code
for (object expression in collection expression) { statements } |
here is what happens when you run it:
collection expression
is evaluated exactly once and the
result is used as the collection object to iterate over. This means
it is safe to write code such as for (object in [NSDictionary
keyEnumerator]) ...
.
object expression
is set to nil
and the loop
immediately terminates.
object expression
is set to the object, then statements
are executed.
statements
can contain break
and continue
commands, which will abort the iteration or skip to the next loop
iteration as expected.
object expression
is set to nil
. This allows
you to determine whether the iteration finished because a break
command was used (in which case object expression
will remain
set to the last object that was iterated over) or because it iterated
over all the objects (in which case object expression
will be
set to nil
).
statements
must not make any changes to the collection
object; if they do, it is a hard error and the fast enumeration
terminates by invoking objc_enumerationMutation
, a runtime
function that normally aborts the program but which can be customized
by Foundation libraries via objc_set_mutation_handler
to do
something different, such as raising an exception.
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If you want your own collection object to be usable with fast enumeration, you need to have it implement the method
- (unsigned long) countByEnumeratingWithState: (NSFastEnumerationState *)state objects: (id *)objects count: (unsigned long)len; |
where NSFastEnumerationState
must be defined in your code as follows:
typedef struct { unsigned long state; id *itemsPtr; unsigned long *mutationsPtr; unsigned long extra[5]; } NSFastEnumerationState; |
If no NSFastEnumerationState
is defined in your code, the
compiler will automatically replace NSFastEnumerationState *
with struct __objcFastEnumerationState *
, where that type is
silently defined by the compiler in an identical way. This can be
confusing and we recommend that you define
NSFastEnumerationState
(as shown above) instead.
The method is called repeatedly during a fast enumeration to retrieve batches of objects. Each invocation of the method should retrieve the next batch of objects.
The return value of the method is the number of objects in the current
batch; this should not exceed len
, which is the maximum size of
a batch as requested by the caller. The batch itself is returned in
the itemsPtr
field of the NSFastEnumerationState
struct.
To help with returning the objects, the objects
array is a C
array preallocated by the caller (on the stack) of size len
.
In many cases you can put the objects you want to return in that
objects
array, then do itemsPtr = objects
. But you
don't have to; if your collection already has the objects to return in
some form of C array, it could return them from there instead.
The state
and extra
fields of the
NSFastEnumerationState
structure allows your collection object
to keep track of the state of the enumeration. In a simple array
implementation, state
may keep track of the index of the last
object that was returned, and extra
may be unused.
The mutationsPtr
field of the NSFastEnumerationState
is
used to keep track of mutations. It should point to a number; before
working on each object, the fast enumeration loop will check that this
number has not changed. If it has, a mutation has happened and the
fast enumeration will abort. So, mutationsPtr
could be set to
point to some sort of version number of your collection, which is
increased by one every time there is a change (for example when an
object is added or removed). Or, if you are content with less strict
mutation checks, it could point to the number of objects in your
collection or some other value that can be checked to perform an
approximate check that the collection has not been mutated.
Finally, note how we declared the len
argument and the return
value to be of type unsigned long
. They could also be declared
to be of type unsigned int
and everything would still work.
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This section is specific for the GNU Objective-C runtime. If you are using a different runtime, you can skip it.
The implementation of messaging in the GNU Objective-C runtime is designed to be portable, and so is based on standard C.
Sending a message in the GNU Objective-C runtime is composed of two
separate steps. First, there is a call to the lookup function,
objc_msg_lookup ()
(or, in the case of messages to super,
objc_msg_lookup_super ()
). This runtime function takes as
argument the receiver and the selector of the method to be called; it
returns the IMP
, that is a pointer to the function implementing
the method. The second step of method invocation consists of casting
this pointer function to the appropriate function pointer type, and
calling the function pointed to it with the right arguments.
For example, when the compiler encounters a method invocation such as
[object init]
, it compiles it into a call to
objc_msg_lookup (object, @selector(init))
followed by a cast
of the returned value to the appropriate function pointer type, and
then it calls it.
8.10.1 Dynamically registering methods 8.10.2 Forwarding hook
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If objc_msg_lookup()
does not find a suitable method
implementation, because the receiver does not implement the required
method, it tries to see if the class can dynamically register the
method.
To do so, the runtime checks if the class of the receiver implements the method
+ (BOOL) resolveInstanceMethod: (SEL)selector; |
in the case of an instance method, or
+ (BOOL) resolveClassMethod: (SEL)selector; |
in the case of a class method. If the class implements it, the
runtime invokes it, passing as argument the selector of the original
method, and if it returns YES
, the runtime tries the lookup
again, which could now succeed if a matching method was added
dynamically by +resolveInstanceMethod:
or
+resolveClassMethod:
.
This allows classes to dynamically register methods (by adding them to
the class using class_addMethod
) when they are first called.
To do so, a class should implement +resolveInstanceMethod:
(or,
depending on the case, +resolveClassMethod:
) and have it
recognize the selectors of methods that can be registered dynamically
at runtime, register them, and return YES
. It should return
NO
for methods that it does not dynamically registered at
runtime.
If +resolveInstanceMethod:
(or +resolveClassMethod:
) is
not implemented or returns NO
, the runtime then tries the
forwarding hook.
Support for +resolveInstanceMethod:
and
resolveClassMethod:
was added to the GNU Objective-C runtime in
GCC version 4.6.
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The GNU Objective-C runtime provides a hook, called
__objc_msg_forward2
, which is called by
objc_msg_lookup()
when it can't find a method implementation in
the runtime tables and after calling +resolveInstanceMethod:
and +resolveClassMethod:
has been attempted and did not succeed
in dynamically registering the method.
To configure the hook, you set the global variable
__objc_msg_foward2
to a function with the same argument and
return types of objc_msg_lookup()
. When
objc_msg_lookup()
can not find a method implementation, it
invokes the hook function you provided to get a method implementation
to return. So, in practice __objc_msg_forward2
allows you to
extend objc_msg_lookup()
by adding some custom code that is
called to do a further lookup when no standard method implementation
can be found using the normal lookup.
This hook is generally reserved for "Foundation" libraries such as
GNUstep Base, which use it to implement their high-level method
forwarding API, typically based around the forwardInvocation:
method. So, unless you are implementing your own "Foundation"
library, you should not set this hook.
In a typical forwarding implementation, the __objc_msg_forward2
hook function determines the argument and return type of the method
that is being looked up, and then creates a function that takes these
arguments and has that return type, and returns it to the caller.
Creating this function is non-trivial and is typically performed using
a dedicated library such as libffi
.
The forwarding method implementation thus created is returned by
objc_msg_lookup()
and is executed as if it was a normal method
implementation. When the forwarding method implementation is called,
it is usually expected to pack all arguments into some sort of object
(typically, an NSInvocation
in a "Foundation" library), and
hand it over to the programmer (forwardInvocation:
) who is then
allowed to manipulate the method invocation using a high-level API
provided by the "Foundation" library. For example, the programmer
may want to examine the method invocation arguments and name and
potentially change them before forwarding the method invocation to one
or more local objects (performInvocation:
) or even to remote
objects (by using Distributed Objects or some other mechanism). When
all this completes, the return value is passed back and must be
returned correctly to the original caller.
Note that the GNU Objective-C runtime currently provides no support
for method forwarding or method invocations other than the
__objc_msg_forward2
hook.
If the forwarding hook does not exist or returns NULL
, the
runtime currently attempts forwarding using an older, deprecated API,
and if that fails, it aborts the program. In future versions of the
GNU Objective-C runtime, the runtime will immediately abort.
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