+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
.
What You Can and Cannot Do in +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:
you can write whatever C code you like;
you can allocate and send messages to objects whose class is implemented in the same file;
the
+load
implementation of all super classes of a class are executed before the+load
of that class is executed;the
+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:
allocation of or sending messages to arbitrary objects;
allocation of or sending messages to objects whose classes have a category implemented in the same file;
sending messages to Objective-C constant strings (
@"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.