8.3. Garbage Collection
Support for a new memory management policy has been added by using a
powerful conservative garbage collector, known as the
Boehm-Demers-Weiser conservative garbage collector. It is available from
https://www.hpl.hp.com/personal/Hans_Boehm/gc/.
To enable the support for it you have to configure the compiler using an
additional argument, -enable-objc-gc. You need to have
garbage collector installed before building the compiler. This will
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
{
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.
