0x01.alloc/retain/release/dealloc实现
alloc:
struct obj_layout {
NSUInteger retained;
};
+ (id)alloc
{
int size = sizeof(struct obj_layout) + 对象大小;
struct obj_layout *p = (struct obj_layout *)calloc(1, size);
return (id)(p+1);
)
alloc类方法用struct obj_layout中的retained整数来保存引用计数,并将其写入对象内存头部,该对象内存块全部置0后返回。
执行alloc后对象的retainCount是“1”,下面通过GNUstep的源代码来确认:
-(NSUInteger)retainCount
{
return NSExtraRefCount(self) + 1;
}
inline NSUInteger
NSExtraRefCount(id anObject)
{
return ((struct obj_layout *) anObject)[-1].retained;
}
由对象寻址找到对象内存头部,从而访问其中的retained变量。因为分配时全部置0,所以retained为0.由NSExtraRefCount(self) + 1得出,retainCount为1可以推测出,retain方法使retained变量加1,
而release方法使retained变量减1.
下面来看一下调用出的retain实例方法。
-(id)retain
{
NSIncrementExtraRefCount(self);
return self;
}
inline void
NSIncrementExtraRefCount(id anObject)
{
if(((struct obj_layout *)anObject [-1].retained == UINT_MAX -1)
{
[NSException raise:NSInternalInconsistencyException format:@"NSIncrementExtraRefCount() asked to increment too far");
}
((struct obj_layout *)anObject)[-1].retained++;
}