autorelease previous object by assignment

Question

If I have a function like this

void setSomeObject( SomeObjectClass obj /*, and some other params*/ )
{
  [_previous autorelease];
  _previous = obj;
}

As far as I understood it the autorelease message is sent to the object itself (not _previous) so at one point, sometime when setSomeObject goes out of scope the original object is autoreleased (if any). Is this correct? I am not using properties but I guess by using them the release of the previous object would be automatic when I do self.previous = obj; ?

Answer 1

No that is incorrect.

[_previous autorelease] sends the autorelease message to _previous. That is the meaning of this syntax.

Answer 2

When you send an -autorelease message to an object, it's added to the active NSAutoreleasePool, which is emptied when the run loop runs. If you say [_previous autorelease], only that object will be autoreleased, and if you then say _previous = obj, that only changes the variable's reference. The old object is still autoreleased.

If you're doing this in a setter method, this is what the pattern generally is:

- (void)setSomeObject:(MyObjClass *obj) {
    if (obj != someObject) {
        [someObject release];
        someObject = [obj retain]; // or copy, depending on what you want
    }
}