Objective C: Newbie question about allocation, retain and release [duplicate]

Answer 1

That leaks the object. Since alloc returns an owning reference, and retain claims ownership a second time, you need to call release to balance the alloc or the object will think you want to hold onto it forever and never be deallocated.

Answer 2

self.m_Object = [[SomeObject alloc] init];

This causes an over-retain. You get one claim of ownership for the alloc, and another through the setter, which is declared as retaining the new value. Since you only have one pointer to the new value, you have too many claims. When the setter is used again:

self.m_Object = anotherObject; 

the original object will receive only one release message, and you will lose the pointer. Since you had two claims on the object, it will not be deallocated, and you will have a leak.

The property access: self.m_Object = obj; is translated by the compiler into [self setM_Object:obj];. That is, the setter method for the property, created by the @synthesize directive, is called. That setter method retains its argument. The other option is to use the ivar directly in your init method:

-init {
    //...
    m_Object = [[SomeObject alloc] init];
    //...
}

then you have only one claim on this object, caused by the use of alloc. Since you also have one reference to the object, this is correct.

Answer 3

The reason is because you defined the property m_Object to retain so it would result in a memory leak because alloc/init call results in a retain of +1 then property will retain giving it at least a retain count of +2. If you would like to make it one line feel free to abuse the auto release pool.

self.m_Object= [[[SomeObject alloc] init] autorelease];