Keeping a strong pointer to a local object

Question

I encountered this thing in a book which I am reading and it got me thinking:

"When you allocate a block, it is created on the stack. This means that, even if you were to keep a strong reference to it, calling it later would result in a crash because the memory would be destroyed as soon as you leave the method in which it was defined."

I thought if I have a strong pointer to something, it is kept alive ? Does this mean this does not apply for objects allocated on the stack?

I am trying to think of an example without using blocks...(eg, of pointer - maybe an ivar- pointing to a stack allocated object which gets destroyed even though the pointer is alive)

Answer 1

Objects are never allocated on the stack in Objective-C. Blocks are special however, since they are stack allocated. So if you want to retain a pointer to a block, you must first copy it by using Block_copy and use the copy, then release it with Block_release . This must be done if the block is to be used after the scope it was declared in is destroyed. More on the matter here: https://developer.apple.com/library/mac/documentation/cocoa/Conceptual/Blocks/Articles/bxUsing.html (under "Copying Blocks"). Yet again though, this does not apply to regular objects.

Answer 2

Blocks can be messaged like objects. To move them from the stack to the heap, just "copy" them.

void (^stackBlock)() = [^(){
    NSLog(@"Hello world");
} copy];