Like primitive variables, blocks are created and stored on the stack. That is, the block will be destroyed along with the stack frame when the function or method that created the block returns. Sometimes, however, your block needs to live longer than that. For example, it could become an instance variable of an object. In this case, you must copy your block from the stack to the heap.

To copy a block from the stack to the heap, you send it the copy message:

ArrayEnumerationBlock iVarDevowelizer = [devowelizer copy];

Now a copy of your block exists on the heap. It is now a heap-based block instead of a stack-based block, and the new block variable is a pointer to the block.

Methods that take blocks as arguments, such as NSArray’s enumerateObjectsUsingBlock: or NSNotificationCenter’s addObserverForName:object:queue:usingBlock:, are expected to copy blocks passed to them to keep them around. In doing so, they create pointers – and strong references – to those blocks.