Advanced Mac OS X Programming: The Big Nerd Ranch Guide

When To Copy

So when do you need to copy a block? Make a copy whenever a block will outlive the scope it is defined in. In particular, these snippets are broken:

i​f​ ​(​r​a​n​d​(​)​ ​%​ ​1​ ​=​=​ ​0​)​ ​{​
 ​ ​ ​ ​b​l​o​c​k​P​t​r​ ​=​ ​^​{​ ​N​S​L​o​g​ ​(​@​"​Y​o​u​ ​a​r​e​ ​a​ ​w​i​n​n​e​r​!​"​)​;​ ​}​;​
}​ ​e​l​s​e​ ​{​
 ​ ​ ​ ​b​l​o​c​k​P​t​r​ ​=​ ​^​{​ ​N​S​L​o​g​ ​(​@​"​P​l​e​a​s​e​ ​t​r​y​ ​a​g​a​i​n​!​"​)​;​ ​}​;​
}​

The braces for the branches of the if statement introduce a new scope, so the blocks are invalid afterwards.

b​l​o​c​k​P​t​r​ ​=​ ​^​{​ ​N​S​L​o​g​ ​(​@​"​H​e​l​p​ ​m​e​"​)​;​ ​}​;​
r​e​t​u​r​n​ ​b​l​o​c​k​P​t​r​;​

Like returning the address of a local variable, returning a block that is still on the stack will cause problems ...