title: what is difference between alloc and allocWithZone:?
body: <p>From <a href="http://forums.macrumors.com/showthread.php?t=1048776" rel="noreferrer">forum discussion</a> , seem like that the big difference is performance factor, allocWithZone: will alloc memory from particular memory area, which reduce cost of swapping. </p>
  
  <p>In practice, almost get no chance to use allocWithZone: , anyone can give simple example to illustrate which case to use allocWithZone: ? </p>
  
  <p>Thanks,</p>