▁is ▁un available ▁in ▁the ▁ 6 4 - bit ▁runtime . ▁Similar ▁functionality ▁can ▁be ▁achieved ▁by ▁using ▁method ▁sw izz ling ▁in ▁categories , ▁that ▁sw aps ▁one ▁method ' s ▁implementation ▁with ▁another ' s ▁that ▁have ▁the ▁same ▁signature . ▁ ▁For ▁the ▁versions ▁still ▁supporting ▁pos ing , ▁all ▁messages ▁sent ▁to ▁the ▁target ▁class ▁are ▁instead ▁received ▁by ▁the ▁pos ing ▁class . ▁There ▁are ▁several ▁restrictions : ▁ ▁A ▁class ▁may ▁only ▁pose ▁as ▁one ▁of ▁its ▁direct ▁or ▁indirect ▁super classes . ▁ ▁The ▁pos ing ▁class ▁must ▁not ▁define ▁any ▁new ▁instance ▁variables ▁that ▁are ▁absent ▁from ▁the ▁target ▁class ▁( though ▁it ▁may ▁define ▁or ▁override ▁methods ). ▁ ▁The ▁target ▁class ▁may ▁not ▁have ▁received ▁any ▁messages ▁prior ▁to ▁the ▁pos ing . ▁ ▁Pos ing , ▁similarly ▁with ▁categories , ▁allows ▁global ▁augment ation ▁of ▁existing ▁classes . ▁Pos ing ▁perm its ▁two ▁features ▁absent ▁from ▁categories : ▁ ▁A ▁pos ing ▁class ▁can ▁call ▁overrid den ▁methods ▁through ▁super , ▁thus ▁incorpor ating ▁the ▁implementation ▁of ▁the ▁target ▁class . ▁ ▁A ▁pos ing ▁class ▁can ▁override ▁methods ▁defined ▁in ▁categories . ▁ ▁For ▁example , ▁ ▁@ interface ▁Custom NS Application ▁: ▁NS Application ▁@ end ▁ ▁@ implementation ▁Custom NS Application ▁- ▁( void ) ▁set Main Menu : ▁( NS Menu *) ▁menu ▁{ ▁ ▁// ▁do ▁something ▁with ▁menu ▁} ▁@ end ▁ ▁class _ pose As ▁([ Custom NS Application ▁class ], ▁[ NS Application