Meaning of assign, copy, retain and other keywords in ios

Assign: Simple assignment, not changing index count-(void) Setmyobject: (ID) newvalue{_myobject=NewValue;} Copy: Create an object with an index count of 1 and then release the old object-(void) Setmyobject: (ID) newvalue{if(_myobject! =newvalue)         {[_myobject release]; _myobject=[newvalue Copy]; }retain: Frees the old object, assigns the value of the old object to the input object, and then increases the index count of the input object to 1-(void) Setmyobject: (ID) newvalue{if(_myobject! =newvalue)         {[_myobject release]; _myobject=[newvalue retain]; }}readonly indicates that this property is read-only, that is, only the getter method is generated, and the setter method is not generated. 

Meaning of assign, copy, retain and other keywords in ios