Clause - 11 stay operator= In dealing with “ Self assignment ”

• Make sure that when the object is self assigning operator= Have good behavior . Technology includes comparison “ Source object ” and “ Target audience ” The address of 、 Careful sequence of sentences 、 as well as copy-and-swap.
• Determine if any function operates on more than one object , When more than one object is the same object , Its behavior is still correct .

“ Self assignment ” Easy to use , Although it doesn't make sense , But if not Processing will cause system error .

1. Law 1 Traditional practice ： stay operator= Make one in “ Certificate test （ identity test ）” achieve “ Self assignment ” Test purpose of .

   widget& widget::operator=( const widget& rhs ){
       if ( this == &rhs ) 	return *this;	//  Certificate test （ identity test ）
       										//  If it's self assignment , Just don't do anything 
       delete pb;
       pb = new Bitmap( *rhs.pb );
       return *this;
   }
   

2. Law 2 Copy copies , Give Way operator= It has exceptional security

   widget& widget::operator=( const widget& rhs ){
       Bitmap* pOrig = pb;				//  Remember the original  pb
       pb = new Bitmap( *rhs.pb );		//  Make  pb  Point to  *pb  A copy of 
       delete pOrig;					//  Delete the original  pb
       return *this;
   }